This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The history of the Earth’s climate is archived within the sedimentary record. That is because there are certain physical, chemical and biological properties of sedimentary rocks that can serve as indicators, or proxies, of the climate that prevailed when they were deposited. Over the past few decades, we’ve discovered more and more proxies, enabling us to probe more aspects of past environments, such as the temperature and composition of the atmosphere and of the oceans, sea level, the presence and extent of ice caps, and even patterns of wind and rainfall. But now there is a heightened interest in such work, since we want to understand what the rapid human-generated rise in carbon dioxide means for the planet.

Isabel Montañez has been reconstructing past global and regional climates throughout her 30-year research career. She is a Distinguished Professor of Earth and Planetary Sciences and Director of the Institute of the Environment at the University of California, Davis. One climate period in particular has attracted her attention: the late Paleozoic Ice Age. That is because in certain key respects, it can serve as an analog of the present day. It was the only time in Earth history when carbon dioxide levels increased abruptly during a period of recurrent glaciation. That ice age eventually gave way to a much warmer greenhouse Earth, devoid of ice. Isabel Montañez, welcome to Geology Bites.

Isabel Montañez: Thank you, Oliver. It’s a pleasure to join you today.

Oliver Strimpel: It’s impressive that we can reconstruct such apparently evanescent things as atmospheric temperature and global sea level through geological time. I said that we use ever more proxies for this purpose. Can you tell us what these proxies are? And let’s start with the traditional ones.

Isabel Montañez: So some of these proxies that we’ve been using for decades now are oxygen isotopes of fossils of marine shells and corals that formed in the ocean, and equally of minerals that form in ancient soils on land. And these can give us information like local surface temperature and the composition of the seawater at the time, or of the rainwater in which they formed. And the carbon isotopic composition, which we typically do with the oxygen, and sometimes elemental concentrations of these minerals: when we combine all those together, they provide information on local temperatures, humidity, rainfall patterns.

And I think one that is particularly important is how we reconstruct CO2 in the past. There are probably nearly a dozen approaches to reconstructing CO2 in the past. All of them involve fossils of living organisms, or minerals that are forming in the oceans or on land. And each of those has its weaknesses and strengths. So we typically use multiple of these proxies. For example, we might use the minerals in soils and the plants that form in the ecosystems associated with those soils, and they will then together give us pretty good constraints on what CO2 in the atmosphere was in the past.

Oliver Strimpel: Can you pick a few of the most important proxies and explain the physical mechanisms or chemical mechanisms by which they managed to record things like CO2 level or temperature?

Isabel Montañez: Yeah. In the oceans, the proxies of CO2 typically involve the isotopic composition of marine shells. And why do they do that? Because these isotopic compositions are very sensitive to the pH of the seawater. And in turn, the pH is a good recorder of how much CO2 the ocean is dissolving. You know, the ocean is like a big sponge, and it is always absorbing CO2. It is limited in how much it can take by the other carbon chemicals in the ocean. So pH is a good indicator of what the balance of those other carbon molecules is. If we can reconstruct pH, and we can reconstruct another component of the ocean, like how much of one of these dissolved carbon molecules was in there, then we can reconstruct CO2 in the atmosphere. And what’s really exciting is we’ve done this mostly using these microfossils called foraminifera, and it’s worked very, very well. It gives some of the best estimates of CO2 in the past. Unfortunately, those foraminifera have only existed for, at best, the last couple of hundred million years, and that means we have this big void prior to that. And literally just a month ago, someone showed how well this works on fossil brachiopods. So that has really now expanded our ability to use that type of CO2 proxy way back to 500 million years.

On land, there are several different approaches, but two of the best are those carbonate minerals that we call pedogenic, or soil-formed, carbonates. They are in any soils that formed in relatively drier conditions, so the southwest of the US would be a good example, Africa, Brazil. And these form around roots, because roots respire CO2 at night, and that CO2 gets dissolved in the pore waters of the soil, and they get oversaturated, and that comes out as a new carbonate mineral. The very cool thing about soils is that there is a balance between how much of the CO2 in the soil is from those roots respiring, or microbes respiring in the soil, and how much has actually infiltrated from the atmosphere. And the balance of the atmospheric CO2 versus the soil-formed CO2 is recorded in the carbon isotopes of the carbonate mineral, as well as of the organic matter in the soil. And that can then be used to reconstruct CO2 in the atmosphere.

And then the other one, which I think is just so elegant, but very straightforward, is that all plants, vascular plants that have formed since about 440 million years ago or so, have breathing pores on their leaves or on their stems, what we call stomata. They are little holes where water comes out, because plants transpire water, but it’s also where they take in CO2 from the atmosphere to photosynthesize. And so there is a very simple inverse relationship between how much CO2 is in the atmosphere and how many of those pores they have on their leaves or stems. If there isn’t a lot of CO2 in the atmosphere, they need lots of those pores to be sure that they get enough CO2. And vice versa, if there’s a lot of CO2 in the atmosphere, they don’t need as many of them. And it’s to their advantage not to have too many, because, as I mentioned, they lose water through these pores, and they don’t want to lose water, especially if it’s getting drier seasonally or with time. And it’s a very tight connection between the amount of CO2 in the atmosphere and those pores that develop. And we simply use how many pores there are per square millimeter on a leaf.

Oliver Strimpel: We’re now talking about proxies based on isotopes. Why should they be sensitive to these environmental parameters? Is it something to do with the reaction kinetics of the different weights of the different carbons in the carbon dioxide from the different isotopes?

Isabel Montañez: Absolutely. The proportion of the heavier versus the lighter isotope incorporated into the mineral, for instance, will have a lot to do with temperature. And if there are other parameters in the soil or in the water that are affecting the rate at which those atoms are being brought in, yes, that will also affect it. It’s a lot easier to move or incorporate the lighter isotope than it is a heavier mass. For plants, it’s a bit more complicated, in that it has to do with their physiological functioning: how well are they taking in CO2, or photosynthesizing and processing it?

Oliver Strimpel: Does any of the material you analyze for these proxies include the actual original organic matter from these skeletons, or from these leaves, that survives, or the soil? Or are you just talking about the replaced material that generally forms a fossil?

Isabel Montañez: Great question. In soils, very, very small minerals will often form effectively cages around this organic matter, and we call it mineral-associated organic matter. Now, millions of years ago, sure, it’s not preserving it perfectly, but it is pretty impressive how much of the molecular structure it can preserve. And another way is that these carbonates that form in the soil actually entomb the organic matter that was in the soil as they were precipitating. For plants, it isn’t in fact the actual cellulose or lipids of the actual leaf. It’s the waxy layer, the cuticular layer that protects the leaf. And there’s a layer on the bottom of the leaf and there’s a layer on the top of the leaf. So effectively, those breathing pores, where we measure their sizes, their depths, and the number of them on the leaf, are casts in that waxy cuticular layer, and that is preserved beautifully. I can show you images from a microscope of something 300 million years old, and a ginkgo from 100 million years ago, and a modern one, and you’d be hard pressed to know the difference.

Oliver Strimpel: Wow, that’s amazing. The actual original wax material.

Isabel Montañez: It is, and we have even extracted from them some of those lipids. So you do get things stuck on there that are preserved.

Oliver Strimpel: Can you tell us about some of the more recently developed nontraditional proxies?

Isabel Montañez: Since the turn of the millennium, there has been an explosion of development of new proxies. I think we can attribute a lot of that to the technology, the machines that give us the possibility to do isotopes across the periodic table. It’s just amazing. It’s something that 40 years ago we couldn’t have even imagined. I’ll give you a couple of examples. Uranium isotopes in carbonates that form in the ocean (and some people are even trying this in lakes) are now allowing us to look at the oxygenation levels on the ocean floor, where a lot of these shells and life might be living. Today our deep ocean is quite oxygenated, but it turns out that when you have global warming, that changes considerably. So these uranium isotopes that are now measurable in these shells, or even just in disseminated carbonates in old sediments, allow us to see how the oxygenation changed, whether anoxic conditions were forming anywhere on the sea floor, whether off a coast or deep in the ocean. And in turn, it tells us about how changes in the ocean circulation were happening. And the very cool thing about uranium isotopes is you can do those in one location on Earth and it’s representative of the whole ocean, because of the well-mixed nature and the residence time of uranium in the oceans.

Another example, going back to the periodic table: we now do the stable isotopic compositions of strontium, calcium, magnesium, lithium, potassium, mercury, and these allow us to look at degrees of weathering on land, which is really important because weathering on land is tightly coupled to CO2 in the atmosphere and to temperature. So this gives us an opportunity to track, again, what’s happening with the changes in greenhouse gases that we’re getting from other proxies, and how the landscape is responding. And that’s important because those responses in turn remove CO2 from the atmosphere. In fact, it’s one of the ways that the Earth has maintained its temperatures, through maintaining its greenhouse gases in these balances of processes. Mercury isotopes are fingerprints of massive volcanic episodes, so we can now tie them directly to the other isotopes. Was the increase in CO2 or methane in the atmosphere driven by volcanism? These same isotopes are telling us something about aridity and seasonality on land in different places, and they can even tell us about other sources of greenhouse gases that aren’t volcanoes, like melting methane out of the oceans. So it really opens up a whole new world for us, beyond just temperatures, effective moisture, and maybe seawater and precipitation composition.

And then lastly, I’d be remiss if I didn’t say there are all of these biologic proxies, primarily what we call biomarkers. Those are biomolecules. So rather than looking at the organic matter that we extract in bulk form, we can actually extract specific biomolecules, like lipids, hopanes and others. And each of them has its own function. So we can look at things like the physiological functioning of the plants: how did they respond to drought? Were they very resilient to deep freezes? But we can also look at microbial interactions with all of these other environmental conditions, and their role in temperatures. They’re another independent source of temperatures as well. So with all of this tool bag (no one group will do all of them), for any given time period there are just enormous amounts of information being developed.

Oliver Strimpel: So how do you try and make sense of all the paleoclimate data you’ve been able to reconstruct from all these proxies?

Isabel Montañez: So our approach is to contextualize that data by using an Earth system model. This model, which was produced by the National Center for Atmospheric Research, or NCAR, is our US model for the IPCC’s work, but also for predicting the climate over the next 100 years. These models are the most comprehensive, inclusive of every process in the ocean, the atmosphere, and on land that we know of that influences climate. Climate is effectively the interaction of the ocean and the atmosphere. So these models not only allow the ocean and the atmosphere to interact and be fully in equilibrium; they allow for cloud differences and development. They allow for aerosols. They model all of the changes in the circulation of the ocean, the effects of sea ice on that circulation, continental ice sheets: how do they influence the temperatures, the air humidity, the sea ice response, and in turn back to the ocean? It has vegetation. Vegetation interacts with the atmosphere and the soils in terms of moving carbon and water, so it influences once again the atmosphere and the ocean. And it just iterates, so that these processes are fully interacting and feeding back on one another. And the proof that these models are really comprehensive and correct is that they are calibrated to the modern world, where we have 100-plus years of excellent data from around the world, and they can reproduce those very, very accurately.

Oliver Strimpel: Okay, so you’ve managed to build this model. How do you make use of it? What kind of questions can you interrogate it with?

Isabel Montañez: So you start out with the boundary conditions, those baseline boundary conditions, and then the models are run to evaluate the climate processes and the ecosystem functioning. What is their sensitivity to this changing CO2 and ice sheet distribution? You don’t usually change much else, because you’re not interested in whether mountains are building or weathering down, or whether the ocean basins are changing in configuration. Those are things on a million-year time scale. But you are interested in what happens if you lose tropical forests, or whether temperatures are changing and rainfall patterns are changing. So that’s what we’re after. We’re interested in seeing what the sensitivity of these is. We’re also interested in whether there are thresholds: are there CO2 thresholds where things just plummet, or where the changes start accelerating? And are there tipping points where those forests disappear, or where the ice collapses, or where the monsoon system effectively collapses or intensifies?

Oliver Strimpel: I said in my introduction that you’ve been especially interested in the late Paleozoic Ice Age because it can serve as an analog to the present day. How far does the similarity go?

Isabel Montañez: I want to stress that it is the process scale we’re after here. The continental configuration was quite different. In fact, it was the opposite of today. It was a supercontinent. Today we have our continents at their maximum distribution away from one another on the globe. Many things are different. That said, the processes in the Earth system, whether in the climate system or the ecosystems on land, and how they interact with the environmental parameters (the atmosphere, the oceans, land, life), have existed and fundamentally functioned as they do today ever since we’ve had ecosystems with a general structure and configuration similar to what we have today. So that’s 400 million years or so, certainly the last 300 million years.

So inferring how climate may change in the future in one particular region from looking at deep time, we can’t do that, because of the differences in continental configuration and other things like that. But evaluating how these processes in the climate system, in the ecosystems, the ice response, the ocean response, respond to increases in CO2, or vice versa, decreases, and the consequent warming or cooling that goes with that on a global scale, and how those responses interact with one another and how they may even feed back on the initial warming or cooling: that is fundamental information that we can glean from looking at past periods. The main thing that makes a time period relevant or not is whether the atmospheric CO2 concentrations were within the range of what we have been seeing since we started burning fossil fuels and what we project into our fairly near future. And that is the case for the late Paleozoic icehouse. In fact, it’s the only time when CO2 was in the range of what we’ve seen since the preindustrial and what we’re going into, except for a transient period in the last several million years.

Oliver Strimpel: But isn’t the fact that there was ice, or recurrent ice, on the poles something that also distinguished that period from others?

Isabel Montañez: Of course. This is the last time, other than the last 34 million years, when there has been continental-scale ice on Earth all the time. In between, there may have been ephemeral, short-lived, maybe small alpine, maybe continental glaciers, but nothing that we can find in the rock record, nor do we produce it in climate models. So this is it. This is the last time that we see the icehouse, as it came on, at its onset, and its response to short-lived warming, essentially the experiment we’re doing now, and ultimately its terminal demise as it turned into a greenhouse.

Oliver Strimpel: I want to come back to the boundary conditions that you mentioned. So when the configuration of the continents was completely different, and presumably ocean circulation as a result was completely different, how come we believe that the sensitivities on a global scale would still be relevant to the situation we find ourselves in today?

Isabel Montañez: So I’m going to give you an example of our modeling that gives us a detailed seasonal response to CO2 changes within that range from the preindustrial to what we anticipate in the future, and then how we can say that we have confidence in that modeling based on the proxies that we’ve developed. We have the CO2 proxies, and we have the temperature proxies for the late Paleozoic Ice Age. And now we had these uranium isotopes, and the uranium isotopes were telling us that up to 25% of the sea floor was going anoxic globally. Yes, there was a different continental configuration, so clearly a different ocean circulation, and there were ice sheets. And we said, this is during an icehouse, where we have big ice, where we are generally cool. In fact, other studies we’ve done have shown that this is the only other time that global temperatures were in line with what humans have been seeing since they evolved, and today. So why are we going 25% anoxic on the sea floor? That’s phenomenal. That has huge implications for the future, if correct.

So with the climate models, we use them with the right boundary conditions to evaluate how the presence of ice, sea ice included, would change the ocean circulation, despite the fact that, again, we are looking at very different continental configurations and ice only in the Southern Hemisphere. With a doubling of CO2, or even a 50% increase, essentially where we are today from the preindustrial, we can see changes, for instance, in the Northern Hemisphere polar region: major changes in sea ice formation, but more importantly in circulation at mid-depths in the ocean. We can see where deep water is forming. So during the times of low CO2, we have deep water formation at both poles, very similar to today, even though the continents are in their very different configuration. We take the CO2, we double it, and we lose the Northern Hemisphere deep water formation: a very different world. And we see that then in the climate on land, and we see it in the sea surface temperatures. We do a 50% increase in CO2 to make it more like today, 400 ppm. And what do we discover? It’s a threshold, because under that we’re not seeing much change. At 400, all of a sudden we see these major changes, and not much more even when we double, go 50% more. And it fits our proxies beautifully. It tells us that about 18 to 25% of the sea floor would go anoxic because of this change.

So again, the boundary conditions were different in terms of continental configuration. There had to be a different ocean circulation, because you have one huge ocean called Panthalassa, and then a huge Mediterranean, which is the other part of the ocean, open to the other one, but still very different. But what have we learned? It doesn’t matter what the ocean is like; it circulates, and it still couples with the atmosphere, and it is still equally sensitive to changes in CO2. Where the water may go down deep to circulate in our conveyor belt may be different, but it’s not about where those continents are. It’s about the CO2 and its warming effect on the ocean, and the ice and sea ice behavior, that seems to be robust regardless of the boundary conditions of the continents.

Oliver Strimpel: Wow, that’s a pretty dramatic conclusion, then, that it just so happens, based on your model, that we’ve gotten to the tipping point.

Isabel Montañez: You can imagine we put that out there, and the default response by scientists is, oh, come on, it was so different back then. And our response is, yes, but step back and ask yourself why the responses are not that different. It’s because there are physics in the system that don’t care where the continents are. They care about the ocean as a whole and how it’s operating. It’s always a conveyor belt. It may slow down, it may accelerate, but it’s still a conveyor belt that is strongly governed by how much sea ice is in the polar regions, how much continental ice is there, how warm the ocean is in that region, how stratified it is. So there are nuanced differences, yes, hugely. But as a global system, it’s still operating as the system as we know it.

Oliver Strimpel: What can we learn then from your late Paleozoic Ice Age, or icehouse, model with regard to our climate future? And I presume one of the differences is that even though CO2 was rising rapidly then, it was not rising at anything like the rate that has happened since preindustrial times?

Isabel Montañez: You hit the nail on the head. I would say there are two key findings. Just by changing the climate initially, if the ecosystem in a given location is changed, we can actually change other parameters in the Earth system substantially. For example, we’ve modeled that if we changed the ecosystem in the tropics, the wet tropics at that time, by removing one type of ecosystem and replacing it with another (based on the fossil record, these shifts really happened), we could change the outflow from the Amazon basin by fourfold, or decrease it by half. So changing the ecosystem in a given location can change our runoff, so flooding. It can change the coastal environment, because it can bring a lot of nutrients to coastal waters. That is not a good thing; it causes eutrophication. It can change the atmosphere’s water, because it can put less in the atmosphere and more in the ground to run off. So that’s a big finding.

We’re looking at a time period similar to today in terms of ice sheets, where CO2 was at its low point, precisely in the range of the lows of the last glacials during the Pleistocene, the last million years. And it rose within these glacial–interglacial cycles, similarly doubling. But there were also these intervals of rapid rises in CO2 that were driven by volcanism, and maybe even by changes in ocean circulation. The important thing here is that we are seeing enormous changes in the climate system, influences on the ecosystems on land and in the ocean, changes in the oxygenation state of the oceans, changes in ice sheet stability, all in response to these changes in CO2 within the range from our preindustrial Pleistocene to our future. And as you said, at rates minimally an order of magnitude slower. And that should be eye-opening: that our Earth system is capable of responding to changes in CO2 analogous to our world and future at all. Because we should be thinking, why would it not be now?

Oliver Strimpel: Isabel Montañez, thank you very much.

Isabel Montañez: It’s very much my pleasure.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you’ll also find a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.