Is Gaia a Superorganism? No, she is a holobiont!

A few days ago, I was discussing with a friend and he used the term "superorganism" for Gaia as the Earth Goddess. When I said that Gaia is not a superorganism but a holobiont, he asked me, "but what is a holobiont, exactly?" I thought about that for a while, and then I said, "a holobiont is a democracy, a superorganism is a dictatorship."

If you have a friend who is a biologist, try to tell her that Gaia is a superorganism. Likely, she won't be happy and she might ask you, rather venomously, "And so, tell me, good sir, how could natural selection have generated this -- ahem-- 'Gaia' of yours? You tell me that there is only one Gaia on this planet and so, in order to evolve, did perhaps different planetary ecosystems in the galaxy competed with each other?" I am not inventing this, it is the scathing criticism of the Gaia theory that Richard Dawkins produced in his book, "The Extended Phenotype" (1982).

Given a certain interpretation of Darwin's idea, Dawkins' position is logical and even unavoidable. There is just one problem: the common interpretation of Darwin's theory is wrong.

Don't misunderstand me: Darwin was one of the greatest scientific geniuses in history. But we need to take into account that he was working on limited data and with limited tools. He himself could never decide exactly between two concepts that he used interchangeably: "natural selection" and "survival of the fittest." They are not the same thing, not exactly at least. If you emphasize the idea of the "survival of the fittest," then you see evolution as a continuous competition among the best in the search for the even better. It is a cutthroat competition, nature in red tooth and claw.

But it doesn't have to work that way. An alternative view of evolution is more compatible with the idea of "natural selection" and it takes its inspiration mainly from the work of Lynn Margulis (1938 - 2011). In brief, the unit of evolution is not the organism (at least, not only the organism) but the holobiont. And evolution is not competitive but mainly collaborative. 

It is a long story, and I invite you to read the fundamental book "Symbiotic Planet" (Margulis, 1998). See also an article of mine on "Cassandra's Legacy" and my Facebook group on holobionts. To summarize, holobionts are collaborative societies of organisms that live together, helping each other. A good example is a human being, a community formed of the main organism (the proper "human"), and a large number and variety of micro-organisms (the microbiota) that live inside and on the surface of the main organism. Every living being on this planet is a holobiont, and there are holobionts formed of smaller holobionts: think of a forest. Trees are holobionts, a forest is a holobiont formed of trees. Holobionts are a self-similar entity operating at various scales.

Then, evolution is a mechanism for keeping a population stable. It operates mainly at the bottom, not at the top. Those organisms which are defective because their genome is damaged, those that Gorshov and Makarieva call "decay individuals" (Biotic Regulation and the Environment, 2000), are removed and the genetic information contained in the population is maintained intact in order to regulate the system. It is not the "survival of the fittest" but the "non-survival of the unfit."  It is a mechanism that sheds entropy away from the system.

Holobionts do compete, but the concept seems to be (this is an interpretation of mine) that while organisms search for perfection, holobionts strive for the good enough. Mostly, holobionts just survive. And those that survive tend to shed away those parts of themselves which are not good for survival. Holobionts continuously exchange genetic information with other holobionts (it is called "holosex"). In this way, they teach survival techniques to each other. You could say that holobionts are a democracy, while organisms are a dictatorship. (you might also say that holobionts are communists, but that means pushing the similitude a little too much.)

So, back to Gaia, now it should be clear that Gaia is a holobiont, just like us (see also this article by Castell et al.) You could call Her a super-holobiont that includes all the smaller holobionts of the biosphere. Then, this idea overcomes Dawkins' objection: Gaia exists not because She competes with other superorganisms. She exists because the sub-holobionts that form Her collaborate with each other. Think of a forest: did it evolve by competing with other forests? No. It came to exist because trees survive better together than alone. The same is true for Gaia. She operates like all other holobionts. She survives because the living beings of this planet survive better (actually, only) if they are together. Gaia is what She is.

The beauty of this concept is that it gives a form and a substance to Gaia that other views don't provide. So far, Gaia has been an abstract term, mostly described in terms of scientific models and equations. But if She is a holobiont, well, the Goddess is one of us! She is a living being just like us. And when you pat, touch, or caress another human being, you are patting, touching, and caressing the Goddess herself. You can do that with your dog, too! We, living creatures of the biosphere, all partake in the same holobionticity!

And so things stand: onward, fellow holobionts!



See my also facebook group on holobionts  and Erik Assadourian's site on Gaianism
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Note: these ideas are not new, after all: 

“I will sing of well-founded Earth, mother of all, eldest of all beings. She feeds all creatures that are in the world, all that go upon the goodly land, and all that are in the paths of the seas, and all that fly: all these are fed of her store. Through you, O queen, men are blessed in their children and blessed in their harvests, and to you it belongs to give means of life to mortal men and to take it away.” Homeric Hymns,

Notes on Gaian Theology: Is the Goddess a Superorganism?

One of the avatars of the Goddess. 

The beauty of the Gaian theology is that, unlike for ordinary theology, you don't have to rely only on second-hand reports about the subject of your studies. Gaia exists, and you can perceive Her all around us. Then, the question is: what or who is She? 

As you know, the modern idea of Gaia as a denizen of the Earth's ecosphere was developed in the 1970s by James Lovelock together with Lynn Margulis. Then, it evolved in various versions and it was misunderstood in various ways. For instance, Toby Tyrrell wrote a whole book trying to demonstrate that there is no such a thing as "Gaia." He succeeded only at showing that one can write an entire book on something he doesn't understand at all.

But it is true that some ways of understanding Gaia are untenable in light of what we know about biology. Sometimes we hear of Gaia described as a "superorganism" and sometimes as engaged in optimizing the ecosystem for living beings. That's no good, as explained, for instance in a 2003 text by Victor Gorshkov and Anastassia Makarieva where they correctly note that, if Gaia is supposed to be a superorganism, then She cannot exist.

But, one moment. Who said that Gaia is a superorganism? Besides, what is a superorganism? The term is sufficiently vague that it can be badly misused and misunderstood. In general, it is intended as any assemblage of biological sub-units that don't individually reproduce but rely on specialized organs for that. An eukaryotic cell is a superorganism, just as an ant colony. And if you, dear reader, are a human being, then you are a superorganism, too. But that doesn't mean Gaia is one. For instance, I have in my hands right now Lovelock's 1988 book "The Ages of Gaia" and I can't find the term "superorganism" anywhere referred to Gaia.

Instead, Lovelock had a very clear idea of what Gaia is and he described that with his "Daisyworld" model, a highly simplified ecosystem consisting of daisies that can be black or white. Note that daisies are not two species, as it is stated very clearly in the book, they are a single species with a certain polymorphism in their pigmentation. The Gaian mechanism in Daisyworld consists in the daisies slight modifying the frequency of one of their alleles -- that is the white pigment allele becomes more frequent -- to cope with a gradual increase in the solar irradiation. They do that to maintain their optimal temperature but that also affects the environment. With more white daisies, the albedo of the planet increases, more sunlight is reflected back into space, and the planet cools down. This is rare in the real ecosystem, but some algae may use this strategy. (image from gingerboot.com)

The daisyworld model is one of those genial ideas that can be completely misunderstood. And it has been misunderstood: it has been seen as a toy, or as irrelevant to the real world, or simply meaningless. But be careful: you may say it is oversimplified, rough, wrong, whatever you want, but all models are wrong and at the same time all models are useful if you understand their limits. And that's the case of Daisyworld a "level zero" model that opens up for us a completely new vision of how the Earth's ecosystem - Gaia - works. A true stroke of genius on the part of James Lovelock, one of the most brilliant minds of our times.

In any case, for what we are discussing here, the point is that the daisies of Daisyworld are NOT a superorganism. They have nothing of the complex structure of sub-units that make a superorganism. They are just a population of loosely coupled individuals. In this case, they act on the environment by slightly modifying their genome, Lovelock had in mind a time scale of millions of years, so there was plenty of time for the genome to change. But that's not a necessary condition, on a shorter time scale we don't need to touch the genome to kick-start the Gaian mechanism. Here is how Gorshkov and Makarieva describe the concept they call "biotic regulation"

Let us suppose that the living objects capable of environmental control are trees, while the regulated global environmental characteristic is atmospheric carbon dioxide concentration. Suppose further that in the course of a major atmospheric disturbance (volcanic eruption, anthropogenic activities) the global atmospheric carbon dioxide concentration becomes significantly higher than the biotic optimum. All trees on the tree-covered planet are thus faced with approximately equal unfavourable environmental conditions. Normal trees immediately begin to work on removing the excessive carbon from the atmosphere in order to restore the optimum concentration of carbon dioxide. This can be done, for example, by depositing the excessive atmospheric carbon in organic form in soil and sediments.

A different Gaian mechanism may not involve the biosphere alone but the whole metasystem formed of the linked geosphere, atmosphere, and biosphere. It is the case of the geological carbon cycle that seems to have been fundamental in keeping the Earth's temperature approximately constant over a time scale of hundreds of millions of years, as I described in a previous post.

None of these mechanisms imply centralized control, altruism, intelligence, planning, or things like that - no superorganism whatsoever. And here She is: Gaia appearing to us. It is an emergent property of the ecosystem that results from internal feedbacks that tend to keep the system in a homeostatic condition.

Back to theology, now we can answer the question posed at the beginning, who is Gaia? As a collective phenomeno of the ecosystem she looks very much like the demons that Jesus encounters in the country of the Gerasenes (Marl 5:9) when they tell him “My name is Legion; for we are many.” So, is Gaia a demoness? Maybe. In ancient times, the concept of daimon (δαίμων) didn't have the ring of evil that later Christianity attributed to it. A daimon is a force, an aggregation, an egregore of nature, generally benevolent although not all-powerful.

Names have the meaning we want to give them: We can say that Gaia is "just" an ecosystem, that she is not a Goddess at all, in the sense that she is definitely not benevolent and merciful, that she is not to be worshipped (of course not!). But there exists also the concept of "reverence" defined as "a feeling or attitude of deep respect tinged with awe." Reverence might be an appropriate attitude towards an enormously powerful creature which (who) can squash humankind into pulp in no time.

And if you don't revere -- worse, if you despise -- the Goddess, woe betide you: the ancient recognized the concept of hubris (ὕβρις) leading to Nemesis (Νέμεσις), the Goddess of vengeance, providing the appropriate retribution to those guilty of overconfidence. Maybe Nemesis is just another name for Gaia, but, eventually, we don't need an angry Goddess to destroy humankind, we seem to be perfectly able to do that ourselves. In the end, it is all in the hands of the Moirai (Μοῖραι) who spin the thread of destiny in their hands.

(h/t Anastassia Makarieva)

Gaia Exists! Here is the Proof

Gaia is neither benevolent nor merciful. She is harsh and ruthless. 

Environmentalists are sometimes defined as "Gaia worshippers," a term supposed to be an insult. That's a little strange because most people on this planet openly worship non-existing entities and that doesn't normally make them targets for insults. Maybe it is because there is an important difference: Gaia exists.

But who or what is Gaia, exactly? The name belongs to an ancient Goddess but the modern version is something different. As you probably know, the term was proposed for the first time by James Lovelock in 1972 and co-developed with Lynn Margulis. As it happens for many innovative ideas, it was the result of a simple observation: if the Sun radiative intensity increases gradually over the eons, how come that the Earth's surface temperature has remained within the boundaries necessary to keep the biosphere alive? There has to be something that keeps it like that. Lovelock proposed that the mechanism was based on regulating the concentration of greenhouse gases, mainly CO2. You understand that this idea goes at the core of the current debate on climate change: it deals with the inner mechanisms that make the Earth's climate what it is and what it may become in the future.

So, Gaia is powerful but She is not supposed to be benevolent or merciful, and not even a Goddess: we could say that She is what She is. But does She really exist? Not everyone agrees on this point. The concept is often referred to as the "Gaia hypothesis" and entire books have been written to demonstrate that there is no such a thing as a control mechanism of the Earth's temperature. Indeed, in the beginning, the idea was mostly qualitative and not proven. Lovelock proposed a clever model called "Daisyworld" that showed how a simple biosphere could control the temperature of a planet. But the Earth's biosphere is not just made out of daisies and something more than that was needed. But over time proofs have accumulated to show that Gaia is much more than a qualitative hypothesis (or an object of worship by people believing in non-existing beings).

Let me show you some data from a 2017 paper by Foster, Royer, and Lunt that can be seen as proof of the existence of Gaia even though they never mention the term. It is not about new discoveries, but it uses available data to look at how the concentration of CO2 and the sun irradiation varied over the past 400 million years, most of the eon we call the "Phanerozoic." The paper is somewhat technical, but clearly written and you can follow the argument even if you are not a specialist in atmospheric physics. Here are the main results:

The top (a) figure shows the average CO2 forcing (red line), compared to the solar forcing (yellow line). "Forcing" means the thermal effect over the Earth expressed as power per square meter (W/m2). It is called forcing because it is a change of a previous condition. A positive forcing warms the Earth, a negative forcing cools it. Values of the order of a few W/m2 may seem to be small, but they may change the Earth temperature of some degrees C.

The surprising result shown in the figure is how the two forcings, sun and CO2, balance each other nearly exactly. You can see that in the bottom panel of the figure: the net forcing is the red line. This is truly impressive. Assuming a sensitivity factor of 0.3, you can calculate that the solar forcing, alone, should have increased the Earth's average temperature of about 2-3 C (nearly 5 F) over 400 million years. The increase would have been considerably larger if feedbacks (e.g. water vapor) are taken into account. But we don't see this increase, not at all. Here are some recent data by Mills et al.

Look at the gray curve: plenty of oscillations but, on the average, the temperature has remained constant over the past 400 million years. If it had increased even of just 2-3 degrees C, the effect would be clearly detectable. If we push back the boundary to more ancient times, to the origins of life on Earth, the effect should have been much larger: the ancient Earth should have been at least 20 K colder than it is today. It should have been a ball of ice. It was not: we know that there was liquid water even in those remote times.

So, the data are clear: the increasing sun irradiance over the Earth's geological history has been compensated mainly by a declining CO2 concentration. Of course, there are other factors affecting climate: other greenhouse gases, changes of albedo, ocean currents, clouds, atmospheric particulate, orbital and axial oscillations. But they seem to play a minor role at the time scale of an eon. And would you believe that this near-perfect compensation occurred by chance? Yes, sometimes things happen by chance, but can the same thing keep happening by chance for 400 million years?

Anyone said "Gaia"? Smile! The Lady is right in front of you. She exists and we are lucky that She is what She is. Otherwise, the biosphere wouldn't have died long ago, burned or frozen.

But what mechanism causes the CO2 concentration to decline as solar irradiance increases? And where does the removed CO2 go? Lovelock had proposed that it was just the biosphere that did the job, it seems now that we need a tight coupling of biosphere and geosphere to obtain the effect we see. In part, CO2 is removed from the atmosphere by photosynthesis and then transformed into the inert substance called "kerogen" (the precursor of fossil fuels), then buried into the crust. In part, CO2 reacts with silicates in the crust to form solid carbonates. It is a long story and not everything is known, but things start to make sense. Lovelock was right.

Now, are events occurring over hundreds of millions of years relevant for us? Absolutely yes. The time scale may change, but the physics remains the same. The impressive point is that there is no fiddling, here, with mysterious models. These are experimental data coupled with simple physical principles that have been known and established for at least a century. They do show that CO2 affects climate, something that many non-worshippers of Gaia refuse to accept.

Comparing the current situation with the record of the Phanerozoic, we can see that the forcing that we are creating with our CO2 emissions (at present about 3 W/m2, and rising) is of the same order of magnitude of the past forcings that caused the Earth to reach the condition of "hothouse Earth," 10-20 degrees warmer than it is today -- and that even for a smaller sun irradiation! If it has happened in the past, it may well happen again. But it would be easier today because the sun is hotter. So, we may well be in deep, deep trouble.

How fast could the transition to hothouse Earth happen? On this point, the Phanerozoic data help us little: we don't have the resolution that would be necessary to detect rapid events such as the incredible burst in atmospheric CO2 concentrations that humans have created during the past few centuries. Some people say that humans will go exctinct in a few decades because of the triggering of the release of methane, another powerful greenhouse gas, from the permafrost. That would be consistent with the several mass extinctions that took place during the Phanerozoic: we know that Gaia is neither benevolent nor merciful.

But the extinction of humankind is not necessarily Gaia's will. The damage we made may still be reversed, especially if we manage to crash the global economic system. That would stop the burning of fossil fuels and the Earth might return to the previous conditions without the utter destruction that some scenarios foresee. Eventually, it surely will, even though that may take a few million years. Gaia may not be benevolent, but she is surely patient.




You Gotta Believe from Nina Paley on Vimeo.



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A comment from my personal troll

Ancient kings hired personal advisors to remind them that they were mortal. This I know well enough by myself, but I thought I could hire a personal troll to remind me of my limits as a scientist. Here is a comment from him, Mr. Kunning-Druger). 

Glad to see this post, professor Bardi, and I see that you and your friends finally admitted what you always refused to admit: climate has always been changing. And all the data you are showing to us that humans have no effect on climate: look at all those variations in CO2 concentrations: where were the SUVs, the coal mines, the oil wells that you and the others have been telling us are the cause of "climate change"? How can that be? And the supposed "coincidence" that you are showing to us, that should "prove" that Gaia exists. Do you think we are stupid to believe that when we know that these numbers come from the same people who wrote "hide the decline" in one of their mails? And all this story of the Goddess, again, it proves what we had been saying all along: those idiotic Greens are just a bunch of adhorers of Nature, they and their little prophetess, that disturbed girl, Greta - just another scam among the many. You think you are doing science, but you do politics with just an attempt to mask it with a little New Age flavor. The reality is that the whole story is a scam to get public money for your fat research grants. We know that and I am going to write to the president of your university to tell him that you are wasting the salary that the government gives to you. You are using it to scam people and you should be fired together with all those silly scientists. (KD).

Gaia: the missing bride

Toby Tyrrel's book "On Gaia" is an interesting book in many respects, but it misses some fundamental features of the self-regulating planetary system that we call "Gaia". (image from Alternative Energy Action Now)

Imagine that a friend of yours invites you to his marriage. You go there and you see everything you expect to see: the church, the flowers, the priest, the groom, and so on. But, as the ceremony goes on, you notice that your friend has overlooked something important: the bride is missing.

With the book by Toby Tyrrel, "On Gaia", you get a similar impression. It is well done in many respects and plenty of details are at the right place: evolution, life, climate, and more. But, as you keep reading it, you notice that the author seems to have overlooked something important: Gaia herself is missing. 

It is said that marriages fail because of excessive expectations of spouses. The same problem seems to be plaguing more than one study on Gaia, including this one. Some people seem to expect really too much from the poor lady and then they end up concluding that she doesn't even exist - as Tyrrell does with this book. His conclusion is wholly negative: there is no such think as a stabilizing feedback system called Gaia and the fact that the Earth has maintained conditions favorable to life for some four billion years is due mainly to "hazard and happenstance" (p 206 of the book).

The problem of excessive expectations appeared early in the history of studies of the stability of the Earth's ecosystem. James Lovelock, the originator of the idea of Gaia (together with Lynn Margulis), proposed that Gaia could "optimize" the ecosystem for the benefit of life. That was too much. The Earth's ecosystem is a complex system of interacting biological and geophysical loops - some tend to stabilize the system, some to destabilize it. The final result is the typical one of all complex system: the tendency of the system to oppose perturbations. That's not the same as optimization - it is homeostasis; something that tends to maintain the system's parameters within certain limits - not necessarily the optimal ones but, at least, in a range that maintains the cycles going.

There are many examples of this behavior; for instance your body is a complex system and it does exactly that: it seeks homeostasis. If the temperature of your body becomes too high, your internal thermostat will bring it down by sweating. But you can't expect the thermostat to be perfect and all powerful: if you fall into a vat of boiling oil, sweating won't help you much. The same is true for Gaia, which is - mainly - a planetary thermostat that tends to keep the planetary temperature within the limits needed for liquid water (and hence life) to exist. You don't have to expect the thermostat to be perfect and all powerful and, indeed, the Earth's history has seen all sorts of catastrophes occurring; when the planet became very hot or very cold, nearly destroying all life on it. But the system has always recovered and has countered all sorts of perturbations. That includes the gradually increasing solar irradiation over the eons that should have had a deleterious effect on life on Earth, had it not been balanced by a decrease in the concentration of greenhouse gases in the atmosphere.

Curiously, however, Tyrrell just can't see the thermostat in action. One reason is that it is very difficult to understand the Earth's feedback system without taking into account geology and, here, the author clearly has troubles in integrating geology in the discussion. Geology, indeed, is the true "missing bride" of the book. Not that geological phenomena are not mentioned in Tyrrell's book, but often in a cursory and insufficient manner. For instance, there is no real discussion of the cycles of the Earth's ecosystem which involve continuous exchanges of matter from the surface to the mantle and back. These cycles renew the atmospheric composition and provide the chemical elements necessary for life. Without a hot core that provides energy for these exchanges, the Earth couldn't be a live planet - it would be dead like Mars.

The problem appears in particular for the main mechanism of the Earth's thermostat: silicate weathering. It is part of the planet-wide carbon cycle, a chemical reaction that removes carbon dioxide from the atmosphere. Its rate depends on temperature, so it has temperature regulating capabilities. (see this post of mine for an introduction and ref. (1) for an in-depth discussion). Tyrrell mentions silicate weathering for the first time only at page 141, quickly arriving to the conclusion that it is only a negative factor for life because of its cooling effect (2)). You need to arrive almost to the end of the main text (p. 191-192) to find a brief discussion of whether silicate weathering can be part of a temperature stabilizing mechanism. Here, too, Tyrrell's conclusion is negative, apparently on no other basis than generic skepticism.

Now, of course one may disagree on all current scientific interpretations, but considering that silicate weathering is a core element of the whole thermostat question, it would surely deserve more discussion before arriving to dismiss Gaia as non existent. This is not the only problem related to geology in the book. Other factors, for instance the effect of the sun's increasing luminosity, are missing or barely mentioned. So, it is really disappointing that the book misses so badly its avowed target - Gaia - especially considering that there are several sections of it that are well done and worth reading, such as the discussion about the temperature effects on biological productivity.

In the end, I think that there is a basic problem in Tyrrell's approach. In the "Conclusions" section, he states that accepting or rejecting the Gaia hypothesis has a strong effect on "how we decide to manage the Earth System" and that "Gaia, by the very nature of the hypothesis, inculcates a predisposition to suspect natural feedbacks to be stabilizing." In other words, Tyrrell emphasizes that Gaia could generate a dangerous feeling of complacency on people and hamper their efforts of fighting climate change.

I beg to differ on this point. Not that I don't share Tyrrell's worries about global warming but in my personal experience the concept of Gaia as a stabilizing factor on climate is alien to the mind of typical science deniers. Rather, most of them seem to use the exactly opposite meme: "climate has always been changing," normally without showing the slightest interest in what exactly causes climate to change - they just don't care. Of course, the internet is so wide that you can find just about anything in it and, as proof of his position, Tyrrell cites a specific site that goes under the name of "The Resilient Earth". However, apart for the title that indeed reminds the concept of Gaia, the contents of the site seem to be the usual mishmash of denialist memes: from "no warming during the past 15 years" to "Al Gore is fat". The general opinion that can be read in deniers' sites about Gaia is that the concept is not just ridiculous but, rather, living proof that climate change is not science but a religion - intended as a derogatory term. (see the figure below as an example of Gaia-bashing. From thepeoplescube.com)

So, I think the least thing we should be worried about is that the concept of Gaia could engender a dangerous "lasseiz faire" attitude. On the contrary, understanding the factors that determine the Earth's temperature can only generate a healthy dose of respect for the delicate balance that has kept climate stable during the past ten thousand years or so. Homeostasis is no guarantee of absolute stability; that holds for an entire planet, just as it does for bicycles (and the latter is something that everyone understands).

In the end, Gaia is not a Goddess, (and surely not a benevolent one). She is not all powerful, she has no ability of optimizing the Earth's environment for life, and she is no guarantee whatsoever that we can keep behaving as planetary hooligans without suffering the consequences of our actions. Gaia is a huge and complex system; a gigantic tangle of geological and biological feedbacks. We are just starting to understand how this system generates its overall tendency to homeostasis and how it has evolved over the billion years of its existence (3). It will continue to evolve until, hundreds of millions of years from now, it will "die" when the Sun becomes too hot for the homeostatic mechanisms to continue operating. In the meantime, we have to keep living on this planet (if we can). Gaia may not be the perfect bride; but we can't keep behaving as if she didn't exist. 



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1. For a review of the climate effects of silicate weathering, see Lee R. Kump, Susan L. Brantley, and Michael A. Arthur, Chemical Weathering, Atmospheric CO2, and Climate Annual Review of Earth and Planetary Sciences Vol. 28: 611-667 (Volume publication date May 2000) DOI: 10.1146/annurev.earth.28.1.611

2. This point illustrates the problems that this book has with geology. At page 142, Tyrrel attributes one of the "big five" mass extinctions, the Late Devonian one, to excessive cooling caused by silicate weathering. But he doesn't say anything about the generally held opinion that mass extinctions appear to be caused by excessive warming (including the Devonian one). See, eg. David L. Kidder, Thomas R. Worsley "Phanerozoic Large Igneous Provinces (LIPs), HEATT (Haline Euxinic Acidic Thermal Transgression) episodes, and mass extinctions"  Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 295, Issues 1-2, 1 September 2010, Pages 162-191

3. For a thorough description of the feedbacks cycles of the Earth's system, you'll do well in reading the book by Tim Lenton and Andrew Watson "Revolutions that Made the Earth". Beware: it is not an easy book to read, but it is surely worth the effort.


About Gaia, see also these posts by yours truly, Ugo Bardi,  

"The Great Chemical Reaction: life and death of Gaia"
"Man Vs. Gaia"
"The Next ten billion years"