The Breathing Goddess: What's Happening to Oxygen in our Atmosphere?

The Goddess herself has created the oxygen she breathes (image by Dezgo.com) 

There is a hugely interesting paper that appeared this year on "Annual Reviews of Earth and Planetary Sciences." It deals with the evolution of oxygen during the Phanerozoic.  The story of the ecosphere is all in there. Unbelievably, some 300 million years ago, the Permian fauna and flora lived in a concentration of oxygen of about 35%, nearly double the current one (about 20%). Even the dinosaurs of the Cretaceous breathed air with about 25%-30% oxygen in it. Ever wondered why the diplodocus was so big? That's one possible reason. 

In comparison, we are oxygen-starved. And we are further reducing the oxygen concentration by burning fossil fuels (it can be observed experimentally). All this may have to do with the different breathing architecture of mammals and birds (aka dinosaurs).

But why these oscillations? They may be the result of geophysical factors. The oxygen concentration in the atmosphere is related to the interplay of carbon sedimentation and sedimented carbon oxidation. The former is mainly due to plate tectonics, the latter to large igneous provinces and similar volcanic events. The winding down of oxygen concentration might be explained by a slowdown of plate tectonics and hence of sedimentation at continental edges, but that's far from being certain. 

I tend to think that it is an indirect result of the sun becoming brighter over the Eons. This increase in irradiation forces the system to reduce the CO2 concentration to keep temperatures within the range needed for life. But that will also indirectly reduce the oxygen concentration because oxygen comes from a photochemical reaction that involves CO2 molecules. Less CO2, less oxygen. And that means we are winding down with the ecosystem complexity: it means, basically, that the biosphere doesn't produce as much oxygen as it used to in earlier times. 

If the oxygen concentration goes below certain limits, metazoa will not be able to survive. In a few tens of millions of years, Earth may have reverted to protozoa only (single-celled creatures), as it had been for more than 2 billion years.

Unless someone starts burning a fraction of the sedimented carbon to increase CO2 concentration AND simultaneously manages to reduce the solar irradiation. Maybe good old Gaia has exactly that in mind!!

https://www.annualreviews.org/doi/full/10.1146/annurev-earth-032320-095425

How to Freeze a Holobiont: The Great Proterozoic Catastrophe

 

Image created by Dall-E

One of the great features of climate science is that you always learn new things. It is the story of how our planet evolved, changed, and arrived where we are, silly naked apes as we are. One of the most fascinating moments of this long history is the "Snowball Earth" period. Some 700 million years ago, in the Sturtian period, at the end of the Proterozoic Eon, the whole planet got frozen, covered by an ice sheet. Yes, Earth was all white, ice-covered, and nicely frozen. The great holobiont that some call "Gaia" nearly died. It may have survived, barely, because some small areas, maybe volcanic hot springs, remained ice-free.

Why this event? An ongoing research effort is aimed at unraveling the story. The latest results show that the great freezing was preceded by the "Franklin Large Igneous Province" (Franklin LIP). It was another spectacular event but for the opposite reasons. It involved a gigantic volcanic eruption that spread molten lava over a huge region that went from the regions now called Alaska and Greenland. It lasted about one million years. Earth's history is never boring!

Now, some recent data show that the Franklin LIP took place immediately before the great Snowball Glaciation. And the proposal is that the LIP caused the snowball. 

Great, but one moment. There have been several Large Igneous Provinces, LIPs, in the later history of Earth. In most cases, the result was warming, not cooling. The "great dying" of the end of the Permian, about 250 million years ago (at the "Permian-Triassic" boundary) was caused by a giant LIP in the region called Siberia today. The demise of the dinosaurs (apart from birds) was caused by another huge LIP taking place in the Deccan region in what is now India (an asteroid may have helped, but the LIP was probably the main cause). Smaller LIPs also caused havoc in the ecosystem. 

The basic idea is that these huge eruptions inject large amounts of CO2 into the atmosphere, generating a rapid and catastrophic warming that, in turn, causes a dieoff in the ecosystem. So, what did the Franklin LIP have that is different from the more recent LIPs? Is this a new demonstration that climate science is all wrong, actually a hoax created by the appliance industry to force us to cook on induction stoves?

Well, no. If you know the basic mechanisms that control CO2 concentrations and consequently Earth's temperature, the story makes plenty of sense. The Franklin LIP was not different (not so much, at least) from the later LIPs. But the world in which these eruptions took place was completely different. 

Let's start from the beginning: How do LIPs emit CO2? It is because the volcanic lava contains carbonates, compounds that can decompose, producing CO2 and oxides. As for all chemical reactions, the equilibrium between CO2 release and uptake depends on temperature. At the high temperatures of molten lava, carbonates tend to decompose, but, when the lava cools down, the reverse reaction occurs. Now, CO2 reacts with silicates to form carbonates. If the mass of lava is truly huge, as it was in the Franklin LIP, then it is no surprise that the CO2 drawdown is massive. If the temperature is hot enough, but not too hot, the drawdown is also rapid, at least on the geological time scale. Starved of CO2, the atmosphere doesn't act any longer as a blanket to keep the Earth warm. Bang! It is snowball Earth. 

Now, there comes the fundamental point: at the time of the Franklin LIP, the interplay between CO2 release and uptake took place in a relatively simple world: There was no life on land; it only appeared a couple of hundred million years later. So, since the LIP took place mainly on land, it didn't interact with the land biosphere. Then, fast forward 400 million years in the future, and you are now seeing the huge, gigantic, enormous, appalling lava landscape that we call the Siberian LIP. Same thing as before, but with a profound difference. By then, life had spread on land and had plenty of time to create huge reservoirs of carbon that resulted from the decomposition of living matter. This carbon we call "coal." In addition, there were also reservoirs of kerogen (solid hydrocarbons dispersed in the soil) and those hydrocarbons that the naked apes living today on Earth call "fossil fuels." 

Imagine the lava spewing out from the Siberian LIP going through this mass of fossil carbon, and you see that there was a factor that didn't exist with the Franklin LIP: coal was burned by the hot lava and turned into CO2. The same thing happened some 190 million years later with the Deccan LIP. Huge amounts of carbon were turned into CO2 and the resulting warming wrecked the whole ecosystem. Bang! Another mass extinction (the poor dinosaurs were boiled alive). That was the rule with all the Phanerozoic LIPs; their correlation with mass extinctions is reasonably well established, although the details can be complicated. You can find more information in this paper by Ernst and Youbi.

And there we stand: the story of life on Earth is an incredible adventure that sees the geosphere, the atmosphere, and the biosphere interacting with each other, usually resulting in a certain degree of stability but sometimes leading to great upheavals. It is what's happening now, with a large tribe of naked monkeys having taken control of the biosphere and playing the role of a Large Igneous Province by burning the huge reserves of fossil hydrocarbons built over the Phanerozoic Eon. For sure, this new "artificial LIP" will not lead to cooling but to warming. A huge and rapid warming. Gaia will probably survive it, but the monkeys... well, it all to be seen

A simple discussion of the recent results on the Franklin LIP  can be found on Anton Petrov's blog.

How Gaia survived Snowball Earth is described in this post by Ugo Bardi https://theproudholobionts.blogspot.com/2022/12/how-gaia-saved-earth-from-cold-death.html

The paper on the Franklin LIP by Dufour et al. that stimulated this post can be found at https://www.sciencedirect.com/science/article/pii/S0012821X23002728

The link to the paper by Ernst and Youbi on mass extinctions is: https://www.sciencedirect.com/science/article/pii/S0031018217302857

If you cannot access these papers, ask me for a copy

Tickling Gaia's Feet. How to Deforest Oneself to Death

Image from Dezgo.com

A few years ago, I published the article reproduced below on my blog "Cassandra's Legacy" I think it is worth republishing it because the debate on global warming has entered a frenzied phase. On one side, the rapid rise of temperatures is evident, and it is worrying people a lot. On the other side, it has caused many people to take refuge in the usual cherry-picking. Hannibal's elephants crossing the Alps is a typical argument that revolves around the idea that "Climate has Always been Changing." 

But it is true that the climate has always been changing. A little, at least. The question is why. In this post, I advance the hypothesis that human actions were an important contribution to the creation of the "Roman Warm Period" (RWP) by deforesting the land, while after the collapse, the forests returned, creating the LALIA (Late Antiquity Little Ice Age). Small variations, but enough to affect humans a lot. These data may be seen as a confirmation of the importance of forests in affecting the atmosphere's temperature as an effect of the operation of the "biotic pump." Tickling Gaia's feet may be very dangerous if She decides to stomp you out. 

Sunday, February 14, 2016

The collapse of the Western Roman Empire: was it caused by climate change?

Image from the recent paper by Buentgen et al., published in "Nature Geoscience" on February 8, 2016. The red curves are temperature changes reconstructed from tree rings in the Russian Altai (upper curve) and the European Alps (lower curve). Note the remarkable dip in temperatures that took place starting with the 6th century AD. But, by then, the Western Roman Empire was past and gone. Its collapse was NOT caused by climate change. 

The relationship between climate and civilization collapse is a much-debated subject. From the recent collapse of the Syrian state to the much older one of the Bronze Age civilization, climate changes have been seen as the culprit of various disasters befalling human societies. However, an alternative view of societal collapse sees it as the natural ("systemic") result of the declining returns that a society obtains from the resources it exploits. It is the concept termed "diminishing returns of complexity" by Joseph A. Tainter.

On this point, there may well exist several causes for societal collapse. Either climate change or resource depletion may sufficiently weaken the control structures of any civilization to cause it to fold over and disappear. In the case of the Western Roman Empire, however, the data published by Buentgen et al. completely vindicate Tainter's interpretation of the collapse of the Roman Empire: it was a systemic collapse, and it was NOT caused by climate change. 

From the data, we can see that there was a cooling episode that probably affected the whole of Eurasia, starting at the beginning of the 6th century AD.  This period is called LALIA (Late Antiquity Little Ice Age), and it seems to have been stronger than the better-known LIA (Little Ice Age) that took place during the 18th and 19th centuries. The LALIA was caused, at least in part, by a series of volcanic eruptions that injected large amounts of particulate in the atmosphere; cooling it by reflecting sunlight. Overall, temperatures went down by a couple of degrees in comparison to the time that we call the "Roman Warm Period."

A brutal cooling, yes, and it surely had effects on human life, as discussed at length in the paper by Buentgen et al. But it had nothing to do with the fall of the Western Roman Empire, whose decline had started at least two centuries before. The Empire started its final disintegration phase at the beginning of the 5th century when it ceased to be able to garrison the fortifications at the borders. Then, Rome was sacked for one first time in 410 AD; and finally destroyed by the Vandals in 455 AD. That was the true end of the Western Empire, even though, for some decades, there were still individuals who claimed the title of Emperors. But all that took place in a period of relatively stable climate, at least from what we can say about the available data. So, the collapse was systemic, related to factors other than climate and, in my opinion, mainly related to the collapse of the Roman financial system; in turn caused by mineral depletion.

But could it be that, after all, there is a correlation between the Roman collapse and climate change? Just it would be the reverse of what it had been sometimes proposed: could the Roman collapse have caused the LALIA cooling (or, at least, contributed to it)? The idea is not farfetched: the population collapse that took place with the fall of the Empire could have led to a considerable level of reforestation of Western Europe, and that would have absorbed CO2 from the atmosphere. That would have been an added factor to volcanic cooling. It is an idea already expressed some time ago by William Ruddiman. It seems to be out of fashion nowadays, but I think that it should be explored more.

In the end, this story can teach us a lot: first of all, how fragile climate is. In the interpretation by Buentgen et al., just three volcanic eruptions - relatively large ones, but not truly gigantic - were sufficient to cause a two-degree cooling extending all over Eurasia. Think of what could be the effect if something similar were to happen in our times! Then, it shows also how the situation, today, has completely changed. Temperatures have taken a completely different trend with the start of large-scale emissions of greenhouse gases in the atmosphere. Incidentally, these data also confirm the "Hockey Stick" data by Michael Mann and others. Global warming is real, the earth's climate is fragile, and we are in big trouble.

Additional note: The data published in "Nature" generated a truly awful article in the "Daily Express" titled "Mini-ice age 1,500 years ago contributed to fall of Roman Empire". There, they put together an incredible mix of unrelated things, showing, for instance, gladiator games that had ceased to exist at least one century before the LALIA. Then, they say that the 6th-century cooling "contributed to the collapse of the Eastern Roman Empire." Which is an interesting extrapolation since the Eastern Empire didn't collapse until about a thousand years after the LALIA!!  At least, they should go back to junior high school, but, on the other hand, think of how they report about climate change: what would you expect from them when they discuss the Roman Empire?

(h/t Graham Readfearn)

How is the Great Holobiont Doing? Alive, but not so Well

 

This is a translation of a recent article I published in an Italian newspaper, "Il Fatto Quotidiano." It refers to the Italian situation, but I think it can be interesting for an international readership. Note how I tried to explain the situation in simple terms, remaining within very strict length limits. The article turned out to be popular, but, as usual, I received my daily dose of insults in the comments. It is like this -- people have an inner rage that they need to unleash in one way or another. I can understand them, but I wish they would find a better outlet for their sacrosanct rage.  Above, an image from the recent floods in Emilia Romagna in Italy, mentioned in the article as the cause of a quarrel between believers and unbelievers.

From "Il Fatto Quotidiano

Climate Change: Where do we stand?

By Ugo Bardi June 05, 2023

The environment and climate are not often mentioned in the media, apart from particular moments such as in the case of the controversy that followed the flooding in Emilia-Romagna. However, on longer time scales, we see that concern about climate change is gradually spreading. The latest Eurobarometer data (you can find them at this link) show that 12 percent of Europeans put climate change among their top concerns, with Italy exactly in the middle. This is not so small as it sounds: 10 years ago, only 6 percent of Europeans gave this answer, and in Italy, 4 percent. Even compared to pre-covid times (now remote), this year we gained a couple of "worry points."

The reaction to the perception of a serious problem can be simply to deny that it exists, but it may also be to exaggerate it. This was the case with the flooding in Emilia-Romagna where it was obvious to some that climate change was to blame while, for others, it was all the fault of the Greens, or perhaps the river rodents called "nutria." More generally, it seems clear that the rise in the number of worried people is going in parallel with that of the number of skeptics. The latter are very active in the discussion, albeit at a rather superficial level, with various accusations of conspiracies of the strong powers and reasoning about things like the Alps with no ice in the Middle Ages, and why don't you consider the effect of the sun, and then today it is raining, so what? On the other side, the reaction is not so much better. "The IPCC says so, hence it's true," or, worse, in Italy there came the proposal to ban by law "climate denialism." 

But instead of launching into talk of conspiracies or invoking censorship, shouldn't we try to better understand what we are talking about? Climate science was not invented by the WEF in cahoots with the Lizard People. And climate models are quite a different matter compared to the two hand-drawn curves that were used as an excuse to lock us at home at the time of the pandemic. Climate science has more than a hundred years of history of studying a difficult and complex subject and is now one of the most active and fruitful fields of study in modern science. It has given us a grand and fascinating picture of the behavior of Earth's climate over a time span of hundreds of millions of years and more. It allows us to interpret how the biosphere was able to survive all this time and to understand how phases of climate instability led to the great mass extinctions. That of the dinosaurs, 66 million years ago, was only one of many and not even the largest. 

Nothing about climate science is beyond criticism. In fact, without criticism, there is no progress. So, let us maintain a healthy skepticism, but let us also avoid destructive polemics that serve only to demonize, not to build. If we take the correct attitude, we see that climate change is not something that models predict for the more or less distant future. It is happening here and now: we can see it, and we can measure it. We have reached a CO2 concentration not seen in millions of years before our time, when temperatures were 4-7 degrees higher than today. And the temperature continues to rise. This year, the development of the condition called "El Niño" in the Pacific Ocean is already causing particularly high temperatures, and it could lead to 2023 breaking all historical records. 

The change is already doing us major damage, for example, making Italian cities unlivable in summer unless in conditioned environments. Not to mention the return of mosquitoes, now victorious everywhere. But the worst damage is being done by the tropicalization of the climate, with prolonged periods of drought alternating with periods of heavy rains. That these intense rains played a role in the disaster in Emilia Romagna is at least likely, although rain was certainly not the only factor at play. Add to that the disappearance of snow in the mountains that used to act as a water reservoir in the summer, and you understand the problems that drought brings to agriculture and why there is talk of ongoing desertification for southern Italy, and perhaps not only for the South. 

These problems can only get worse if we continue to behave as we have been doing, which is to ignore the impact of human activities on the ecosphere. CO2 emitted by fossil fuel combustion is probably the main factor causing warming, but others, such as deforestation and loss of biodiversity, have their weight. 

But let's end on a few optimistic notes. The first is that the global transition to renewables is going great. We have passed the $1 trillion per year level of investment in the transition. If we keep it up, we can reasonably hope to get rid of fossil fuels in a reasonable timeframe. Plus, we are seeing some "greening" of the planet, almost certainly caused by the fertilizing effect of CO2 (see this link). So it seems that the goddess Gaia is trying to lend us a hand in avoiding the worst. But we have to work on it, otherwise, the old lady might decide she can't stand us anymore and make us go the way of the dinosaurs.

Is Gaia going to die of heat stroke?

 

Figure from the recent paper by Duffy et al. on "Science"

What is the maximum temperature that the Earth's Holobiont (aka "Gaia") can withstand before collapsing? Some data from a recent paper in "Science" indicate that we are not so close to the "Venus scenario" that would sterilize our planet. Nevertheless, the system is under stress

As you see in the figure at the beginning of the post, the optimal average temperature of planet Earth for the standard C3 photosynthesis (that of trees) is around 16 C, not far from the current value of around 18 C. (note that the graph takes into account not just temperature, but CO2 concentrations, supposed to be affecting temperature). Switching to the C4 mechanism (grass and others) moves the maximum to about 28 C;  10 degrees higher than it is now. 

So, we have a comfortable range in terms of plant life, and consider that the "zero" in the curve doesn't mean that all plants die; just that they are less efficient, especially in the hot equatorial zones. Note also how the "respiration" curve keeps growing as a function of temperatures; in the paper, they say that it keeps growing up to 60-70 C. 

Complicated story, but in any case, Gaia is not going to die soon. Note that during the Eocene, some 56 million years ago, the Earth's temperature was indeed some 8-10 degrees higher than it is now, and the planet was covered with lush forests. It is believed that C4 photosynthesis didn't appear on Earth before 35 million years ago. So, even an extra 10 C of warming will not destroy the biosphere: Gaia has a thick skin.

Eventually, the increasing solar irradiation will kill the Great Earth Holobiont but, hopefully, that will take a few hundred million years (at least). About humans, though.... well, it is another story. Do we still have a few decades left? Maybe. 

Gaia on the Move: the Rise of the Savanna Monkeys

This text had already been published as an appendix to a longer post on the evolution of forests. It is republished here as a stand-alone post on the role of humans in the evolution of the world's forests (link to the image above)

Primates are arboreal creatures that evolved in the warm environment of the Eocene forests. They used tree branches as a refuge, and they could adapt to various kinds of food. Modern primates do not shy from hunting other species, maybe even ancient primates did the same. From the viewpoint of these ancient primates, the shrinking of the area occupied by tropical forests that started with the "Grande Coupure," some 30 million years ago, was a disaster. They were not equipped to live in savannas: they were slow on the ground: an easy lunch for the powerful predators of the time. Primates also never colonized the northern taiga. Most likely, it was not because they couldn't live in cold environments (some modern monkeys can do that), but because they couldn't cross the "mammoth steppe" that separated the Tropical forests from the Northern forests. If some of them tried, the local carnivores made sure that they didn't succeed. So, "boreal monkeys" do not exist (actually, there is one, shown in the picture, but it is not exactly a monkey!).  

Eventually, monkeys were forced to move into the savanna. During the Pleistocene, about 4 million years ago, the Australopithecines appeared in Africa, (image source). We may call them the first "savanna monkeys." In parallel, perhaps a little later, another kind of savanna monkey, the baboon, also evolved in Africa. In the beginning, australopithecines and baboons were probably practicing similar living techniques, but in time they developed into very different species. The baboons still exist today as a rugged and adaptable species that, however, never developed the special characteristics of australopithecines that turned them into humans. The first creatures that we classify as belonging to the genus Homo, the homo habilis, appeared some 2.8 million years ago. They were also savanna dwellers. 

This branch of savanna monkeys won the game of survival by means of a series of evolutionary innovations. They increased their body size for better defense, they developed an erect stance to have a longer field of view, they super-charged their metabolism by getting rid of their body hair and using profuse sweating for cooling, they developed complex languages to create social groups for defense against predators, and they learned how to make stone tools adaptable to different situations. Finally, they developed a tool that no animal on Earth had mastered before: fire. Over a few hundred thousand years, they spread all over the world from their initial base in a small area of Central Africa. The savanna monkeys, now called "Homo sapiens," were a stunning evolutionary success. The consequences on the ecosystem were enormous.

First, the savanna monkeys exterminated most of the megafauna. The only large mammals that survived the onslaught were those living in Africa, perhaps because they evolved together with the australopithecines and developed specific defense techniques. For instance, the large ears of the African elephant are a cooling system destined to make elephants able to cope with the incredible stamina of human hunters. But in Eurasia, North America, and Australia, the arrival of the newcomers was so fast and so unexpected that most of the large animals were wiped out. 

By eliminating the megaherbivores, the monkeys had, theoretically, given a hand to the competitors of grass, forests, which now had an easier time encroaching on grassland without seeing their saplings trampled. But the savanna monkeys had also taken the role of megaherbivores. They used fires with great efficiency to clear forests to make space for the game they hunted. Later, as they developed metallurgy, the monkeys were able to cut down entire forests to make space for the cultivation of the grass species that they had domesticated meanwhile: wheat, rice, maize, oath, and many others. 

But the savanna monkeys were not necessarily enemies of the forests. In parallel to agriculture, they also managed entire forests as food sources. The story of the chestnut forests of North America is nearly forgotten today but, about one century ago, the forests of the region were largely formed of chestnut trees planted by Native Americans as a source of food (image source). By the start of the 20th century, the chestnut forest was devastated by the "chestnut blight," a fungal disease that came from China. It is said that some 3-4 billion chestnut trees were destroyed and, now, the chestnut forest doesn't exist anymore. The American chestnut forest is not the only example of a forest managed, or even created, by humans. Even the Amazon rainforest, sometimes considered an example of a "natural" forest, shows evidence of having been managed by the Amazonian Natives in the past as a source of food and other products. 

The action of the savanna monkeys was always massive and, in most cases, it ended in disaster. Even the oceans were not safe from the monkeys: they nearly managed to exterminate the baleen whales, turning large areas of the oceans into deserts. On land, entire forests were razed to the ground. Desertification ensued, brought upon by "megadroughts" when the rain cycle was no more controlled by the forests. Even when the monkeys spared a forest, they often turned it into a monoculture, subjected to be destroyed by pests, as the case of the American chestnuts shows. Yet, in a certain sense, the monkeys were making a favor to forests. Despite the huge losses to saws and hatchets, they never succeeded in completely exterminating a tree species, although some are critically endangered nowadays. 

The most important action of the monkeys was their habit of burning sedimented carbon species that had been removed from the ecosphere long before. The monkeys call these carbon species "fossil fuels" and they have been going on an incredible burning bonanza using the energy stored in this ancient carbon without the need of going through the need of the slow and laborious photosynthesis process. In so doing, they raised the concentration of CO2 in the atmosphere to levels that had not been seen for tens of millions of years before. That was welcome food for the trees, which are now rebounding from their former distressful situation during the Pleistocene, reconquering some of the lands they had lost to grass. In the North of Eurasia, the Taiga is expanding and gradually eliminating the old mammoth steppe. Areas that today are deserts are likely to become green. We are already seeing the trend in the Sahara desert. 

What the savanna monkeys could do was probably a surprise for Gaia herself, who must be now scratching her head and wondering what has happened to her beloved Earth. And what's going to happen, now?  There are several possibilities, including a cataclysmic extinction of most vertebrates, or perhaps all of them. Or, perhaps, a new burst of evolution could replace them with completely new life forms. What we can say is that evolution is turbo-charged in this phase of the existence of planet Earth. Changes will be many and very rapid. Not necessarily pleasant for the existing species but, as always, Gaia knows best. 

How Gaia Saved the Earth from a Cold Death

 

The Goddess Gaia in the form of the winter deity Khione, daughter of Boreas, the North Wind, and the Athenian princess Oreithyia (image by "Nobody-Important"). 

Earth is a fragile planet and it might freeze to a snowball if not taken care of. So far, the Goddess has done a good job at that but, at least a couple of times during the past few billion years, the Earth actually froze. Might that happen again? It seems that we were close to that just a few tens of thousands of years ago. Now, the problem doesn't exist anymore, with humans pumping zillions of tons of greenhouse gases into the atmosphere. And, who knows? Humans could be the tool used by the Goddess to avoid another "snowball Earth." But now we may have too much of a good thing and the Earth risks boiling. Hopefully, Gaia can take care of that, too.   

It is always amazing to realize how complex is the system that we call the "Ecosphere". And how the system's complexity keeps its parameters within the limits needed for life to exist and prosper. It is the concept of "Gaia" as it was proposed by James Lovelock and Lynn Margulis. The ecosystem is in homeostasis and tends to maintain relatively constant parameters by means of a tangle of internal feedbacks, as all complex adaptive systems ("CAS") do. 

But homeostasis doesn't mean perfect stability. The system's parameters may oscillate - even wildly - before the internal feedbacks can bring them back to the "good" values. Sometimes the system gets close to its limits and it may well be that, at times in its long history, the ecosystem risked going over the edge and then Gaia could "die." This seems to be a common destiny for extrasolar planets, as recently argued by Chopra and Linewaver.

A recent paper by Galbraith and Eggleston on Nature starts from these concepts, noting how the concentration of CO2 in the atmosphere never went below ca. 190 ppm during the past 800,000 years. That happened in correspondence with the lowest temperatures ever observed during that period: the planet was going through a harsh ice age.

This figure from a recent paper by Galbraith and Eggleston on Nature shows an interesting fact: the concentration of CO2 in the atmosphere never went below ca. 190 ppm over the past million years or so. Possibly, it touched the danger limit for the ecosystem to survive. For lower concentrations, plants wouldn't have been able to perform photosynthesis and the biosphere would have largely disappeared.

About these ice ages, there is an interesting point related to the system's feedback. The more ice there is, the more reflective the planet's surface becomes (more exactly, the planetary albedo increases). But, the more reflective the planet's surface is, the cooler the planet becomes. So, we have an enhancing feedback that may transform the whole planet into a single, frozen ball: "snowball earth". It has happened, although possibly not completely, at least twice in the history of Earth. It was during the period we call, appropriately "Cryogenian," from 720 to 635 million years ago. It was not a real "snowball" -- not all of Earth was covered in ice. But what was not under the ice was a frozen desert. To give you some idea of the fascination of this subject, here is an excerpt from the abstract of a paper by Hoffmann et al. on "Science"

"....the small thermal inertia of a globally frozensurface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO2rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The sub-glacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the icecover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. "

Can you imagine the Earth in these conditions? A wasteland of dry deserts and ice sheets. At that time, there were no multicellular creatures and life may have survived in hot pockets, maybe volcanic lakes, where it was still possible to find liquid water. 

We may have been dangerously close to a new snowball Earth episode during the past million years or so. Not a trifling matter because today the ecosphere is much more complex than it was at the time of the Cryogenian. A new snowball Earth would likely cause all vertebrate lifeforms to go extinct. It is not just a question of being too cold: the limit of concentration of CO2 that permits plants to perform photosynthesis at a reasonable rate is considered to be around 150 ppm, at least for the most common kind of plants. Under that value, all multicellular plants die, and with them all animal life. Only single-celled creatures could eke out a precarious existence in those conditions. 

But something prevented the ice sheets to expand all the way to envelop the whole Earth and, at the same time, prevented the CO2 concentration to go below 190 ppm. What was that? Several hypotheses are possible. Galbraith and Eggleston favor a biological one, saying that:

In terrestrial ecosystems, carbon fixation by plants is limited by low ambient CO2 (ref. 31). On this basis, ref. 12 proposed that CO2-limitation had significantly reduced plant-mediated silicate weathering during low-CO2 intervals of the past 24 million years, thereby enforcing a lower bound on the ocean–atmosphere carbon inventory over >10^5 yr timescales. Subsequent experiments have been consistent with this ‘carbon starvation’ mechanism, showing reduced weathering by tree-root-associated fungi under low CO2 (ref. 32). Although the feedback on silicate weathering would appear too slow to play a role on the 104 yr timescale of glacial CO2 minima 30, it may be possible that strongly reduced weathering rates lowered ocean alkalinity (thereby decreasing CO2 solubility) on a millennial timescale. Alternatively, reduced photosynthesis rates during the LGM (last glacial maximum) would have slowed the accumulation of terrestrial biomass14, consistent with estimates for lower terrestrial primary production rates33. By slowing the accumulation of carbon in vegetation and soils, this would have provided a stabilizing feedback via an increase of the ocean–atmosphere carbon pool.

Complicated stuff, right? But, basically, the idea is that CO2 is slowly drawn down from the atmosphere by a reaction with rocks (silicates), forming carbonates. This reaction is called "weathering" and it is favored by plants, whose roots provide a good environment for it to take place. Fewer plants, less CO2 drawdown. At the same time, a smaller global biomass means that the quantity of CO2 stored in it becomes lower and this extra carbon most likely ends up in the atmosphere as CO2. So, there are two feedbacks embedded in the system that tend to stabilize its temperature. But, as you may understand from the text by Galbraith and Eggleston, it is even more complicated than this! In any case, these stabilizing geobiological feedbacks oppose the ice/albedo feedback and tend to slow down the glaciation before the two sides of the ice sheet touch each other at the equator. 

But suppose that the Earth really became the snowball that some studies claim to have observed: how did it recover? If it is frozen, it is frozen. Maybe not completely dead, but poor Gaia was reduced to a minor sprite inhabiting hot springs. How could Earth return to the lush ecosphere we are used to?

There is an explanation: it is because volcanoes do not care whether the Earth's surface is frozen or not. They continue pumping CO2 and other greenhouse gases into the atmosphere. Again from Hofmann et al. 

“If a global glaciation were to occur, the rate of silicate weathering should fall very nearly to zero (due to the cessation of nor-mal processes of precipitation, erosion, and runoff), and carbon dioxideshould accumulate in the atmosphere at whatever rate it is releasedfrom volcanoes. Even the present rate of release would yield 1 bar ofcarbon dioxide in only 20 million years. The resultant large green houseeffect should melt the ice cover in a geologically short period of time”[(69), p. 9781]. Because Snowball Earth surface temperatures are below the freezing point of water everywhere, due to high planetary albedo,there is no rain to scrub CO2(insoluble in snow) from the atmosphere."

Note one subtle detail: if temperatures were to go below the freezing point of CO2 (-78 C) even in small regions at the poles, that would form a nearly infinite CO2 sink. And that would be "snowball forever" -- maybe it would have made the Goddess Khione happy, but it didn't happen. Possibly, that was too cold even for a Winter Goddess!

In any case, it seems that CO2 was pumped into the atmosphere by volcanoes, maybe it was the work of the volcanic form of Gaia, the goddess Pele, known for her habit of taking lava showers. 

When the CO2 concentration arrived at levels hundreds of times those of the present-day atmosphere, the result was a cataclysmic rapid collapse of the glaciers and a rise in temperatures. Not only the Earth's ecosystem was saved from a cold death, but it rebounded spectacularly: it was now the time of the "metazoa," the formal term indicate animals. There came the Cenozoic, in which we are still living, with its incredible variety of lifeforms when plants and animals colonized the continental lands. 

You see how the job of Gaia is not so simple. it involves a delicate balance of many factors. Some tend to stabilize the system, while others tend to destabilize it. During the past 15 million years or so, cooling factors took the lead and slowly pushed Earth to lower and lower CO2 concentrations and, with that, lower temperatures.

 Image from Wikipedia Commons. The x scale is in million years from the present. Note the rapid cooling of the past million years or so.

We do not know exactly what caused the cooling, there are several theories. But one thing is sure, Gaia started feeling that it was too cold for her, even in her form of Khiome, goddess of ice. She could die and, this time, perhaps for good. 

(Image source)

So, it became imperative for Gaia to mobilize some of the geosphere carbon and push it into the atmosphere in the form of a greenhouse gas that would warm the Earth back to comfortable temperatures. The Goddess Pele was too slow for that, maybe she is now a little tired after blowing CO2 into the atmosphere for four billion years. So, maybe Gaia thought of a more creative solution. 

Why not use those clever monkeys which had just evolved in Earth's savannas to dig carbon out of Earth's crust, combine it with oxygen, and then pump it back into the atmosphere?  It worked: in just a few hundred years, the monkeys managed to bring back the CO2 concentration to the levels that were typical of Earth as it was a few tens of millions of years ago. 

It may be that, now, Gaia faces the opposite problem: those monkeys have pumped so much CO2 into the atmosphere that now we risk pushing the planet on the opposite side of a climate collapse, to a "hothouse Earth" that might kill the biosphere. Something like that happened with the great extinctions at the end of the Permian and the Cretaceous. Alas, life is difficult, but Gaia can cope. Does that mean getting rid of those pesky carbon-burning monkeys? Maybe. After all, Gaia is a Goddess, she ought to know what she is doing and she has no qualms when it is time to do what's to be done. She can find ways. 

(Image Source)

James Lovelock, 1919-2022. One of the great minds of the twentieth century

The great rainforest holobiont is part of the even greater holobiont we call "Gaia"

James Lovelock left us at 103, after a life dedicated to science. His main contribution was the concept of “Gaia,” which will forever accompany his name.

There are many ways to be a scientist: some are collectors who collect facts as if they were stamps. Others are theorists, who spend their lives building castles in the air that never touch the real world. There are those who spend their lives criticizing, and many who see science as a competition to prove they are better than others.

Lovelock was in another category: he never wrote an equation, he never worried about cheap brawls between scientists, and never even an employee of a university or a research institute. He was a creative, one who was not afraid to build measurement tools using his hands, a characteristic of creatives who often combine manual and mental skills. Lovelock was part of the tradition of the great creative scientists of the past, walking on the same path that Charles Darwin had started tracing with his theory of evolution by natural selection. (like Lovelock, Darwin, never wrote an equation!)

For a scientist, being creative is risky. The creative seeks the perfect blend of data and intuition and does not always succeed. An intuition without data is nonsense, while data without intuition is nothing more than a telephone directory. But Lovelock managed to get the right blend with Gaia.

Like all creatives, from Newton onwards, Lovelock hoisted himself on the shoulders of giants, taking from them what he needed for his synthesis. Lynn Margulis and William Golding are equally responsible for the idea of “Gaia,” in the sense of the terrestrial ecosystem. But it was Lovelock who acted as the spearhead, launching the idea as early as 1972, after studying the data coming from the first probes that had landed on Mars. His basic intuition, that oxygen is the “signature” of the existence of biological life, was right. Then, he expanded this idea to explain how the whole planetary ecosystem self-regulates by a series of feedback mechanisms.

As always happens, also in science original and innovative ideas tend to be attacked with a vehemence that goes beyond the need for proper verification. Lovelock's idea had an undertone of mysticism, of "New Age," of hippies smoking weed, that kind of thing. And, above all, it went directly against the dominant paradigm of the time, that of “neodarwinism” which couldn’t conceive how the creature called “Gaia” could emerge without being in competition with others for the same resources.

You can imagine the controversy that came up. And, even today, officially we must use the term "Gaia hypothesis" to avoid the risk of being mistreated by the defenders of the orthodoxy. And yet, perhaps unexpectedly, Lovelock's idea “Gaia " was never completely discredited, despite the crossfire of critics.

Of course, Lovelock was not always right, and his ideas had to be refined, tuned, and sometimes radically changed. He had to back down from some interpretations that turned out to be too radical: for instance, he argued that an ice age is a perfect condition for Gaia to exist to maximize the ecosystem’s “metabolic rate.” It seems clear, nowadays, that it is not the case. Then, one of the regulation mechanisms he had initially proposed, the “CLAW hypothesis,” based on the role of phytoplankton in generating cloud condensation mechanisms, turned out to be probably wrong or, at least, not relevant. And sometimes his interpretations of Gaia as endowed with a certain volition of hers went a little too far on the side of mysticism.

But these mistakes are not crucial. The point is that the idea of Gaia is fundamental to understanding how it's possible that such a fragile thing as biological life has existed on Earth for at least three billion years. It was not by accident, but by the self-regulating capabilities of the system that allowed it to survive the various catastrophes that hit Earth during this long period. Then, you may call this capability with a different name. It doesn’t matter: "Gaia" remains a fundamental idea for today's science, still a source of new ideas, new insights, and new discoveries.

And I think the idea of Gaia also goes beyond the dry terms that science uses to describe phenomena such as “complex adaptive systems” or “self-regulating feedback systems.” I think that we can say that “something” exists, out there, that’s beyond our capabilities of understanding. If we want to call that “something” Gaia, it is perfectly legitimate. And if we wish to see “her” as a Goddess, it is legitimate, too. Who said that science must always be right? So, we can thank Gaia for having been so kind to James Lovelock, and giving him a long and productive life. May he rest in peace in the arms of the Goddess he created, and who created him.

Gaia's One Billion Years Task: Colonizing the Land

 

Gaia as the sea Goddess Grammamare according to Hayao Miyazaki's interpretation in the film "Ponyo"

Imagine a time machine that brings you back to the Earth of one billion years ago, right in the middle of the eon called the "Proterozoic." First of all, you need an oxygen respirator, otherwise you'll die of suffocation in a few minutes. You also need a wide-brimmed hat and an outfit that covers your limbs in such a way as to protect your skin from the ultraviolet radiation. It is your planet, but in this period it is not especially friendly to a metazoan as you are.

You walk a few cautious steps onward. In front of you, the blue sea. You turn around: an expanse of dry rocks that continues all the way to the horizon. No traces of anything green that you can see: no plants, no insects, no birds, nothing like that. Above you, the sun is bright in the blue sky. You notice that it is a little less bright than you are used to seeing it, in your time. No traces of clouds: it is what you expected: no trees means no evapotranspiration of water vapor, no volatile organic compounds to function as nucleation sites for the water droplets that form clouds. 

You walk toward the sea. There are mainly rocks, but also some sandy places: small patches of beach. If there is a beach, there has to be a river, somewhere, that created it. You see it, not far away. It is completely dry, its bed going straight through the rocky landscape from the hills in the distance. Rains, when they arrive, must be torrential downpours that come and go quickly. 

You kneel on the beach, in front of the sea, lifting some water with your cupped hands. You know that it should be less salty than the seawater you are used to in your time, and you are tempted to taste it to confirm. But that is not a good idea. That water is brimming with microorganisms, most of them unlike anything your immune system is used to. You drop the water on the surface of a rock, where it forms a dark spot that rapidly evaporates and disappears. 

Standing up in front of that alien sea, you look at the gentle waves coming and going. You know that there are no fish in there. No crabs, no seashells, no seaweed, nothing like that. But there are enormous numbers of microorganisms. They are photosynthesizing, eating each other, reproducing by splitting themselves in two. They can live only in water. Is there life on the dry rocks on the shore? Maybe some of those microscopic creatures survive there, maybe even thrive, perhaps algae or even ancestors of modern lichens. But they are just eking out a precarious existence. They are invisible to the naked eye, and their time has not come yet.

On the horizon, an enormous orange moon rises as the sun slowly fades on the opposite side. You keep looking at the dark waters in front of you. Just under the surface, you glimpse something that looks like a pair of large eyes. You think you see her just for a moment, Gaia in her form of sea goddess, languidly swimming in the calm sea. 

____________________________________________________

Back to your time machine. You dial 350 million years before your time, the start of the Carboniferous Period. You press the button. 

You emerge out of the machine, breathing the fresh air, smelling something you had never smelled before. Whatever it is, the air is humid, rich in oxygen. You are in a small clearing, in front of you, there is a pond surrounded by a lush forest. Trees, tall trees, forming a full canopy under the low clouds, swept by a gentle wind. The place is eerily silent: no birds, no insects, nothing like that. Yet, you recognize the place: this is your planet, Earth, not yet the way it will be in the future that is your time, but a familiar world. 

As you stand, a noise comes to you: a buzz. You see something flying away, an insect of some kind. It starts raining. It is a warm, gentle downpour that wets you rapidly, but ends quickly. It has been enough to disturb the creatures living under the low bushes. You have a glimpse of them scuttling away: tetrapods, early amphibians. They jump into the water of the pond and then disappear. They are your ancestors, the ancestors of all the metazoans that will move on land in the future that's your time. 

As you walk, splashing your boots on the mud, you wonder how Gaia pulled this incredible trick: transforming the bare rock of entire continents into lush forests. While you think that, you have a glimpse of a pair of bright eyes staring at you from the canopy. You look up, and they disappear, leaving only a Cheshire-cat smile of the Goddess of the Forests, then she vanishes among the branches.

Images of the Goddess courtesy of "Mon Seul Desir"

The Great Cycle of Earth's Forests

 

 Forests appeared on Earth some 400 million years ago, and they have been thriving over that long period. But, during the past 150 million years, they started to show signs of distress, reacting to the decline in atmospheric CO2 concentrations and to the competition with grasslands. As Earth changes, will forests be able to cope and survive? It is an extremely slow trend, but we cannot rule out that forests will conclude their cycle and disappear in a geologically short time. This text is an attempt to reconstruct the story of forests and to imagine what their future could be in deep time. (image courtesy of Chuck Pezeshky) 

A forest is a magnificent, structured, and functional entity where the individual elements -- trees -- work together to ensure the survival of the ensemble. Each tree pumps water and nutrients all the way to the crown by the mechanism called evapotranspiration. The condensation of the evaporated water triggers the phenomenon called the "biotic pump" that benefits all the trees by pumping water from the sea. Each tree pumps down the carbohydrates it manufactures using photosynthesis to its mycorrhizal space, the underground system of roots and fungi that extracts mineral nutrients for the tree. The whole "rhizosphere" -- the root space -- forms a giant brain-like network that connects the trees to each other, sometimes termed "the Wood Wide Web." It is an optimized environment where almost everything is recycled. We can see it as similar to the concept of "just in time manufacturing" in the human economy. 

Forests are wonderful biological machines, but they are also easily destroyed by fires and attacks by parasites. And forests have a competitor: grass, a plant that tends to replace them whenever it has a chance to. Areas called savannas are mainly grass, although they host some trees. But they don't have a closed canopy, they don't evapotranspirate so much as forests, and they tend to exist in much drier climate conditions. Forests and grasslands are engaged in a struggle that may have started about 150 million years ago when grass appeared for the first time. During the past few million years, grasses seem to have gained an edge in the competition, in large part exploiting their higher efficiency in photosynthesis (the "C4" pathway) in a system where plants are starved for CO2.

Another competitor of forests is a primate that left its ancestral forest home just a couple of million years ago to become a savanna dweller -- we may call it the "savanna monkey," although it is also known as "Homo," or "Homo sapiens." These monkeys are clever creatures that seem to be engaged mainly in razing forests to the ground. Yet, in the long run, they may be doing forests a favor by returning the atmospheric CO2 concentration to values more congenial to the old "C3" photosynthetic mechanism still used by trees. 

Seen along the eons, we have an extremely complex and fascinating story. If forests have dominated Earth's landscape for hundreds of millions of years, one day they may disappear as Gaia gets old. In this post, I am describing this story from a "systemic" viewpoint -- that is, emphasizing the interactions of the elements of the system in a long-term view (it is called also "deep time"). The post is written in a light mood, as I hope to be able to convey the fascination of the story also to people who are not scientists. I tried to do my best to interpret the current knowledge, I apologize in advance for the unavoidable omissions and mistakes in such a complex matter, and I hope you'll enjoy this post. 

The Origin of Forests: 400 million years ago

Life on Earth may be almost 4 billion years old but, since we are multicellular animals, we pay special attention to multicellular life. So, we tend to focus on the Cambrian period (542-488 million years ago), when multicellular creatures became common. But that spectacular explosion of life was all about marine animals. Plants started colonizing the land only during the period that followed the Cambrian, the Ordovician, (485 - 443 million years ago). 

To be sure, the Ordovician flora on land was far from impressive. As far as we know, it was formed only by moss (perhaps lichens, too, but it is not certain). Mosses are humble plants: they are not vascularized, they don't grow tall, and they surely can't compare with trees. Nevertheless, mosses could change the planetary albedo and perhaps contribute to the fertilization of the marine biota -- something that may be related to the spectacular ice ages of the Ordovician. It is a characteristic of the Earth system that the temperature of the atmosphere is related to the abundance of life. More life draws down atmospheric CO2, and that cools the planet. The Ordovician saw one of these periodic episodes of cooling with the start of the colonization of the land. (image from Wikipedia)

There followed another long period called the "Silurian" (444 – 419 My ago) when plants kept evolving but still remained of the size of small shrubs at most. Then, during the Devonian (419 -359 million years ago) we have evidence of the existence of wood. And not only that, the fossil record shows the kind of channels called "Xylem" that connect the roots to the leaves in a tree. These plants were already tall and had a crown, a trunk, and roots. By the following geological period, the Carboniferous (359 - 299 My ago), forests seem to have been widespread.  

A major feature of these ancient trees was the development of an association with fungi. Their roots formed what we call a "mycorrhizal" symbiotic system. The fungi receive carbohydrates that the tree manufactures using photosynthesis, while the tree receives from the fungi essential minerals, including nitrogen and phosphorous. We don't know the details of how this symbiotic relationship evolved over hundreds of millions of years but, below, you can see a hypothesis of how it could have happened. (Source) (in the figure, "AM" stands for "arbuscular mycorrhiza" - the oldest form of symbiotic fungi).

Another major evolutionary innovation that may have been already operating in the Paleozoic forests is the "biotic pump." As an effect of the pressure drop created by the condensation of evapotranspirated water, forests can create pump water vapor from the ocean and create the "atmospheric rivers" that bring water inland. That, in turn, creates the land rivers that bring that water back to the sea. As forests create their own climate, they can expand nearly everywhere. The image shows clouds created by condensation over the modern Amazon rainforest (source).  

If we could walk in one of those ancient forests, we would find the place familiar, but also a little dreary. No birds and not even flying insects, they evolved only tens of million years later. No tree-climbing animals: no monkeys, no squirrels, nothing like that. Even in terms of herbivores, we have no evidence of the kind of creatures we are used to, nowadays. Grass didn't exist, so grazers couldn't exist either. Herbivores were browsers surviving on leaves or on decaying plant matter. Lots of greenery but no flowers, they had not evolved yet. You see in the image (source) an impression of what an ancient forest of Cladoxylopsida could have looked like during the Paleozoic era.

The Paleozoic forests already had one of the characteristics of modern forests: fires. There had never been fires on Earth before for at least two good reasons: one was that there was not enough oxygen, and the other was that there was nothing flammable. But now, with the oxygen concentration increasing and plants colonizing the land, fires appeared, lighting up the night. They would remain a characteristic of the land biosphere for hundreds of millions of years.

Image Source. The "fire window" is the region of concentrations in atmospheric oxygen in which fires can occur. Note how during the Paleozoic, the concentration could be considerably larger than it is now. Fireworks aplenty, probably. Note also how there exist traces of fires even before the development of full-fledge trees, in the Devonian. Wood didn't exist at that time, but the concentration of oxygen may have been high enough to set other kinds of dry organic matter on fire. 

Wildfires are a classic case of a self-regulating system. The oxygen stock in the atmosphere is replenished by plant photosynthesis but is removed by burning wood. So, fires tend to reduce the oxygen concentration and that makes fires more difficult. But the story is more complicated than that. Fires also tend to create "recalcitrant" carbon compounds, charcoal for instance, that are not recycled by the biosphere and tend to remain buried for long times -- almost forever. So, over very long periods, fires tend to increase the oxygen concentration in the atmosphere by removing CO2 from it. The conclusion is that fires both decrease and increase the oxygen concentration. How about that for a taste of how complicated the biosphere processes are? 

 The Mesozoic: Forests and Dinosaurs

At the end of the Paleozoic, some 252 million years ago, there came the great destruction. A gigantic volcanic eruption of the kind we call "large igneous province" (sometimes affectionately "LIP") took place in the region we call Siberia today. It was huge beyond imagination: think of an area as large as modern Europe becoming a lake of molten lava. (image source)

It spewed enormous amounts of carbon into the atmosphere in the form of greenhouse gases. That warmed the planet, so much that it almost sterilized the biosphere. It was not the first, but it was the largest mass extinction of the Phanerozoic age. Gaia is normally busy keeping Earth's climate stable, but sometimes she seems to be sleeping at the wheel -- or maybe she gets drunk or stoned. The result is one of these disasters.  

Yet, the ecosystem survived the great extinction and rebounded. It was now the turn of the Mesozoic era, with forests re-colonizing the land. Over time, the angiosperms ("flowering plants") become dominant over the earlier conifers. With flowers, forests may have been much noisier than before, with bees and all kinds of insects. Avian dinosaurs also appeared. They seem to have been living mostly on trees, just like modern birds. 

For a long period during the Mesozoic, the landscape must have been mainly forested. No evidence of grass being common, although smaller plants, ferns, for instance, were abundant. Nevertheless, the great evolution machine kept moving. During the Jurassic, a new kind of mycorrhiza system evolved, the "Ectomycorrhizae" which allowed better control of the mineral nutrients in the rhizosphere, avoiding losses when the plants were not active. This mechanism was typical of conifers that could colonize cold regions of the supercontinent of the time, the "Pangea."  

During the Mesozoic, the dinosaurs appeared and diffused all over the planet. You surely noted how the Jurassic dinosaurs were often bipedal (See the illustration showing an early form of Iguanodon). They are also called "ornithopods," it is a body plan that allows herbivorous creatures to browse on leaves on the high branches of trees. A bipedal stance makes the creature able to stand up, balancing on its tail, reaching higher heights. Some dinosaurs chose a different strategy, developing very long necks for the same purpose: the brontosaurus is iconic in this sense even though, traditionally, it was shown half-submerged in swamps (the illustration is from the New York Tribune of 1919). The idea that brontosaurs lived mainly in swamps is not so bright, if you think about that. Why should a semiaquatic creature need a long neck? Think of a hippo with the neck of a giraffe: it wouldn't work so well. 

A much better representation of long-necked dinosaurs came with the first episode of the "Jurassic Park" (1993) movie series when a gigantic diplodocus eats leaves. At some moment, the beast rises on its hind legs, using the tail as further support. 

If you are a dinosaur lover (and you probably are if you are reading this post) seeing this scene must have been a special moment in your life. And, after having seen it maybe a hundred times, it still moves me. But note how the diplodocus is shown in a grassy environment with sparse trees: a Savanna. That's not realistic because grass didn't exist yet when the creature went extinct at the end of the Jurassic period, about 145 million years ago. 

To see grass and grazers, animals specialized in eating it, we need to wait for the Cretaceous (145-66 million years ago). Evidence that some dinosaurs had started eating grass comes from the poop of long-necked dinosaurs. That's a little strange because, if you are a grazer, the last thing you need is a long neck. But new body plants rapidly evolved. The Ceratopsia were the first true grazers, also called "mega-herbivores". Heavy, four-legged beasts that lived their life keeping their head close to the ground. The Triceratopses gained a space in human fantasy as prototypical dinosaurs, and they are often shown in movies while fighting tyrannosauruses. You see that scene in Walt Disney's movie "Fantasia" (1940). It may have happened for real.

Note the heavy bone shield over the head. With so much weight on board, Trixie couldn't possibly rise on its hind legs to munch on leaves on tree branches. Note also the beak, it looks perfectly adapted for collecting grass. It means that the Cretaceous landscape was probably similar to our world. We don't know if there existed the kind of biome we call today "savanna" -- a mix of grass and trees, but surely the land was shared by forests and grass, each biome with its typical fauna. 

The Great Cooling and the Rise of C4 Grass

At the end of the Cretaceous period, 66 million years ago, a new large igneous province appeared in the Deccan region, in India. It generated another climate disaster with the associated mass extinction. Most dinosaurs were wiped out, except those we call "birds" today. A large meteorite also hit Earth at that time. It caused only minor damage but, millions of years later, it gave human filmmakers a subject to explore in many dramatic movies. 

In time, the Deccan LIP faded away, and the era that followed is called the "Cenozoic." The ecosystem recovered, forests re-colonized the land, and mammals and birds (the only survivors of the Dinosauria clade) fought to occupy the ecological niches left free by their old masters. The early Cenozoic was a warm period of lush forests that offered refuge to a variety of animals: birds made their nests in branches, while squirrels and other small mammals jumped from branch to branch, or lived at the bottom. It is during this period that primates evolved: the huge forests of those times offered refuge for a variety of species that had probably already developed sophisticated social behaviors.  

Grass also survived the end-Cretaceous catastrophe. As a result, some mammals evolved into new "megaherbivores" or "megafauna" that occupied the same ecological niche that the triceratopsides had colonized long before.  Here is a brontotherium, a large herbivorous mammal that lived some 37-35 million years ago, during the late Eocene period (image from BBC).

The megabeasts of the Cenozoic do not have the same fascination of the giant dinosaurs, but this creature has a nice-sounding name, and it looks a little like Shrek, the ogre of Spielberg's movie. Note how the beast is correctly shown walking on a grassy plain. The Eocene is supposed to have been mostly forested, but grass existed, too. The brontotherium was an opportunistic grazer, apparently able to subsist on various kinds of food, not just grass. 

During the warm phase of the Cenozoic, Earth reached a maximum temperature around 55 million years ago, some 8-12 deg C higher than today. The concentration of CO2, too, was large. That is called the "early Eocene climatic optimum". It doesn't mean that this period was better than other periods in terms of climate, but it seems that Earth was mainly covered with lush forests and that the biosphere thrived.  

Then, the atmosphere started cooling. It was a descent that culminated at the Eocene-Oligocene boundary, about 34 million years ago, with a new mass extinction. It was a relatively small event in comparison to other, more famous, mass extinctions, but still noticeable enough that the Swiss paleontologist Hans Georg Stehlin gave it the name of the "Grande Coupure" (the big break) in 1910. One of the victims was the Brontotherium -- too bad, it was a nice-looking beast. 

Unlike other cases, the extinction at the Grande Coupure was not correlated to the warming created by a LIP, but to rapid cooling. You see the "step" in temperature decline in the figure. 

Why the big cooling? The answer is not completely known. Surely, cooling was correlated to a decline in the CO2 content in the atmosphere and that, in turn, may have been generated by the collision of the Indian plate with Eurasia. It was a gigantic geological event that generated the Himalayan mountain belt. It exposed huge amounts of fresh rock to the atmosphere, and the result was the removal of CO2 because of silicate erosion and weathering. 

The Himalaya hypothesis is one of those explanations that seem to make a lot of sense, but it has big problems. Another possible explanation is that Earth just outgassed less CO2 than before. The CO2 that plants need for their photosynthesis is generated mainly at the mid-oceanic ridges where the hot mantle (the molten rock layer below Earth's crust) outgasses it, as it has been doing for billions of years. It may well be that the mantle is getting a little colder over the eons, so it outgasses less CO2 than before. It may be true, but it seems to be a weak effect -- not enough to explain the CO2 decline of the Cenozoic.

In my opinion, the most likely hypothesis is that the CO2 concentration declined because of higher biological productivity not just on land, but also in the sea (as seems to be implied in a recent study )

In other words, the early Cenozoic may have been so booming with life of all kinds that it absorbed more CO2 from the atmosphere than the mantle could replace by outgassing. The result was the cooling phase. The abrupt step at the "Grande Coupure" may be related to the evolution of a specific life form: baleen whales, which changed the equilibria of the whole marine ecosystem, drawing down even more CO2 from the atmosphere.

This interpretation agrees with the fact that ice ages are often observed after LIPs. It may be one of the many cycles of the ecosphere. When a major LIP appears, the rise of CO2 is disastrous at the beginning but, in the long run, it gives the biosphere a chance to rebound and expand in a CO2 rich system.  Then, the rebound generates its own doom: the abundant biological productivity draws down CO2 from the atmosphere, cools the planet, and the system finds itself CO2-starved again. In this interpretation, the Eocene cooling and the Grande Coupure were long-term consequences of the Deccan LIP that had destroyed the dinosaurs, millions of years before. I hasten to note that this is just one of the several possible interpretations but, in my opinion, it makes a lot of sense.    

The Eocene cooling had profound effects on forests. First, the CO2 decline gave an advantage to those plants which utilized a more efficient photosynthesis mechanism called the "C4" pathway. Earlier on, the standard photosynthesis mechanism (called "C3") had evolved in an atmosphere rich in CO2. The C3 mechanism is efficient in processing carbon dioxide, but it is hampered by the opposite process called "photorespiration," which becomes important when the CO2 concentration is low. Using the C4 mechanism, plants can concentrate CO2 in the cells where photosynthesis occurs and avoid the losses by photorespiration. 

C4 plants appeared shortly after the Grande Coupure and diffused mainly in grasses, Trees, instead, didn't adopt the new mechanism. The explanation is subtle: photosynthesis needs water, and the process that goes on in leaves is strongly connected to the evapotranspiration mechanism. The C4 mechanism needs less water than the C3 one, so evapotranspiration is hampered. The result is that C4 trees -- if they exist -- cannot be as tall as the ordinary C3 ones, and so they are not favored by natural selection in forests. In an atmosphere of very low CO2, forests are disadvantaged because of the higher photosynthesis efficiency of grasses. 

During the period that followed the Grande Coupure, temperatures and CO2 concentrations remained stable, but at relatively low levels. The result was that many forests disappeared, replaced by grasslands and savannas. Herbivorous species evolved teeth more specialized for grazing and became "mega-herbivorous" species. The landscape must have become similar to the modern one, with patches of forests alternating with savannas. 

A typical savanna ecosystem: the Tarangire national park in Tanzania. (Image From Wikipedia). Compare with the forest image at the beginning of this post. 

Despite the expansion of savannas, rainforests continued to exist in the tropical regions. Conifer forests kept a foothold in the Northern regions, helped by their Ectomycorrhiza system that avoided the runoff of nutrients in winter. The boreal forest is also called "Taiga." 

Then, a new cooling phase started, apparently a continuation of the previous trend: cooling begets more cooling. It was the beginning of the "Pleistocene," a period of unstable climate with ice ages and interglacials following each other, triggered by small oscillations in solar irradiation caused by the characteristics of Earth's orbit. These oscillations are called "Milankovich Cycles" -- they are not the cause of the ice ages, just triggers. (Image Source).

The oscillations are caused by ice having a built-in albedo feedback so that the more ice expands, the more sunlight is directly reflected into space. That causes the temperature to decline and ice to expand even more. Taken to its extreme consequences, this mechanism may lead to the "Snowball Earth" condition, with ice covering the whole planet's surface. It may have happened for real during the "Cryogenian" period, some 600 million years ago. Fortunately, there were mechanisms able to re-heat Earth and return it to the conditions we consider "normal." 

During the Pleistocene, the CO2 concentration in Earth's atmosphere plunged to very low levels, especially during the glacial periods, when it reached levels as low as around 150 parts per million (ppm). Earth may have inched close to a new snowball Earth but, fortunately, that didn't happen. In part, it may be because the sun, today, is about 5% hotter than it was during the Cryogenian. But we will never know how close Gaia got to freeze to death. 

During the Pleistocene, the advancing ice sheets swept away all plants, but even in non-glaciated areas, forests suffered badly.  Tropical rainforests didn't disappear, but they were much reduced in extension. In the North, most of the Eurasian boreal forests were replaced by the "mammoth steppe," a huge area that went from Spain to Kamchatka. where mammoths and other mega-herbivores roamed. 

It is not impossible that an ice age colder than the Pleistocene average could have led to the eventual extinction of the forests, completely replaced by grasses. But that didn't happen, and things were going to change again with the appearance of the Savanna Monkeys -- a completely new species that came to dominate the ecosystem. 

The rise of the Savanna Monkeys

Primates are arboreal creatures that evolved in the warm environment of the Eocene forests. They used tree branches as a refuge, and they could adapt to various kinds of food. Modern primates do not shy from hunting other species, maybe even ancient primates did the same. From the viewpoint of these ancient primates, the shrinking of the area occupied by tropical forests that started with the "Grande Coupure," some 30 million years ago, was a disaster. They were not equipped to live in savannas: they were slow on the ground: an easy lunch for the powerful predators of the time. Primates also never colonized the northern taiga. Most likely, it was not because they couldn't live in cold environments (some modern monkeys can do that), but because they couldn't cross the "mammoth steppe" that separated tropical forests from the Northern forests. If some of them tried, the local carnivores made sure that they didn't succeed. So, "boreal monkeys" do not exist (actually, there is one, shown in the picture, but it is not exactly a monkey!). 

Nevertheless, eventually, monkeys were forced to move into the savanna. During the Pleistocene, about 4 million years ago, the Australopithecines appeared in Africa, (image source). We may call them the first "savanna monkeys." In parallel, perhaps some time later, another kind of savanna monkey, the baboon, also evolved in Africa. In the beginning, australopithecines and baboons were probably practicing similar living techniques, but in time they developed into very different species. The baboons still exist today as a rugged and adaptable species that, however, never developed the special characteristics of australopithecines that turned them into a very different kind of animals. The first creatures that we classify as belonging to the genus Homo, the homo habilis, appeared some 2.8 million years ago. They were also savanna dwellers.

This branch of savanna monkeys won the game of survival by means of a series of evolutionary innovations. They increased their body size for better defense, they developed an erect stance to have a longer field of view, they super-charged their metabolism by getting rid of their body hair and using profuse sweating for cooling, they developed complex languages to create social groups for defense against predators, and they learned how to make stone tools adaptable to different situations. Finally, they developed a tool that no animal on Earth had mastered before: fire. Over a few hundred thousand years, they spread all over the world from their initial base in a small area of Central Africa. The savanna monkeys, now called "Homo sapiens," were a stunning evolutionary success. The consequences on the ecosystem were enormous.

First, the savanna monkeys exterminated most of the megafauna. The only large mammals that survived the onslaught were those living in Africa, where they had the time to adapt to the new predator. For instance, the large ears of the African elephant are a cooling system destined to make elephants able to cope with the incredible stamina of human hunters. But in Eurasia, North America, and Australia, the arrival of the newcomers was so fast and so unexpected that most of the large animals were wiped out. 

By eliminating the megaherbivores, the monkeys had, theoretically, given a hand to the competitors of grass, forests, which now had an easier time encroaching on grassland without seeing their saplings trampled. But the savanna monkeys had also taken the role of megaherbivores. They used fires with great efficiency to clear forests to make space for the game they hunted. In the book "1491" Charles Mann reports (. p 286) how "rather than domesticating animals for meat, Indians retooled ecosystems to encourage elk, deer, and bear. Constant burning of undergrowth increased the number of herbivores, the predators that fed on them, and the people who ate them both"  

Later, as they developed metallurgy, the monkeys were able to cut down entire forests to make space for the cultivation of the grass species that they had domesticated meanwhile: wheat, rice, maize, oath, and many others. 

But the savanna monkeys were not necessarily enemies of the forests. In parallel to agriculture, they also managed entire forests as food sources. The story of the chestnut forests of North America is nearly forgotten today but, about one century ago, the forests of the region were largely formed of chestnut trees planted by Native Americans as a source of food (image source). By the start of the 20th century, the chestnut forest was devastated by the "chestnut blight," a fungal disease that came from China. It is said that some 3-4 billion chestnut trees were destroyed and, now, the chestnut forest doesn't exist anymore. The American chestnut forest is not the only example of a forest managed, or even created, by humans. Even the Amazon rainforest, sometimes considered an example of a "natural" forest, shows evidence of having been managed by the Amazonian Natives in the past as a source of food and other products. 

The action of the savanna monkeys was always massive and, in most cases, it ended in disaster. Even the oceans were not safe from the monkeys: they nearly managed to exterminate the baleen whales, turning large areas of the oceans into deserts. On land, entire forests were razed to the ground. Desertification ensued, brought upon by "megadroughts" when the rain cycle was no more controlled by the forests. Even when the monkeys spared a forest, they often turned it into a monoculture, subjected to be destroyed by pests, as the case of the American chestnuts shows. Yet, in a certain sense, the monkeys were making a favor to forests. Despite the huge losses to saws and hatchets, they never succeeded in completely exterminating a tree species, although some are critically endangered nowadays. 

The most important action of the monkeys was their habit of burning sedimented carbon species that had been removed from the ecosphere long before. The monkeys call these carbon species "fossil fuels" and they have been going on an incredible burning bonanza using the energy stored in this ancient carbon without the need of going through the need of the slow and laborious photosynthesis process. In so doing, they raised the concentration of CO2 in the atmosphere to levels that had not been seen for tens of millions of years before. That was welcome food for the trees, which are now rebounding from their former distress during the Pleistocene and reconquering the lands they had lost to grass. In the North of Eurasia, the Taiga is expanding and gradually eliminating the old mammoth steppe. Areas that today are deserts are likely to become green. We are already seeing the trend in the Sahara desert. 

What the savanna monkeys could do was probably a surprise for Gaia herself, who must be now scratching her head and wondering what has happened to her beloved Earth. And what's going to happen, now?  

The New Large Igneous Province made by Monkeys

The giant volcanic eruptions called LIPs tend to appear with periodicities of the order of tens or hundreds of million years. But nobody can predict a LIP and, instead, the savanna monkeys engaged in the remarkable feat of creating a LIP-equivalent by burning huge amounts of organic ("fossil") carbon that had sedimented underground over tens or hundreds of millions of years of biological activity. 

It is remarkable how rapid the monkey LIP (MLIP) has been. Geological LIPS typically span millions of years. The MLIP went through its cycle in a few hundreds of years. It will be over when the concentration of fossil carbon stored in the crust will become too low to self-sustain the combustion with atmospheric oxygen. Just like all fires, the great fire of fossil carbon will end when it runs out of fuel, probably in less than a century from now. Even in such a short time, the concentration of CO2 is likely to reach, and perhaps exceed, levels never seen after the Eocene, some 50 million years ago. It is not impossible that it could reach more than 1000 parts per million. 

There is always the possibility that such a high carbon concentration in the atmosphere will push Earth over the edge of stability and kill Gaia by overheating the planet. But that's not a very interesting scenario, so let's examine the possibility that the biosphere will survive the carbon pulse. What's going to happen to the ecosphere?

The Savanna Monkeys are likely to be the first victims of the CO2 pulse that they themselves generated. Without the fossil fuels they had come to rely on, their numbers are going to decline very rapidly. From the incredible number of 8 billion individuals, they are going to return to levels typical of their early savanna ancestors: maybe just a few tens of thousands, quite possibly they'll go extinct. In any case, they will hardly be able to keep their habit of razing down entire forests. Without monkeys engaged in the cutting business and with high concentrations of CO2, forests are advantaged over savannas, and they are likely to recolonize the land, and we are going to see again a lush, forested planet (arboreal monkeys will probably survive and thrive). Nevertheless, savannas will not disappear. They are part of the ecosystem, and new megaherbivores will evolve in a few hundreds of thousands of years. 

Over deep time, the great cycle of warming and cooling may restart after the monkey LIP, just as it does for geological LIPs. In a few million years, Earth may be seeing a new cooling cycle that will lead again to a Pleistocene-like series of ice ages. At that point, new savanna monkeys may evolve. They may restart their habit of exterminating the megafauna, burning forests, and building things in stone. But they won't have the same abundance of fossil fuel that the monkeys called "Homo sapiens" found when they emerged into the savannas. So, their impact on the ecosystem will be smaller, and they won't be able to create a new monkey-LIP. 

And then what? In deep time, the destiny of Earth is determined by the slowly increasing solar irradiation that is going, eventually, to eliminate the oxygen from the atmosphere and sterilize the biosphere, maybe in less than a billion years from now. So, we may be seeing more cycles of warming and cooling before Earth's ecosystem collapses. At that point, there will be no more forests, no more animals, and only single-celled life may persist. It has to be. Gaia, poor lady, is doing what she can to keep the biosphere alive, but she is not all-powerful. And not immortal, either. 

Nevertheless, the future is always full of surprises, and you should never underestimate how clever and resourceful Gaia is. Think of how she reacted to the CO2 starvation of the past few tens of millions of years. She came up with not just one, but two brand-new photosynthesis mechanisms designed to operate at low CO2 concentrations: the C4 mechanism typical of grasses, and another one called crassulacean acid metabolism (CAM). To say nothing about how the fungal-plant symbiosis in the rhizosphere has been evolving with new tricks and new mechanisms. You can't imagine what the old lady may concoct in her garage together with her Elf scientists (those who also work part-time for Santa Claus). 

Now, what if Gaia invents something even more radical in terms of photosynthesis? One possibility would be for trees to adopt the C4 mechanism and create new forests that would be more resilient against low CO2 concentrations. But we may think of even more radical innovations. How about a light fixation pathway that doesn't just work with less CO2, but that doesn't even need CO2? That would be nearly miraculous but, remarkably, that pathway exists. And it has been developed exactly by those savanna monkeys who have been tinkering -- and mainly ruining -- the ecosphere. 

The new photosynthetic pathway doesn't even use carbon molecules but does the trick with solid silicon (the monkeys call it "photovoltaics"). It stores solar energy as excited electrons that can be kept for a long time in the form of reduced metals or other chemical species. The creatures using this mechanism don't need carbon dioxide in the atmosphere, don't need water, they may get along even without oxygen. What the new creatures can do is hard to imagine for us (although we may try). In any case, Gaia is a tough lady, and she may survive much longer than we may imagine, even to a sun hot enough to torch the biosphere to cinders. Forests, too, are Gaia's creatures, and she is benevolent and merciful (not always, though), so she may keep them with her for a long, long time. (and, who knows, she may even spare the Savanna Monkeys from her wrath!). 

We may be savanna monkeys, but we remain awed by the majesty of forests. The image of a fantasy forest from Hayao Miyazaki's movie, "Mononoke no Hime" resonates a lot with us. But can you see the mistake in this image? What makes this forest not a real forest? 

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Note: You always write what you would like to read, and that's why I wrote this post. But, of course, this is a work in progress. I am tackling a subject so vast that I can't possibly hope to be sufficiently expert in all its facets to avoid errors, omissions, and wrong interpretations. Corrections from readers who are more expert than me are welcome! I would also like to thank Anastassia Makarieva for all she taught me about the biotic pump and about forests in general, and Mihail Voytehov for his comments about the rhizosphere. Of course, all mistakes in this text here are mine, not theirs.