Monday, May 21, 2012

The great chemical reaction: life and death of Gaia

 "This text is a written version of a talk that I gave in Desio (near Milano, Italy) at a meeting organized by the Centro Culturale Lazzati on Jan 30th 2012. It is much shortened with respect to the actual talk, but it tries to maintain the spirit and the rhythm of that presentation. 

You know, ladies and gentlemen, every time I give a talk I try to say  something different - otherwise it would be boring for me and, perhaps, for you, too. So, this time I thought I could do something closer to what's my job. After all, I teach chemistry. So, shouldn't I teach you a little bit of chemistry? Then, I thought that I could start by presenting to you a chemical reaction. Here it is:

Well, after you give speeches for a while, you become somewhat telepathic. So, I know what you are thinking. Yes: I can read your minds and I know that this slide is making you happy; isn't it? By the way,  the exit door is down there. Maybe you can scream something like "I forgot to turn off the gas stove!" as you run away.

Well, nobody is running away and that's nice. I said that I know what you are thinking and it is true - without exaggerating, of course! You are thinking that chemical reactions are boring. And I agree with you: chemical reactions are very boring. I can tell you that: I studied chemistry, I teach chemistry, I've been working in chemistry for all my life. I should know!

So, why do chemists like the things that they hate - so to say? Are they masochist or what? Well, no. Maybe I am asking you to believe something a little too extreme, but let me tell you something: chemistry is not boring! Chemistry is fascinating, it is interesting, it is even fun. And chemical reactions are not what chemistry is about. Chemical reactions are just a shorthand that hides the really interesting things. If you look at the symbols, well, it is boring. If you look at what the symbols describe, if you look inside, well it is not the same. It may be an interesting story, as I was saying it may be fun, it may be fascinating. You know, when I was a freshman in chemistry, I had to attend chemistry labs. There were many nice girls in my class and they were all wearing lab coats in the lab - not exactly sexy as garments. But that was just the outside: what was fascinating was the inside!

So, I hope today that I could show you that the specific reaction that I am showing to you today is hiding something hugely interesting. It is called "silicate weathering" and is the basis of life on Earth. The way I have written it, it is very simplified - it is much more complex than that. But we can take it in this form in order to understand it. If that reaction were not running all the time on our planet, I wouldn't be here, you wouldn't be here and not even those nice looking girls that I met during my time as a student would ever have existed. Nothing alive on this planet would exist. The entity we call "Gaia" would not exist.

Let me explain. What do we mean exactly with "Gaia"? I think the best I can do is to show you an image.

I am sure you recognize what this is; it is "Pandora" from the film "Avatar." Now, we can say that Pandora is a sort of an Earth on steroids. It is lush, it is full of life, full of creatures: dragons, monsters, waterfalls, trees, mountains, clouds; all that. Of course, Pandora is a fantasy world; but we are discovering plenty of new words in the Galaxy; many are about the same size of our Earth and at the right distance from their suns; so they could well host organic life similar to ours - like Pandora does in the Avatar movie. We can't say for sure if such words exist, but one thing we can say is that - if they exist - the reaction I was showing to you before must be running on there. A world without that silicate weathering reaction running is like Mars or Venus. No silicate weathering reaction, no life.

Let me explain: in order for life to exist, there have to be some materials that make it exist. And the most important material that makes life exist is a special molecule that we call carbon dioxide and that we write as CO2, pronounced see-oh-two. You know that carbon dioxide is what plants use to carry on photosynthesis, which is what keeps alive everything on this planet. If Pandora is so lush and beautiful, it has to have CO2 in the atmosphere, just as our Earth does. Plants make CO2 react with hydrogen extracted from water and out of this reaction they create all organic matter which is then be used to make living beings. In a sense, CO2 is Gaia's food, it is also Gaia's blood, Gaia's lymph and more.

But, then, if CO2 is Gaia's food, there is a problem. CO2 is a reactive molecule and here is where the reaction I wrote kicks in:

You see that this reaction contains carbon dioxide; CO2, on the left side. And you see that this CO2 reacts with something written as "CaSiO3" which I can read as "calcium silicate". Now, the reaction (keep in mind that it is very simplified) says that carbon dioxide reacts with the silicates of the crust to create carbonates (CaCO3) and silica (SiO2). So, a gas, carbon dioxide, reacts with rocks to create more rocks - those carbonates are what we commonly call "limestone". So the carbon which once formed CO2 becomes carbonate, which is solid. Let me show you:

This is a weathered rock somewhere. See? CO2 reacts with the rock and corrodes it. In doing so, CO2 disappears. Clearly, it is a very slow process. You don't see rocks being washed away by rain, unless you are willing to wait for a very, very long time. How long? Well, we are talking of geological times; millions of years, but that's not what we are worried about. The question is; if CO2 is consumed by the reaction, how long would it take for the atmosphere to lose all of it? (and note that plants would start dying much before CO2 were to disappear completely).

On this point, there is an answer that you can find in Robert Berner's 2001 book which has a rather impressive title "The carbon cycle of the phanerozoic". Berner says that all the CO2 in the Earth's atmosphere would be consumed by the weathering reaction in about ten thousand years. In part, it would be replaced by CO2 degassing from the oceans, but even that source would be exhausted in about 300,000 years. These numbers are, of course, just orders of magnitude but for what we are concerned here, the uncertainty doesn't matter much. Life on Earth has been going on for more than three billion years and there must have been CO2 in the atmosphere all this time. No CO2, no life. There is no escape to that. So, CO2 was not consumed by the weathering reaction, nor by the formation of fossil fuels and coal, which also removes it from the atmosphere.

So, you see that we arrived to a paradox. The weathering reaction should have consumed all the CO2 in the atmosphere long ago but there is still plenty of it; enough, at least, to keep photosynthesis going and with it all life on Earth. But paradoxes are almost always pathways to understanding deeper truths and this one is no exception. Let's go back (once more!) to the weathering reaction:


You probably remember from what you studied in high school that chemical reactions never go fully in one direction. They can go both ways and often they are in an equilibrium condition in which reactants and products remain in constant concentrations. And you may remember that there are conditions that can shift the equilibrium from one direction to another. About the weathering reaction, we said that it goes from left to right, as you can see from the picture of the weathered rock seen before. But, if we could make the reaction go from right to left, then the carbonates (limestone) decompose and become a source of CO2. If that were possible, we'd have a way to bring CO2 back in the atmosphere. We need, therefore, to close the "geological cycle" of CO2 (something different than the well known biological cycle - that wouldn't be enough by itself to keep CO2 in the atmosphere).

How could that happen? Well, another thing that you surely learned in high school is that the equilibrium of a chemical reaction depends on temperature. There are good reasons based on thermodynamics that say that a solid compound decomposes at high temperatures. That's what happens to carbonates, provided that you can reach temperatures of the order of several hundred degrees Celsius - possibly over a thousand. Now, where can you find these temperatures on Earth?

Very easy: look at your feet. Think of making a hole of a few tens of kilometers and there you are. You find an area of the Earth called the "mantle" which is semi-molten rock composed mainly of  silicates, but also carbonates. Here is the structure of the inside of our planet as we know it today.

You have to go deep down, but eventually you reach temperatures where carbonates are decomposed into CO2 that would then be degassed out by volcanoes, geysers, hot springs, all that. That's exactly what happens in the great CO2 cycle that goes under the name of "plate tectonics". Here is it:

Let me explain a little this image. It shows how the ocean floor moves and is gradually pushed inside the depths of the Earth in a process called "subduction". Everything that stands on the ocean floor is destined, eventually, to disappear into the mantle. But this is also a cycle, you can see in the figure how material from the mantle is pushed up to the surface to form new ocean floor at those regions which are called "mid-ocean ridges". A very slow process, it takes tens of millions of years for a piece of rock that surfaces at the mid ocean ridge to go back to the mantle. But it does occur.

Now, this is also the CO2 cycle. You see, we said that the reaction of carbon dioxides with silicates produces carbonates. These carbonates end up on the ocean floor, often in the form of the shells of dead marine organisms. And the final result is that this carbonate is pushed into the mantle - where it is hot enough to decompose it into oxide and CO2. Then, the CO2 returns to the atmosphere in the form of volcanic eruptions.

The beautiful thing of all this is that the cycle is that it is the "control knob" of the Earth's surface temperature. Really, the CO2 cycle is a thermostat that keeps the Earth not too warm and not too cold; just right. It has been doing that for billions of years. As a thermostat, it must be said that it has not always functioned so well: we have had ice ages and those hot periods called sometimes "planetary hothouses". But, on the whole, the Earth's temperature has always remained within the limits that make life possible. Otherwise, we won't be here.

So, how does the thermostat work?  First of all, you know that CO2 is a "greenhouse gas". It traps the heat emitted by the Earth's surface acting a little like a blanket that keeps the planet warm. So, the more CO2 there is, the more we expect the Earth to be warm. As a consequence, the temperature can be regulated by controlling the concentration of CO2 in the atmosphere. But how can that be done? Well, there is the trick: the speed of chemical reactions depends on temperature. It is true also for the silicate weathering reaction:

High temperatures make the reaction go faster. So, if the Earth's becomes warmer, then there is more CO2 consumed and that reduces the temperature because the concentration of CO2 goes down - and remember that it is a greenhouse gas! The opposite takes place if the Earth becomes cooler - the reaction slows down, the CO2 concentration increases because of all those volcanoes emit it and, in the end, the temperature returns to the previous values. See? Simple and effective.

Of course, as I said, the control is far from perfect. It involves times of the order of millions of years, so it takes a huge time lapse for the planet to recover from a perturbation. For instance, a very large volcanic eruption took place some 250 million years ago in Siberia. It emitted so much CO2 that the resulting increase in temperatures almost killed all life on Earth. The silicate weathering reaction, eventually, absorbed all that CO2 and brought temperatures back to more acceptable values for the biosphere. But it took millions of years. So, if we look at the temperature record, we see that it oscillates and that shouldn't surprise us too much. Here are the data we have for the past 550 million years or so, the period we call "Phanerozoic":

As I said, the regulation is not perfect, but the fact that temperatures oscillate around a constant value tells us that there is a regulation ongoing. You see, the point is that the planet badly needs that regulation, because the sun's irradiation is far from being constant. It increases of about 10% every billion years because of reasons that have to do with the evolution of stars. So, in a period of half a billion years, as the Phanerozoic, we'd expect the planetary temperature to go up as the result of the sun becoming more and more bright. Instead, we don't see it. What we see, instead, is a gradual reduction of the concentration of CO2, as we see here (these data are, again, from the work of Berner):

Yes, it is irregular, but there is no doubt that the concentration of CO2 has gone down, on the average, during the past half billion years. And if we make a little calculation that takes into account the increase in solar luminosity (you can find it in Berner's book) we can see that the numbers do click together. The variation of CO2 concentration is what has kept the Earth not too warm and not too cold, just right, during the geological past.

Now, I guess you are asking yourselves what's going to happen in the future. As you surely noted, the CO2 concentration has been going down and continues to do so (apart from human intervention in terms of burning fossil fuels, but that's not part of the regulation system). Something that could happen is that the Earth's core cools down so much that it will stop the tectonic movement that decomposes the carbonates and closes the CO2 cycle. In that case, the CO2 concentration would go to zero and kill the biosphere. But, according to the data we have, that will not be the cause of the death of the biosphere which, instead, will be destroyed by the increasing solar irradation. Eventually, we'll arrive to a point where the system can't reduce the concentration any more. Yes, and before we arrive to that point, there won't be enough CO2 for plant photosynthesis. And without photosynthesis, there can't be any life on Earth - everything must die.

That's indeed the ultimate destiny of the Earth's biosphere. Of Gaia, if you like. If Gaia is a living being then, as all living beings, it must die. It will be a slow process - very slow by human standards. But it is going to happen. In the simulation below, by Franck and others, you can see the slow winding down of the biosphere which should become extinct a billion and a half years from now. You see also that vertebrates should disappear much earlier, perhaps in less than a billion years

And here is an image of the ultimate destiny of the Earth. To be sterilized by the sun as it becomes more and more bright. The oceans will evaporate and - eventually - the surface will melt under the tremendous heat.

That is, clearly, a far away future. Maybe, by then, our descendants, if there will be any, will have found another place to live, around another star or somewhere in the galaxy. But our main concerns are not about such a remote future. Our main concern is that even the near future may give to our close descendants, a lot of problems with the Earth's temperature.

The problem is that we have been tinkering with the thermostat without understanding exactly what we were doing. And we have been emitting into the atmosphere a large amount of gases which had been removed from the atmosphere as part of the regulating mechanism. Gases which had been stored underground in the form of what we call "fossil fuels": coal, oil, and natural gas. The perturbation made to the system is very large and extremely rapid if compared with anything that has occurred in the past history of Earth.

You probably have seen this picture and it is very, very worrisome. The fact is that such high CO2 concentrations have never occurred on Earth during the past few millions of years. When we had such concentrations, tens or hundreds of millions of years ago, the sun was less hot than it is now and, nevertheless, the Earth was a much warmer place than it is today. We might be able to adapt to a much warmer planet, but the process wouldn't be painless. Just think that the melting of the continental icecaps would submerge all of our coastal cities.

We can't hope that the silicate thermostat will save us from CO2 caused warming. This reaction


is damn slow by our standards. It will, eventually, remove from the atmosphere the CO2 we have emitted, but it will take tens of thousands of years, at the very least. Look at these simulations by Dave Archer and you see what the problem is:

See? part of the CO2 we have emitted in the atmosphere will still be there in 40,000 years from now. Actually, it will stay there much longer. So, you see how important it is the reaction that I showed to you. The silicate weathering reaction is what keeps "Gaia" alive - better said, it is Gaia. And don't make the mistake of thinking that Gaia is a goddess and that, somehow, she cares about us. No, it is more correct to say that Gaia doesn't give a damn about us - which is what you'd expect from a chemical reaction, after all. It is us who have been tampering with this chemical reaction and it will be us who will have to face the consequences.

In the end, we can't hope to force the planet to do what we want it to do. So, we must learn to live with the flow of the Earth's cycles. For that, we must know a little chemistry my idea today was to show to you a bit of this chemistry. But more than chemistry, we must learn our limits, otherwise we won't survive for long.

This is our Earth, not a fantasy planet, let's try to keep it the way we found it:


Ugo Bardi is a member of the Club of Rome, faculty member of the University of Florence, and the author of "Extracted" (Chelsea Green 2014), "The Seneca Effect" (Springer 2017), and Before the Collapse (Springer 2019)