12/18/2020

How The Past Hints About Our Climate’s Future

17:49 minutes

a graph that shows fluctuating levels of co2 going back 100 million years. it also shows projected levels of co2 a few hundred years in the future, which are much higher than previous appears on the graph. it also has projectsions for middle road emissions, which are more inline with the graph, and a sustainable projection, which is on the lower end compared to the rest of the graph
Past carbon dioxide concentrations (left) compared to possible future emissions scenarios (right): The rate of current emissions is much faster—occurring over decades—unlike geological changes, which occur over millions of years. If emissions continue unabated, carbon dioxide levels could meet or exceed values associated with past warm climates, such as the Cretaceous period (100 million years ago) or the Eocene epoch (50 million years ago), by the year 2300. Credit: Jessica Tierney/University of Arizona

Ask a climate scientist how much the earth will warm as a result of the carbon dioxide we’re emitting right now, and the answer will be a range of temperatures: likely anywhere from 1 to 5 degrees Celsius. 

But all the models we have to predict the future are based on data from the past, most of it collected in the last 140 years. As carbon dioxide rises further past the unprecedented-in-human-history 400 parts per million (ppm), we are increasingly in a world never before seen by human eyes—or measured by thermometers. 

While we are certain the Earth’s climate will warm as CO2 increases, it’s harder to pin down exactly how sensitive the climate is. Scientists are working hard to narrow down our uncertainties about the coming temperature changes, sea level rises, and new patterns of rainfall and drought

And paleoclimatologists can examine ancient rocks, sediments, ice, and fossilized shells for clues about how past climates changed in response to different levels of carbon dioxide. Climates from past epochs have not only experienced that 400 ppm mark, but also levels higher than 1,000 ppm—and correspondingly, higher temperatures and higher seas. In Science last month, a team of researchers made the case for using more data from these climates, millions of years ago, to help us map out the future we face.

Science Friday producer Christie Taylor talks to University of Arizona geoscientist Jessica Tierney, who is lead author on the new research. 


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Segment Guests

Jessica Tierney

Jessica Tierney is an associate professor of Geosciences at the University of Arizona in Tucson, Arizona.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. The climate is changing. And more importantly, we are changing the climate by burning fossil fuels and pouring carbon dioxide into the Earth’s atmosphere. But as scientists puzzle out how drastically temperatures will rise, they haven’t got a lot to work with, just 140 years of direct measurements of the greenhouse gas, temperature, and rainfall.

But there is hidden information locked up in fossils and rocks from millions of years ago. And that’s where they are turning for help. SciFri producer Christie Taylor has more.

CHRISTIE TAYLOR: It’s been several years since our atmospheric carbon dioxide reached 400 parts per million. And human beings have actually never even been alive for such high concentrations of CO2. But you know who was? The dinosaurs. 100 million years ago, CO2 was well over the 1,000 PPM mark. And more recently, 3 million years ago, Earth’s atmosphere had the same level of CO2 as we have now, in a changing climate.

So could data from those times, ancient rocks, marine sediments, ice cores, be the time machines we need for understanding temperature, sea level, and everything else we might be in for as the climate changes? That’s what a group of scientists concluded in research published in Science last month, that the only way to narrow down the range of what could happen is to plug in the numbers on what we know did happen, long before the first thermometer or CO2 detector had been invented by human minds.

Here to explain more is Dr. Jessica Tierney, an associate professor of geosciences at the University of Arizona in Tucson. She’s lead author of the new research. Welcome to Science Friday, Dr. Tierney.

JESSICA TIERNEY: Thanks for having me.

CHRISTIE TAYLOR: So the paper that you co-authored for Science last month states that past climate informs our future. And you’re talking about how climates from millions of years ago can help us understand what’s happening to us right now under human induced climate change. So why do we need to look that far back?

JESSICA TIERNEY: Right. So as we look into the future, we’re looking at a future with high CO2 levels in the atmosphere. Already, we’ve surpassed 400 PPM CO2. The last time that CO2 was that high was 3 million years ago, actually, which is a long time ago, right? So we’re kind of trotting into the unknown here. And the problem is we can’t actually understand the potential climate changes that we’re going to experience if we only look at, for example, the historical record of temperature change or the historical record of precipitation change because that’s occurred over a very narrow range of CO2.

So if we go back deeper into Earth’s history, though, 3 million years ago we saw 400 PPM. But if we go back even farther, we find ancient warm climates, the so-called greenhouse climates where CO2 levels are up near 1,000 PPM. And that’s a really warm climate. So in order to really understand where we are headed, we have to look back.

CHRISTIE TAYLOR: When you’re talking about that historic climate data, you’re talking about temperatures that we have recorded with instruments by human agents. Why isn’t that data enough?

JESSICA TIERNEY: Well, if you think about the historical change in temperature, we’ve seen a little bit over a degree of global warming in Celsius units. And some regions have warmed more than others. But what we’re looking at into the future, the end of the 21st century, we’re talking about warming that could be 3 degrees Celsius or it could be as high as 5 degrees Celsius.

And that level of climate change is way outside of what we’ve experienced in historical time. It doesn’t sound like big numbers. But for context, the last glacial maximum, which was the last ice age where huge glaciers covered North America, that was a 6-degree Celsius temperature change. You know, pretty big, but it’s still a single digit number. So you can see how dynamic the Earth system is.

We go back to the Pliocene 3 million years ago and the temperature change is only 3 degrees Celsius, warmer than present. And yet, the Greenland ice sheet was completely melted. It was gone.

CHRISTIE TAYLOR: You mentioned this range of temperature changes we might see as a result of this increase in carbon dioxide in the atmosphere. What are the biggest unknowns there creating that range?

JESSICA TIERNEY: Yes, so the first sorts of uncertainty, of course, is how much carbon are we going to put into the atmosphere. And so what will the CO2 levels be? Obviously, that depends on us and the sort of decisions that we make. But in addition, even for a set emission scenario, let’s say, there is actually a wide range of predictions from climate models, which we use to kind of forecast out into the future.

And that actually relates to the sensitivity of the climate model to changes in CO2. So it turns out that different models have different sensitivities. And so some models are very sensitive. For a doubling of CO2, maybe they experience 5 degrees Celsius of warming. So that would be a high sensitivity. Others are quite low. Let’s say they experience only 2 degrees C warming.

So for the same projected emissions, the low sensitivity model will give you a much lower warming answer than the high sensitivity model. So the problem is, what is climate sensitivity? In order to narrow those projections, you would need to narrow that number. And in order to figure that out, this is where the ancient record of climate change, a paleo climate record can really play a strong role because we have all these different climates to sample.

We can go to warm climate. We can go to a cold climate. We can determine what was the change in CO2, what was the change in temperature, and then come up with a range of plausible climate sensitivity. So we can either rule in or rule out, for example, a high sensitivity model based on the study of these ancient climates.

CHRISTIE TAYLOR: So I feel like this is the question that is going to be burning in everyone’s mind. How do you look back that far in time and have a sense of the accuracy of what you’re finding?

JESSICA TIERNEY: Right, great question. So we no longer have thermometers when we’re going back into ancient climate history.

CHRISTIE TAYLOR: No time machines?

JESSICA TIERNEY: No time machines, unfortunately. We all wish that we could go back, as geoscientists. We actually have to use the remnants of living things, often, that are preserved in rocks and sediments. And we have to use chemical tricks to actually decipher what is the temperature and what is the carbon dioxide. The ice cores that are from Antarctica, they trap bubbles of ancient air directly.

And so that’s really cool because from an ice core, we can actually directly measure ancient CO2. So that’s kind of easy. But that only takes us back a little less than a million years. So one of the archives that we really rely on for some of these ancient warm climates that have occurred in the last tens of millions of years are marine sediment cores. They’re collected from the middle of the deep ocean with a very, very large ship with a very, very large coring apparatus.

And so you’re going out there collecting these cores. And they go back millions of years, this tube of mud. In that tube of mud are tiny fossils of single celled organisms called foraminifera. And these guys in a protist, they live in the upper areas of the ocean. And they’re useful to us because they’ve been around for a long time in the geological record. Their shells are made of calcium carbonate, chalk, essentially. And they’re well preserved in sediments.

So we can find them and we can measure different aspects of their chemistry, which can tell us about both temperature and CO2, actually. Measuring different isotopes, in particular, is a big thing for us in climate science, what we call stable isotopes, each atom has a slightly different number of neutrons. And basically, these stable isotope serve as tracers for temperature and also CO2, which is really interesting and kind of a newer technique. But now, using those shells, we can actually measure that chemistry in the lab and come back with numbers of temperature and CO2.

CHRISTIE TAYLOR: That’s amazing. So how far back can we look?

JESSICA TIERNEY: So the Marine sediment core record goes back about 100 million years. That’s a long time. That takes us back to the Mesozoic and the time of the dinosaurs, and covered several really warm climates, including the Eocene warm periods 50 million years ago and also the middle Cretaceous, which is a very steamy time. The reason that the sediments don’t go back further is simply because we’re getting them from the deep ocean. And, well, because of plate tectonics the ocean crust is, over these timescales, being recycled.

So it actually gets abducted under another plate and then recycled. So we can’t find marine sediments in the ocean that are older than that. Of course, we can go and switch to rocks on land. And rocks on land can take us back even farther.

CHRISTIE TAYLOR: Do we know exactly what the Earth was like for its entire history at this point, or are there gaps?

JESSICA TIERNEY: Right. So I think we have a sense of the broad scale changes, for example, when the Earth was experiencing, overall, a cold climate, which we define, in part, by whether there is large ice sheets at the poles, and then when it’s experiencing warm climates where we don’t see a lot of continental ice sheets left behind. And we know those changes fairly well for, I would say, even the last billion years, we’ve got 700 million years ago, Snowball Earth.

CHRISTIE TAYLOR: What was that?

JESSICA TIERNEY: Yeah, Snowball Earth is pretty spectacular. That’s the coldest known climate that we’ve observed on Earth. And basically, in that time, ice extended all the way to the equator. And so it was, basically, the Earth froze over. We have evidence of ice sheets in tropical locations, persisting there for quite a long time.

And then, eventually, actually, when the Earth came out of Snowball Earth, it warmed up really rapidly and went to the other extreme, went to some extremely warm climates in the aftermath. But that’s pretty incredible. And those incidents of Snowball Earths were really recorded by the fact that we do physically see evidence of ice sheets in what would be tropical latitudes. So it’s pretty wild.

And then moving forward in time, we have extreme warm climates. For example, probably the first one that we understand really well is the Permian Triassic transition, which is also the most extreme extinction in Earth’s history, when you have more than 90% of all the species on Earth died. And it’s also a very warm climate associated with really high CO2 levels, extremely high temperatures, in this case, in response to persistent volcanic eruptions which will emit CO2. And so that’s something on the other extreme.

CHRISTIE TAYLOR: So how does this understanding of deep time, geologically speaking, you know, under some of these worst predictions for carbon emissions, what does this tell us our future Earth could look like?

JESSICA TIERNEY: So the first thing we can tell from this long-term Earth history is the importance of CO2. So every time in Earth’s history when the climate gets hot, it’s CO2. And so there’s this really tight link between CO2 and global temperature that persists across millions and even billions of years, basically the entirety of Earth history.

And then looking to the future, one of things we can learn from the past is actually how does the Earth system recover from a very rapid emission of carbon dioxide? So a lot of people ask me, you know, what’s going to happen? Is the Earth just going to spiral off, become Venus in the future in response to CO2 emissions that humans are doing? And the answer is no, it won’t. It will actually recover.

But the timescale of recovery is geological and not a human timescale. So it can be hard to understand. The full recovery is on the order of 100,000 years. And how do we know that? So we’ve actually seen events in Earth’s history where, all of a sudden, a large amount of CO2 was admitted to the atmosphere. So one of the best studied events is something called the Paleocene Eocene Thermal Maximum. It’s what we call a hyperthermal event that occurred 55 million years ago, and basically, a bunch of CO2 emitted into the atmosphere through volcanism in the North Atlantic.

And we see that the Earth’s temperature skyrockets up an increase of around between 4 and 6 degrees Celsius, or something. And then we can watch what happens in the aftermath, right? So the first thing that happens in this event is that the oceans acidifying. So a lot of the CO2 we are emitting right now is taken up by the ocean. And in fact, the ocean is sort of helping us out that way because the CO2 that’s taken up by the ocean is no longer in the atmosphere and no longer warming the atmosphere.

And the pH of the ocean will drop to the point where, actually, the calcium carbonate sediments on the bottom of the ocean, the chalk sediments that are actually made up of those foraminifera shells I was talking about earlier, start to dissolve. And the good news about that process is that when this chalk dissolves, it neutralizes the acid. So it actually neutralizes the CO2. So it is part of the recovery.

But you can imagine that that has consequences for things that are living in the ocean and experiencing these low pH. But you see that there’s a recovery, it’s very slow, as the rest of the CO2 is essentially neutralized by those calcium carbonate sediments. And then on very long timescales, actually, CO2 starts to react with rocks on land. And that neutralizes it and brings the system back to where it was before.

But as I said, that’s taking place over a timescale of about 100,000 years. We’re talking about timescales that are much longer than, sort of, living things are used to.

CHRISTIE TAYLOR: Just a reminder that this is Science Friday from WNYC Studios. I’m Christie Taylor talking to Dr. Jessica Tierney about how past climates can inform our future. Is this a tricky thing to communicate when we live, also, in a world where climate change has been politicized and there’s this contingent of people who say, well, the Earth was hotter than this before, therefore climate change isn’t something we need to act on. How do you work with that?

JESSICA TIERNEY: Yeah, so for sure the Earth has been so much hotter than now. But I like to remind people that we weren’t around then and we’re not adapted to it. Fundamentally, humans, we evolved only 300,000 years ago in the Pleistocene. We’re an ice age species. We’re used to having a lot of ice on planet Earth. We’re used to the temperature ranges that we’ve evolved in.

If you just dropped us into the middle Cretaceous, I’m not sure how well we would do, you know? It’s very warm. And different set of organisms were adapted to do well in that time. And so suddenly shoving the Earth into a high CO2 planet, yeah, sure, it’s happened before. But these ecosystems and us, we’re not ready for it. And so you think about the magnitude of sea level rise that accompanies warm climates, right?

Even in the Pliocene, which we consider a moderately warm greenhouse climate, you know, Greenland’s gone, West Antarctica is gone. And so you’ve got like 10 meters, 20 meters of sea level rise. That’s extraordinary, not to mention when you go back to these ancient warm climates and there’s no land ice at all.

So the question is– yes, sure it’s been warm before but, I mean, first of all, we’re not prepared for it. And secondly, there’s the issue of the rate of change. So when I talk about these ancient climates, a lot of them are the consequences of millions of years of changes in greenhouse gases. And then the Earth system can kind of adjust and catch up, right?

We have never observed anything in the geological record that is as fast as the anthropogenic perturbation. Quite honestly, we don’t have a perfect analog, right? I mean, we know what happens, but it’s happening so much faster. And so in terms of the preservation of our society and the ecosystems that we have on this planet, you ought to be concerned about that.

CHRISTIE TAYLOR: And that’s all the time we have. Dr. Jessica Tierney, an associate professor of geosciences at the University of Arizona in Tucson and lead author on new research in Science last month about the power of paleo climates. Thank you so much for being with me, Jess.

JESSICA TIERNEY: Thanks.

CHRISTIE TAYLOR: For Science Friday, I’m Christie Taylor.

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