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The Habitable Planet: A Systems Approach to Environmental Science 

Many Planets, One Earth Interview with Paul Hoffman

Interviewer: What regulates the average temperature of the Earth over billions of years?

PAUL: This only really became a question in the middle of the 20th century when people realized that the sun is a normal star, and in its main sequence, which lasts for 10 billion years or so, the radiant energy that’s emitted by the sun slowly increases by a significant amount over time and that’s because of the accumulation of helium in the sun’s core. So, over the 5 billion years of our solar system, solar radiant energy has increased by almost 30 percent. It’s surprising, therefore, that the geological evidence indicates that the surface temperature of the Earth hasn’t changed very much over at least 3 ½ billion years, which is the time span of the geological record that really gives us good evidence on surface temperature. There’s no evidence of any overall change. There have been ups and downs but no overall change. So that means that there must be something internal to the Earth that is adjusting—that is self-adjusting so that the Earth always maintains a stable temperature—not unchanging but limited in the amount of change despite this large increase in solar radiation. A guy by the name of James Walker, an atmospheric scientist at University of Michigan, is the person who is mainly responsible, I think, for putting us on the right track. And that’s because of the ability of carbon dioxide to modulate the surface temperature and also for carbon dioxide to adjust to changes in the solar forcing.

Interviewer: How does Walker’s theory of climate work?

PAUL: Carbon dioxide is emitted to the atmosphere by volcanism primarily—some other forms of out-gassing, too—but primarily from volcanic activity. Carbon dioxide is consumed by rock decomposition—that’s the breakdown of various igneous rocks—granites and basalts to form soil. Geologists call this process weathering. And that process consumes carbon dioxide. The carbon dioxide gets dissolved in ground water and river water. And in combination with various cations [positively charged ions] like calcium and magnesium and sodium and potassium, that carbon dioxide is washed by rivers and in ground water into the ocean, and in the ocean, organisms use those cations and the bicarbonate ions and precipitate calcium carbonate and some calcium magnesium carbonate. And that calcium and calcium magnesium carbonate is deposited as sediment on the floor of the ocean and ultimately sinks back into the mantle through the plate tectonic process called subduction—gets heated up and decarbonation reactions take place, and it’s converted back into CO2, which comes back out of volcanoes. So, that’s the sort of complete cycle, the geological cycle, of carbon.

The amount of carbon dioxide in the atmosphere is controlled by the balance between the carbon dioxide that’s coming out of volcanoes and the carbon dioxide that’s being consumed by weathering processes. You can increase the carbon dioxide if volcanism became greater. You could reduce it if weathering became faster on the global scale. But that doesn’t give us a thermostat. The way the thermostat works—and this was the key point of James Walker’s idea—is that the weathering reactions are themselves dependent on temperature. So, if the Earth got warmer for any reason, those reactions would go faster simply because of the rise in temperature and also because with the rise in temperature, there would be more water vapor in the atmosphere and, therefore, there would be a more active hydrological cycle. There would be more rain in rainy areas. And more rain means that those chemical reactions would also go faster because they’re catalyzed by temperature and the presence of water.

And so, if the Earth got warmer for some reason, the weathering rates would go up and carbon dioxide would be consumed at a faster rate. That would either lower carbon dioxide or would limit the rise of carbon dioxide and would ultimately lead to a new stable climate―perhaps at a slightly elevated CO2 and therefore a higher temperature because of the greenhouse effect. But it would be a stable climate. Similarly if the Earth cooled down for some reason, what would happen is that weathering rates overall would get slower. And so, if the volcanic outgassing rates stayed the same, carbon dioxide would start to accumulate in the atmosphere. That would counteract the cooling and would tend to stabilize the climate again not at exactly the same temperature as before, but it would prevent a runaway cooling or warming. And so over geological time, as the solar radiation got greater and greater, what happened is this thermostat―due to the temperature dependence of the silicate weathering―allowed the carbon dioxide level in the atmosphere―and therefore the effectiveness of the greenhouse effect―to be lowered over time by exactly the right amount that would be needed to maintain the surface temperatures in the Earth suitable for life.

Interviewer: Is this the same reaction that you see when you see a statue being eroded or the façade of a building affected by acid rain?

PAUL: There are two different kinds of rocks that are commonly used in building stones. One is granite or basalt―igneous rocks. And the other kind of rock that’s commonly used is limestone or marble. They both dissolve and decay and decompose under the influence of acid rain. Limestone and marble decompose more rapidly. They simply dissolve into their constituents―calcium ions and bicarbonate ions. They get washed into the ocean, and they combine again under the influence of organisms and precipitate as calcium carbonate. But the CO2 that was dissolved when the limestone or marble dissolves is released again when the calcium carbonate precipitates, because after all, all you’re ultimately doing is taking calcium carbonate, dissolving it and then remaking it. So, there’s no net change. There’s no consumption of CO2 during the dissolution and ultimate burial of limestone or marble. But basalt and granite is another matter. When basalt and granite decompose, because you’re forming bicarbonate, you’re removing two molecules of CO2, only one of which gets released. The other one goes to form the carbonate of the calcium carbonate. And if that gets permanently buried in the sediment, it’s gone until it comes back out of a volcano.

Interviewer: How did Walker’s ideas help explain the possibility of a “Snowball Earth” deep freeze?

PAUL: When James Walker worked out this idea of the carbon dioxide thermostat, he thought that would prevent a snowball Earth from every occurring. But why did he ever think it was possible in the first place? Well, that was because of the work of climate physicists in the 1960s. This was before James Walkers’ work. The climate physicists were actually mainly interested in the question of the stability of the Arctic Sea ice. It’s interesting that even that long ago, people were concerned about the possibility of the rise of carbon dioxide due to human activities, which couldn’t really be effectively policed by the silicate weathering, the weathering feedback, because the weathering feedback is too slow. That’s a process that’s very good under geological time scale of millions of years. But it can’t prevent a buildup of carbon dioxide over 100 years or a few hundred years as we’re doing with the present human experiment. So, in the 1960s, climate physicists were already interested about the possibility that because of the buildup of CO2 from industrial sources, is it possible that the arctic sea ice might disappear? People in northern Europe were particularly interested in this because they thought that that might improve the Russian climate, for example. And so they carried out models — these were actually some of the first climate models in a three dimensional Earth. Now it wasn’t a physical model. It wasn’t even a computer model. They didn’t have computers. These were calculations. And what they were calculating was what would happen to the surface temperatures on the Earth as a result of a change in the carbon dioxide concentration. In particular, they were interested in certain other feedbacks, positive feedbacks, in the climate system, one of which is due to the effect of snow and ice on the planetary albedo.

“Albedo” is the Greek word for whiteness. The albedo is important because only a certain fraction of the solar rays that we receive are reflected—reflected by clouds, reflected by continental surfaces, reflected by ice and snow. And if they’re reflected, then those rays go back out. Some of them are trapped by the atmosphere, but some of them are just lost to space. So, obviously, the surface temperature is strongly dependent on the albedo, which is defined as the fraction of the incoming radiation that the Earth absorbs and is not reflected. Here’s where snow and ice are very important. Fresh snow has an albedo of about 0.9. That means 90 percent of the incoming radiation is reflected. In contrast, sea water, which covers most of the Earth’s surface, has an albedo of 0.1. That means only ten percent is reflected—90 percent is absorbed. So, obviously, there is a huge difference in how much of the radiation is reflected, depending on whether you have water or land, which has an albedo of about 0.3, or sea ice, which has an albedo of around 0.4 or 0.5, depending on how dark it is or continuous glacial ice, which, if it’s not covered by fresh snow, has an albedo of 0.6 or 0.7 or a freshly snow-covered surface, which has an albedo of 0.9. So, the planetary albedo is very important in determining the surface temperature of the Earth.

Now here’s the positive feedback. If the Earth gets colder for some reason—for any reason—say CO2 levels go down slowly, the Earth gets colder so presumably the area that’s covered by ice and snow gets greater. As a result, the albedo becomes higher. More of the radiation is being reflected; less is being absorbed and so there’s an additional cooling effect. You had your initial cooling, but as a result of the increase of snow and ice cover, there’s an additional cooling that’s just due to the change in the reflectiveness of the surface. Now, if you imagine Earth, it’s a globe and most of the ice and snow is in the polar regions. As the Earth gets colder, those polar caps get larger and larger, and they start to move incrementally towards the equator. But as they move equater-ward, for every increment of growth, the area that becomes covered with ice and snow becomes larger and larger. Therefore, the feedback becomes stronger and stronger. And there becomes a point where the feedback is self-sustaining—you can’t stop it—that’s the tipping point. Once you go beyond that point, there’s no stopping the advance of the ice and snow, and models suggest that from that point, which is around 30 or 40 degrees of latitude with ice and snow down at sea level, beyond that point, the advance of the ice would occur very rapidly. And the entire tropical ocean, which is half the surface area of the Earth, would become ice covered in a matter of months to years. That would be a catastrophic advance of the ice in the final stages. So, there’s an instability in the system. There’s a tipping point beyond which the ice can’t be stopped. And that means there’s a bifurcation. You’re either in an entirely ice-covered world, or you’re in a world that is only partially covered with ice but is short of that tipping point.

Interviewer: So, which came first―the snowball theory or the possibility that this tipping point could be reached?

PAUL: Well, the theory came first. It was out of the theory that the idea of the existence of the tipping point came. Now, the climate physicists that came up with the realization that this instability existed, they assumed it had never happened because they thought that if it had happened, we wouldn’t be here because life would be extinguished. They also thought there would be no escape. They assumed that if the Earth ever had frozen over, the albedo would be so high that we would have to wait billions and billions of years for this solar radiation to become great enough to melt the snowball. They thought there would be no way of escape. So, naturally they assumed that lucky us. It could have happened. But it didn’t.

Interviewer: Now that we’re in a snowball, how does Earth come out of it?

PAUL: This is quite ironic because James Walker, although he was thinking that a snowball should be prevented by the silicate weathering feedback, and the slow geochemical carbon cycle, also realized that if the Earth ever had become a snowball it wouldn’t last forever—that there was an escape. And the escape is plate tectonics, and in particular the way plate tectonics drives the carbon cycle. Here was his reasoning. He said that if a snowball occurred, if the Earth was entirely covered with ice, the oceans were frozen over and the planetary albedo was say 0.6, if you calculate the radiative energy balance, given the present day solar radiation and an albedo of 0.6, the average surface temperature of the Earth would be 50 degrees below zero on the Celsius scale or about 223 degrees Kelvin. Way, way below freezing everywhere. Under that condition, there wouldn’t be very much weathering because weathering goes faster when you have a higher temperature and liquid water. So if everything is frozen, weathering rates would be extremely slow. But plate tectonics would continue. Volcanoes continue to pump CO2 into the ocean water from mid-ocean ridges and into the atmosphere along volcanic arcs and places like Iceland. What happens is that, slowly over time, the carbon dioxide and maybe other greenhouse gases in the atmosphere build up and create a stronger and stronger greenhouse effect. And ultimately, according to theory, the greenhouse effect, due to the accumulation of carbon dioxide, becomes so strong that it’s able to counteract the high albedo of the ice-covered Earth and precipitate what is believed to be a violent deglaciation or meltdown of all the ice under the influence of an enormously elevated carbon dioxide load in the atmosphere.

Interviewer: What would be the necessary conditions for the initiation of a snowball?

PAUL: The runaway ice albedo feedback is a way of taking a very cold world and making it into a snowball, but it doesn’t explain how you get to that tipping point to begin with. I mean it’s not an easy matter to get the ice line at sea level―you can think of this as the migrating front of a floating sea ice or ice shelf down to 30 or 40 degrees latitude. At the last glacial maximum, it was only around 50 degrees of latitude. So you have to have a very extreme cooling event to ever get you close to that snowball ice albedo runaway. There have been a number of theories that have been put forward. They’re all highly speculative. One interesting one is that in the spiral arms of our galaxy, there are concentrations of dust and molecules known as giant molecular clouds. There are smaller molecular clouds as well, but there are giant molecular clouds. It’s estimated that the Earth has probably passed through one of these giant molecular clouds a few times during its history, and it’s most likely to pass through them when we’re going through the spiral arms of the galaxy. When the Earth enters one of these giant molecular clouds, it is believed that the effect of all these ions in the upper troposphere—that’s the upper part of the dense lower atmosphere―that it might have the effect of significantly increasing the nucleation of clouds there. And clouds, of course, also reflect sunlight and increase the Earth’s albedo. Some models suggest that when the Earth enters a molecular cloud, a giant molecular cloud, there is a sufficient increase in the cloudiness of the upper troposphere that the surface temperatures are depressed sufficiently to induce a snowball Earth. The important thing about this process is that it occurs on a timescale that’s short enough―a few tens of thousands of years―that silicate weathering feedback can’t compensate. So any change in the radiative forcing―no matter how you do it―whether you do it with molecular clouds or by turning the sun’s energy up or down, if it’s too slow, then silicate weathering feedback, as Walker predicted, would simply modulate the climate and prevent a runaway climate state. So it has to be a change the occurs rapidly. One possibility is a giant molecular cloud. It seems from theory that that’s possible. However, there’s no test for that that’s been carried out so far that provides any empirical support.

Interviewer: Are there any other theories about what might have triggered the snowball?

PAUL: Another one which has a little bit more empirical support, mostly from carbon isotopes, is the idea that during this period just before the Cambrian, there was a long interval, maybe one or two hundred million years, where the continents were at unusually low latitudes. This would, of course, make the Earth colder because you’d be increasing the weathering rates because you’re putting more of the continents in places where it’s warm and wet. Therefore the weathering rates would be greater, and you’d be consuming carbon dioxide in a faster rate and, therefore, the Earth would stabilize at a colder temperature. But not only would you be weathering more, you’d be creating a lot of sediment. You’d have big river deltas from all the rivers draining out of these equatorial continents, and a lot of organic matter would be buried in these delta sediments, and that organic burial would, of course, remove carbon dioxide from the atmosphere, but it would also produce a growing reservoir of methane in the sediment. The methane could be destabilized, and could come back out at some point and if the methane starts to leak out of the sediment, then the methane concentration in the atmosphere might start to slowly rise.

That would be an interesting situation where the Earth would be a little bit like a drug addict. What would happen if the methane concentration slowly rises because methane is a very strong greenhouse gas—several times stronger molecule by molecule than carbon dioxide—is that the silicate weathering feedback, sensing the presence of the methane, the weathering rates would increase and so the carbon dioxide would start to go down to compensate for the rise of the methane. But the rise of the methane would simply supplant the carbon dioxide.

There wouldn’t be any overall change in the mean temperature of the climate, but the climate would be less stable for a number of reasons. First of all, methane tends to come out in bursts. Secondly, the methane concentration is not buffered by the ocean the way carbon dioxide is. And methane has a very short residence time in the atmosphere. So if it comes out in a burst, it rapidly is removed again. The methane concentrations can go up and down, up and down. And if methane is becoming more important and CO2 less important, then the up and down and up and down has a stronger influence on climate. If you’re already in a cold state because of the increase in weathering because of the low latitude continents, you’re in a cold climate but a climate that has greater variability, then you’re increasing the risk of it getting so cold that you reach that tipping point for the ice albedo runaway. So that’s another theory. And there is some support for it because the carbon isotopes provide a signal, first of a long period of a high rate of organic burial and then, a shorter period in which that organic matter is coming out, likely in the form of methane.

Interviewer: What was the Earth like during these frozen periods?

PAUL: Once the ocean is frozen over, you have a solid surface planet. And so you now have an Earth that’s more like Mars than it is like Earth. The best terrestrial analog would be in the middle of a large continent―like the middle of Asia, in the deserts of Asia where you have a sort of pretty bright reflective surface, and you’re a long way from the moderating influence of the ocean. What happens there is you get an exaggerated temperature fluctuation between day and night and between winter and summer. If you’ve been to the continental deserts, you know that it’s hot in the daytime, and it’s bitter cold at night. And it gets very cold in the winter and very hot in the summer. So first of all, you would have a real exaggeration of the temperature fluctuations on a diurnal and a seasonal basis and there would be a number of other peculiar meteorological effects. In the winter hemisphere, it would get exceedingly cold. The surface temperatures would get so cold that there would be very, very cold air immediately adjacent to the surface, and you’d have an inversion. You’d have strong winds that would be as a result of the fact that the air would be descending in the winter hemisphere and rising in the southern hemisphere. But they wouldn’t feel the surface at all in the winter hemisphere because they’d be protected by this inversion, so the surface would get even colder still. And so, you’d have extreme contrasts between the seasons and between day and night.

Interviewer: Can you go into a little more detail of what the Earth was like during a snowball period?

PAUL: The ice on the ocean, because of the very cold temperatures, gets extremely thick. But it’s thicker in the high latitudes than it is at the equator and that’s because the thickness of the ice depends on the surface air temperature. That determines how thick an ice layer you need to diffuse out that geothermal heat—the heat that’s coming out of the interior of the Earth. Now because of this gradient in the ice thickness―thin at the equator,thicker at the high latitude―this thick ice flows. It flows exactly like a glacier on land or a glacial ice shelf. It flows down the thickness gradient so it flows from higher latitudes towards the equator. It flows under the influence of gravity due to its own weight. It’s floating, but it’s flowing, just like a glacier. It’s actually quite a dynamic world. The ice is flowing. The atmosphere is circulating. The ocean is circulating slowly as a result of melting in some places and freezing in other places. That now means that the ice is a flowing mass, and it’s shearing past the continents because where the ice abuts against a continent or an island, it’s frozen onto a rigid sea bed. But where the ice is floating, it’s flowing. And so there’s continual shearing that’s taken place. The ice is continually cracking. I mean if you were there, it would be a very noisy place because the ice would be cracking. The sea water would be coming into the cracks, keeping it apart, because remember sea water is denser than the ice. Organisms would be living in the cracks, doing photosynthesis, doing heterotrophy. The cracks would then seal over with new ice, but then new cracks would form. This would be a very vital and dynamic environment.

Interviewer: Can you describe your fieldwork in Namibia?

PAUL: When a geologist picks a field area, he picks it on the basis of interesting rocks that are well exposed, so you can study them easily and well―easy logistics because you want to spend as much time as possible doing your work and not sitting around cooling your heels―and political stability because if you’re serious about your work, you want to be able to work there for many years because it’s a cumulative process of working out exactly what’s going on. Namibia provided that. The reason why the rocks are so interesting there is that we have glacial deposits, but the glacial deposits are sitting at two horizons in the middle of a thick succession that’s composed almost entirely of marine carbonates. What is now Namibia was an area that in the period just before the Cambrian looked like the Great Bahama Bank―a vast area hundreds of kilometers across of shallow, warm water, where carbonate sediment is continually forming on a big shallow water platform, which has an apron of deeper water carbonates that flank it. Namibia looked just like that. Now the importance of this is that because of the solubility of calcium carbonate, calcium carbonate forms preferentially in the warmest part of the surface ocean. So, we know that Namibia was the warmest part of the ocean at that time or one of the warmest parts, and it was glaciated. And not only was it glaciated, there were no mountains there. These were not mountain glaciers, these were glaciers that formed at sea level in the warmest part of the ocean. So there, you are front and center with the main paradox of these glaciations—that you have glaciers at sea level in the warmest parts of the world, which implies that the rest of the world was glaciated as well.

Now, it’s even better. In Namibia, we have not only the platform, but we have the submarine apron as well, all beautifully preserved. And that allows us, for example, to get an idea of how much sea level fall there was as a consequence of the buildup of ice sheets on the land. During the last ice age, the last Pleistocene Ice Age, sea level fell by about 130 meters as a result of the buildup of one or two kilometers of ice over most of North America and Northern Europe. In Namibia, we’re getting the impression that the sea level fall was around 500 meters. If you do the calculation, that means that the average thickness of ice on all continents and continental shelves globally — 40 percent of global surface area―the average thickness was around 1.1 kilometers. That’s about the same thickness as the west Antarctic ice sheet today. That was the average thickness on every continent.

Interviewer: Why did you first go to Namibia?

PAUL: These glacial deposits have been studied for decades, and I only started studying them in the early 1990s, so I was a relative newcomer. I wanted to go to an area that hadn’t been studied so much before but was particularly advantageous. And of course, Namibia only became independent in 1990. Prior to that, there was a 20 year civil war―so not very much geological field work was done in northern Namibia at that time, which was under occupation by South African defense forces. Namibia offered this wonderful situation as far as the geology is concerned―extraordinarily well exposed because it’s on the actively eroding great escarpment of southern Africa, along the skeleton coast of Southwest Africa, so there’s very little vegetation. It’s the Namib Desert, so the rock exposure is excellent and very little work had been done. I believed that with independence, Namibia had every chance for success as a multiracial representative democracy and indeed that has proved to be the case. The roads are well maintained there. There’s an excellent geological survey. And I might just mention that the founding president of Namibia, Sam Nujoma, upon his retirement at the age of 72, after his constitutionally mandated second term as president, retired and enrolled as a freshman in geology at the University of Namibia.

Interviewer: How many times have you been to Namibia? What is your typical expedition like?

PAUL: I’ve been to Namibia every year now since 1992, which was my first year there. In the early days, I used to spend almost three months there every year — June, July, and August, and the last few years, it’s been more like a month or five weeks most years. I have a vehicle in Windhoek, the capital. So we can arrive, get the vehicle, do our shopping, and be in the field site the next day. It’s one full day’s drive from Windhoek up to the remote northwest of the country where we do our work. We can camp right by the geology. It’s open range country. The people who live there are very hospitable and helpful, and it’s just a wonderful place to do serious geological field work.

Interviewer: What happened when you first started talking about the snowball Earth at geologists’ meetings?

PAUL: You know it was the funniest thing. After this paper was published, I would go to scientific meetings and all my old friends would avoid me. Nobody wanted to talk to me. I think it was because they didn’t want to be put on the spot of making a commitment as to whether they liked this idea or not. Because like all ideas that are like this, they’re dangerous. They’re dangerous because it’s dangerous to support them because they might be wrong. But it is dangerous to debunk them because they might be right. And there is sort of, you know, a guilty truth that most scientists haven’t read the paper to begin with. They’re afraid of being put on the spot and they want to sit on the fence. So there’s this strangest feeling that I had that I was being avoided by all my former friends.

Interviewer: Do you see that kind of debate as really important, that you really have to keep people on their toes?

PAUL: Well, you know, it’s hard because a Snowball Earth is an Earth that is so different from the present Earth, and one that we haven’t thought about for very long, that it is really hard to note exactly how it operates and what the hypotheses actually predict. And how best to test it. I have been thinking night and day about this problem for, you know, 8 or 10 years now and so it’s hard to expect people who have been thinking about it for far less time and maybe only part time to have thought through things as thoroughly. It’s difficult to grasp because it lies outside the parameter space of what we’ve been generally thinking about,.

Of course, physicists were the first ones to demonstrate, in theory anyway, that it’s possible for global glaciation to occur. But they didn’t believe it had actually occurred. They thought the Earth was just lucky that it didn’t happen because it could have happened but obviously it didn’t. And the reason why they thought it didn’t happen was because they assumed that life would be extinguished. But, of course, this was back in the 1960s and at that time, we didn’t know about the life that exists in the deep oceans for example, around hydrothermal vents which wouldn’t be much affected by freezing surface temperatures. But also, they didn’t know about all the life that lives quite happily in Antarctica, in ice, around ice, beneath ice, those organisms which could probably survive a snowball Earth. And also, we have to remember that complex multicellular animals hadn’t evolved yet. In fact, it appears that the beginning of their evolution coincides remarkably closely in time with these extreme climate events. We don’t know yet whether there’s any causal connection between the two but there is a remarkable coincidence in timing.

Interviewer: To a geologist, what are some of the features that these formations have in common?

PAUL: One of the striking features of these glaciations that occurred just before the Cambrian is that directly above the glacial deposits, you get a very continuous layer of calcium magnesium carbonate. It’s a rock called dolomite. This layer is anywhere from a few meters to many decimeters in thickness, and it’s very continuous, and it has some rather idiosyncratic sedimentary structures, which are seen worldwide. The question is: what’s the significance of this layer of calcium magnesium carbonate? According to the snowball Earth hypothesis, that layer represents the carbon that melted the snowball Earth. It’s not pure carbon, obviously. It’s calcium magnesium carbonate. But the carbon in the carbonate used to be the carbon dioxide in the snowball atmosphere. And calcium and magnesium are the result of weathering in the greenhouse aftermath of the snowball Earth. And so the deposition of that layer represents an extreme weathering that takes place under the influence of the high temperatures that prevail once the ice has melted, high temperatures that came about because of the big buildup of CO2. Once you start to melt the snowball, the ice disappears in a few thousand years; whereas it takes a few hundreds of thousands of years to draw down the CO2 through weathering. And the result of the weathering is a layer of calcium or calcium magnesium carbonate.

Interviewer: A lot of the evidence for big changes in the global carbon cycle comes from stable carbon isotopes analysis. Can you explain how this works?

PAUL: When you measure the ratio of the two stable isotopes of carbon―carbon 13, the heavy one, and carbon 12, the light one, which is 99 percent of the carbon―what you notice is that in that last 20 or 30 meters, the carbon 13, the heavy one, gets rarer and rarer. That’s an indication that there’s a new source of carbon that is very depleted in carbon 13, the heavy isotope. And almost certainly that source has to be some form of organic carbon. The source that is most depleted is methane. And methane is very, very depleted in the heavy isotope. One way of interpreting that shift is that that represents or signals or records a buildup over a time scale of around a million years in the concentration of methane in the atmosphere and that isotope shift is seen worldwide beneath these glacial deposits.

Interviewer: What else can you infer from the isotope anomalies?

PAUL: The isotopic composition of carbon in seawater obviously depends on the composition of the carbon that’s coming into the atmosphere from volcanoes and the carbon that’s leaving the ocean atmosphere system in the form of sediment. The volcanic carbon, for the most part, doesn’t change its isotopic composition very much. But the carbon that leaves the ocean does so in two forms: as carbonate minerals and as organic matter. They have a very large difference in isotopic composition—about three percent difference in the ratio of carbon 13 to carbon 12. And so, if you change the relative proportions of the organic matter relative to the carbonate, you will change the isotopic composition of seawater. In fact, the composition of seawater today is what it is because the fraction of the carbon that is being buried in sediment that is buried in the form of organic matter is around 20 percent. Maybe 100 to 200 million years before these snowball glaciations, the isotopic composition of seawater implies that the fraction of carbon being buried as organic matter was not 20 percent but more like 40 percent or even 50 percent―so a much higher proportion of the carbon that was leaving the ocean was doing so in the form of organic matter rather than mineral carbonate. And so, presumably, that means that there was more and more organic matter that was building up in the sediment, much of it being converted into methane. The methane initially was stored in the sediment in the form of methane hydrate―this ice-like material, which contains methane. But there’s a limited capacity of methane that can be stored in sediment, and if you reach that capacity, and you’re still producing methane, then the methane starts to leak. Normally when methane leaks into the ocean, it’s very quickly oxidized. But if you have a lot of low latitude continents, and you have a lot of organic matter being buried, it’s possible that some of those areas, the water itself, would become anoxic like the Black Sea today. If methane is being released from the sediment in areas where the water is anoxic, the methane won’t get oxidized in the water cull. It will leak into the atmosphere, and if methane starts to build up in the atmosphere, of course, it’s a very powerful greenhouse gas, and over time CO2 will go down, and then the Earth becomes in this drug addict-like state where it’s dependent on a continual supply of methane. Because if the methane is cut off for some reason, the methane, because of its short residence time, will quickly disappear, and you’ll lose your greenhouse warming effect.

Interviewer: It’s like the person who goes out on a warmish winter’s day with too many layers of clothes and gets very, very hot, and then he takes off all his outer garments, and suddenly he’s very cold because he’s been so used to the hot. I think people can understand that pretty well.

PAUL: When geologists first proposed that something about the atmosphere must be trapping heat, they made the first analogy to a greenhouse, and the greenhouse referred to was one used in experiments that were carried out by a Swiss nobleman named de Saussure in the late 18th Century. De Saussure invented the solar oven, a glass box within another glass box, and in the inner glass box, the air temperature was sufficient to boil water just from the rays of the sun. That’s because of the trapping effect of the glass. Now it’s not an exact analogy, of course, with the real greenhouse effect in the atmosphere, which is more due to the absorptive properties of certain molecules like CO2 and methane. But the name stuck.

Interviewer: Tell me briefly―what does your actual field work consist of?

PAUL: We’re interested in measuring the carbon isotopes systematically, both because they tell us something about the carbon cycle and also because we use them for correlation. When we go to the field, we select certain sections that we know well — that we’ve measured bed by bed — so we know each layer. And we decide on how much detail we want our carbon isotopic record, and then we systematically collect small hand samples, maybe every meter or maybe every half meter or maybe every ten or twenty centimeters. We collect a sample, and we write a number on it so we know where the sample comes from. We trim it to get all the weathered material off it, and we collect it just large enough so that we can saw it in the laboratory to get a smooth face, and then we select the best, least altered, part of the sample. We get a dental drill, and we drill a little hole to get a small amount of powder. That powder is taken upstairs and is put through a series of chemical reactions that can convert it into CO2 gas, which is then ionized and accelerated down a mass spectrometer, which causes these ions to be deflected under the influence of a magnetic field by a degree of deflection that is very sensitive to the mass differences between the different ions. You can select out the carbon 13 from the carbon 12 and measure it with extreme accuracy as a ratio — not an absolute abundance — but as a ratio of carbon 13 to carbon 12 or indeed any other pair of isotopes, which differ in mass by one neutron.

Interviewer: Just give me a sense of the power of this tool for geology and in general. Could you do this without the mass spectrometry?

PAUL: Oh no. Iisotope analysis is in many ways the most profound revolution in the Earth’s sciences of the 20th Century. First of all, radiometric dating — radioactive isotopes — allowed us for the first time to date the age of the Earth and the age of the solar system. Before the 1960s, it was a continuing controversy about the age of the Earth. It allowed us not only to date the age of the Earth but to date most igneous rocks and, therefore, to date the stratigraphic record where igneous rocks like volcanic ash layers are deposited contemporaneously with sedimentation. It also allows us to study all sorts of biogeochemical cycles―the biogeochemical cycle of carbon, the biogeochemical cycle of sulfur, and many other geochemical cycles because there are processes in those cycles which fractionate the isotopes. We can also do all sorts of things in tectonics. We can measure the rate at which continental crust was extracted from the mantle because of fractionations of isotopes that occur when the mantle melts under high pressure mantle conditions, for example. We can tell not only about the early history of the solar system but also of the history of pre-solar grains—grains of dust, which were produced and which experienced a history before they were incorporated in our solar system can be studied through isotopes. It’s almost limitless, the applications of isotope geochemistry in cosmochemistry and in solar system studies, in planetary history, and in biogeochemistry.

Interviewer: Going on to the aftermath, you’ve seen enormous wave ripples, indicating wind speeds vastly above what we see now. Can you tell us a little bit about that research?

PAUL: One of the idiosyncratic features found in cap carbonates in many areas―Namibia, Canada, Svalbard, Australia―is a sedimentary structure called a giant wave ripple. These are just like normal wave ripples; they have rounded troughs and sharp linear crests, and they build vertically, and they’re related to oscillatory wave motion. Under the influence of winds, waves occur, and that causes the particles in the water to undergo a sort of elliptical trajectory. And that causes grains on the surface to swash back and forth, and you get these ripple forms that build up. Now the unusual thing about the ones that are in cap carbonates is that they’re huge. Instead of having relief of a centimeter or two and a spacing of a few centimeters, the relief is 30 or 40 centimeters, and the spacing is two or three meters. So these are just like normal wave ripples, but they’re very large. We’ve done calculations to indicate the wave speeds that would be required to build up these very, very large scale ripples, and the key is that these must be winds that blew across whole ocean basins. They were like trade winds. They’re not like hurricane winds because hurricanes are very small features. These are prevailing winds, and the wind speeds we calculate at around 20 to 30 meters per second. That’s about three times the speed of normal trade winds. So these aren’t unusually strong winds, but they’re unusually strong as trade winds, of winds that blow continually over vast stretches of the ocean. Now that’s just an intuitive speculation because this really hasn’t been studied in a proper model but certainly something unusual was afoot.

Interviewer: What do you look for in the fossil record?

PAUL: The truth of the matter is that there isn’t much of a fossil record. Fossils are extremely rare during this interval. There is no evidence for multicellular animals. There is certainly evidence of multicellular algae that predate these extreme glaciations, but the record is very, very spotty. I mean you might have one locality where you have excellent preservation, where you can document these things maybe once every hundred million years on average. The only type of fossil that is present in any abundance in a more or less continual way is something called archicarps. These are acid-resistant, organic walled microfossils. And there’s some kind of planktonic form. We’re not even sure whether organisms or just reproductive cysts of algae, whether they’re non-skeletal heterotrophs like protus. We really don’t know. But we know that they’re quite abundant. During the period of the glaciations and between the two purported snowball events, the diversity of these forms is extraordinarily low. For some reason, something is inhibiting the diversity of these things. In China, immediately following the second in the last snowball event, suddenly these animals start to increase in diversity — they change rapidly, and they grow all sorts of spiny ornaments — spiny processes on their outer margins. Some people have suggested that this isin response to predation. And that would be very important because those predators would be the first predators we know of that were capable of eating a eukaryotic cell because these archicarps are likely algae — they’re going to be eucharyotes — not bacteria. Up until this time, heterotrophs had been eating bacteria but they hadn’t been eating eukaryotes because eukaryotes are huge compared to bacteria. So, these would represent the first true Metazoans, the first animals capable of eating a eukaryotic algae for lunch. At least that’s the interpretation―that this change in the archicarps is a response to this kind of predation. Now that would imply, if that’s correct, and it’s a speculation, that would imply that the first true Metazoans appeared in the immediate aftermath of the last snowball event. We don’t have any idea at this point why a snowball glaciation and its greenhouse aftermath might have created a circumstance that would have been selectively favorable for this great change in the course of biological evolution but the coincidence in timing is tantalizing.

You can’t do photosynthesis unless you have photons and liquid water in the same place. In a snowball Earth, it would be very limited―the places where that condition would occur. So there were stresses associated with the glaciation, and there are obviously stresses associated with the greenhouse aftermath. It got very hot, and it was a very acidic environment because of the high levels of carbon dioxide producing carbonic acid. These would be very stressful, so you could imagine that that would be an environment in which there would be strong selective pressure and, therefore, that might be an incentive for evolutionary change. But the arrival of multicellular animals is not just a change; this is a biological innovation. This is a change to a world in which you had organisms which achieved a level of complexity and behavior that had never been seen previously. And we really don’t know what kinds of conditions allow for biological innovation as opposed to change.

Interviewer: Could it be that the climate somehow pushed the innovations?

PAUL: There are all sorts of things that are interesting to speculate about. Many people think that the evolution of multicellular animals was somehow related to a rise in the level of atmospheric oxygen because animals, if they are going to do anything, need oxygen for energy to burn sugar, and they also need oxygen to make collagen, which is the connective tissue that holds large complicated animals together, which would fall apart.

There is some evidence—it’s not clear-cut or certain—but there’s some evidence for a rise of oxygen at around this time. It’s not known whether this had anything to do with the glaciations, but it might have. After all, a rise in oxygen is going to be a good way of getting rid of methane, which can make a glaciation by eliminating the methane greenhouse, and some people have suggested that. If it was advantageous for organisms to be adaptive to a rise of oxygen—because remember oxygen is a poison for organisms—so the first organisms that did oxygenic photosynthesis, the first task they have to have is to develop enzymes to protect them from the destructive effects of this oxygen. I mean we take antioxidants for a reason.

Is there anything about the glaciation and its aftermath that might have provided a selective pressure that would have prepared some organisms to take advantage of a rise of oxygen for any reason? Joe Kirschvink, the guy who invented the term snowball hypothesis and was the first to declare the hypothesis, recently has pointed out that if oxygen levels were extremely low during the snowball glaciation, UV radiation would impinge directly on the ice, and as a result hydrogen peroxide would be produced and would actually get buried in the ice and then release when the ice melts. And of course, hydrogen peroxide is a very powerful oxidant. So if there was a lot of hydrogen peroxide in the glacial aftermath, then that would be a very strong selected filter against any organisms that weren’t capable of handling that oxidant. And that might be an interesting pre-adaptation for organisms to take advantage of a more oxidizing world. But this is highly speculative at this point.

Interviewer: Why do you keep going back out in the field?

PAUL: Every time I go back to the field, I find things that aren’t what I expected. That’s the main reason to go to the field―to test your expectations―because your expectations are basically the predictions of what you currently believe. You do the best you can with the observations you have in hand, and the best theory you can get your hands on, and you make predictions, and you say, well, where do I go to most effectively and most rapidly test those predictions. I’ll keep going to the field so long as field work shows that some of those predictions are wrong because that makes me rethink, and in many cases, change my mind about something. That’s why I continue to go back every year. It’s part of the continual scientific process of synthesizing observations, making hypotheses, which make predictions that can be tested with new observations.

Interviewer: Why has the snowball hypothesis, which first received so much criticism, become so widely accepted?

PAUL: The reason I think the snowball hypothesis is attractive is that it is the only hypothesis that simultaneously and plausibly explains a set of observations about which everyone is in agreement―that glacial deposits are widespread on every continent, that there’s paleomagnetic evidence that ice was flowing into the ocean close to the equator, that glaciation occurred in areas that were the warmest parts of the Earth, and that there’s a number of different lines of evidence suggesting that there was severe perturbations of the ocean, in particular, that the ocean went anoxic for long periods of time, plausibly due to an ice cover and the association of the glacial deposits with thick sequences of carbonates, meaning that the warmest parts of the world were below freezing. There’s no other hypothesis that explains all those different categories of observations. As Darwin said, “I can’t believe that a false hypothesis would explain so many different categories of observations.”

Series Directory

The Habitable Planet: A Systems Approach to Environmental Science 


Harvard Smithsonian Center for Astrophysics in association with the Harvard University Center for the Environment. 2007.
  • Closed Captioning
  • ISBN: 1-57680-883-1