Climate varies on all time scales, from a year to the age of the Earth. Humans, however, best relate to phenomena comparable to their lifetime: decades to a century. Looking at Earth’s history through 100-year increments you can get an understanding of how climate changes on a relatable scale. That’s important because we actually don’t have a lot of information about this from what we call the instrumental record: we have measurements like temperature and pressure for about a century and a half, but before that we didn’t have very good measurement devices so the observations are pretty shaky. Arguably, in some regions of the world we still don’t have very good coverage, and need more satellite and field programs. To go back further, paleoclimate scientists use “proxies:” indirect observations from sources—like ice core samples from glaciers, tree rings, sediment cores from lakes and oceans, cave deposits, or corals—to try and form a picture of how the climate has changed over the past.
This allows us to understand how the climate varies over longer time spans. The tradeoff is that the observations are more uncertain, but by putting together a great many of them we can start forming this tapestry of climate, and with that we can get an understanding of how climate changes on its own or in response to external influences. It has long been known that—to take one example—the energy output from the sun is not constant over time, which changes the energy of the planet. Volcanic eruptions also alter planetary energy balance. And of course we know that our human activities are perturbing the energy balance of the planet in a major way. So one of the key tasks in climate science is to understand the changes that we are witnessing today, and asking how much of that is manmade and how much of that is natural.
What have you found? Is there wild variation?
It depends how far back you go. Paleoclimatology is a little bit like a detective story. One of my colleagues uses the analogy of Cold Case vs CSI. CSI, for example, is a TV show that looks at very fresh evidence that is left on the scene then tries to reconstruct what happened. You can do that when it’s very recent—you have a lot of evidence. Whereas Cold Case is trying to understand a crime that happened a long time ago and where a lot of evidence is gone, so you have to rely on other things. The same holds true of climate: if you look at the very recent past, for our purposes the last 2000 years, which is very recent from a geologic standpoint, there’s a lot of information there, so we can reconstruct things in a fair amount of detail. The farther back you go, the more patchy the observations are. But we also look at bigger changes. Think about ice ages for example, which were these huge swings in global climate. Those definitely left an imprint in the geologic record that we can still see today. Similarly, we know that there were once crocodiles living at the poles, we know that Antarctica wasn’t glaciated until about 20 million years ago, so we know that there were “hyperthermals,” such as the one that happened about 55 million years ago, where it was much warmer than today. We have these pieces of evidence from various times in the past where the temperature was very different from today. And in most of these cases, greenhouse gases like the ones our civilization is so busy producing had a major impact on the habitability of our planet.
In what way is your research used?
One of the challenges of Paleoclimatology is to translate its findings into actionable items. This is still a frontier area of research. Ultimately, the goal of these things is to establish how much of these variations are manmade, and how much are natural. One way that this is done is in these climate assessments, for example: the Intergovernmental Panel on Climate Change (IPCC) that puts together these reports every few years from all of the evidence from literature, including Paleoclimatology. The paleoclimate record is used for a number of things, including to estimate the sensitivity of the Earth’s climate to perturbations like the burning of fossil fuels. In that context, my personal research has been used to gauge how anomalous recent changes in El Niño are.
Would you be comfortable saying the deviations over the last 100 years are due to fossil fuels?
Yes. All signs point to the fact that we are living in extraordinary times. The current levels of warmth are very high, though perhaps not completely unprecedented in the past 2000 years. There were times, about a thousand years ago, where in some regions of the globe it was about as warm as it is today. But the rate of warming, globally speaking, seems to be much faster than anything we’ve seen recently, and that holds true if you go back into the geological record and you see ice cores, for example, that capture variations over the last 800,000 years or so. The rate of increase of CO2 is completely off the charts and so is the rate of increase in temperature, as best as we can reconstruct it. Past climates are just like history: we study history to understand human nature, and to understand government, and society and how they fare in times of crises. Hopefully, if we are wise, we learn from these mistakes. So it is with climate. We look at Earth’s history to teach us how the Earth reacted to various crises—like meteorites killing the dinosaurs or episodes where volcanoes were much more active than today—and things of that nature. From that we learn some things about the way our planet works, and that’s how we use the past to tell us something about the present and also the future.
Is there an episode in history that we can learn from so that we can know what we have in store?
There are a number of ways to address that. One of the things that people do with Earth’s history is look at analogs for the current situation. For example, there is this episode called the Eemian, which was about 125,000 years ago, give or take: that’s the last interglacial period, or the last time the Earth was in a non-glaciated state. We know that, globally speaking, the sea level was a few meters higher than it is today. It gives us essentially a kind of benchmark to look at past variations in sea level and from that we can understand what is going to happen to our cities and coastlines as we head into this new uncharted territory of global warming.
There are other ways to look at this, which I’m very interested in, and that is to look at the response of past societies to climate variations. One of the reasons why I like the so-called Common Era, the last 2000 years of history, is because there also happens to be a lot of shifts in human history that happened at that time. There are a lot of societies, for example, that thrived and then collapsed in large part, we think, because of climate variations. A famous example is the Vikings who went to Greenland around 960 AD, so at the end of the past millennium, they set up some colonies there and thrived for a few hundred years and then their colonies collapsed. We think they just happened to colonize Greenland at a time where it could be called Greenland because it was relatively green: it was a relatively warm time in that region of the world during those centuries, then it cooled off. There are other reasons definitely why their colonies collapsed, but that was probably the straw that broke the camel’s back.
There are other examples closer to us, the Anasazi, for example, who occupied a lot of territory in the four corner region in the United States. They thrived for about 600 years, built the greatest pre-Columbian buildings in North America, and then they collapsed abruptly sometime in the 12th century after about six centuries of being around. They were obviously adapted to an arid environment—they had figured out ways to grow food, they had figured out ways to do irrigation, even to store grain for times of famine, and yet they were undone by what we think is a mega-drought—a drought very similar in intensity to the one we have today, but persisting for several decades on end.
Those types of histories give us a lot of lessons about our own societies, and they give us a mirror to look at how we deal with environmental change, and the answer is not very well, even today. They give us a warning of how vulnerable we are to climate variations. There’s this idea that technology will solve all our problems, but the lesson from the Twitter revolutions of the Arab spring, which largely stemmed from a rise in food prices triggered by a drought, is not cause for comfort. Social media has been weaponized by ISIS and Russia, fake news is rampant, and our increased interconnectedness turns out to lead to more instability, not less. Taking the long view, I am confident that we can solve our environmental problems, but only if we don’t get distracted.
Dr. Julien Emile-Geary is associate professor of Earth Sciences at USC
Image: New York Times