The State of the Climate—and of Climate Science

Four scientists discuss where the climate is and where it's going.

Jun 30, 2009 5:00 AMNov 12, 2019 4:20 AM
Robin Bell, Ken Caldera, Bill Easterling, and Stephen Schneider. | Timothy Archibald


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In the list of world challenges, global warming might be at once the most alarming and the most controversial. According to some predictions, climate change caused by human activity could cause mass extinction in the oceans, redraw the planet’s coastlines, and ravage world food supplies. At the same time, a significant portion of the American public questions whether global warming will really cause any major harm; many still doubt that human-driven warming is happening at all. How can we settle the debate? And can we intervene in the process or find ways to adapt to the new conditions? In conjunction with the National Science Foundation and the San Francisco Exploratorium, DISCOVER brought together four experts to discuss the reality and meaning of climate change. In a highly nuanced exchange of ideas, these researchers weighed the various scenarios and laid out a road map for navigating the warmer world to come. The conversation was moderated by DISCOVER’s editor in chief, Corey S. Powell.

POWELL: One question I hear all the time is whether the current change in climate is truly extraordinary. Even if humans are contributing to global warming, isn’t this just like the natural variations that have happened many times in the past?

Robin Bell:

A little background first. I spend a lot of time studying the ice sheets at the bottom of the planet—how they form and how they collapse. The poles are like the planet’s air conditioner. When things are working well, the poles keep the planet nice and cool and we don’t think about it. When things stop working, the poles can start to melt and there’s a puddle on the floor. Today both poles are getting warmer; in Greenland and Antarctica you can see the surface of the ice dropping, and you can see there’s less mass when you measure the ice from space. The process has been ongoing, but it looks like it’s happening faster than it was. We know the ice sheets have come and gone in the past. Why is this any different? One of the most compelling reasons is that in the past the ice sheets from the two poles didn’t move together—one would lead and the other would follow. This time, both the north and south are spewing ice into the global ocean, accelerating at the same time.

Ken Caldeira: Another indication of how unusual all this is can be seen by looking at ocean chemistry. When we drive our car and carbon dioxide comes out of the tailpipe, within a year it has spread throughout the atmosphere and is integrated with the surface ocean. When carbon dioxide dissolves in seawater, it forms carbonic acid, and in high enough concentrations carbonic acid is corrosive to the shells and skeletons of many marine organisms. To measure the impact, people go out in ships and drill holes in the ocean floor, where shells of marine organisms have settled throughout geologic history. What we see is that if we continue in our current trends in burning fossil fuels, the ocean will become more acidic than it has been at any time in the past 65 million years. The last time the ocean was as acidic as it has the potential to become in the coming decades, we saw a mass extinction event.

POWELL: Yet as you note, the earth got warm in the past, too.

Caldeira: That’s true, but it got warm over millions of years, and ecosystems had a chance to adapt. What we’re seeing are rates of increase in greenhouse gases and warming that exceed natural rates by a factor of 100. So what we’re doing is really unusual when seen from a geologic perspective.

POWELL: Humans are doing in centuries what natural processes do over millions of years?

Caldeira: Yes, and the other timescale mismatch is that what we do over the next decades will affect life on this planet for hundreds of thousands of years, if not millions of years. We are at a critical juncture in earth history. If we don’t do the right thing and there are geologists around 50 million years from now, they’ll be able to look at cores and see the remnants of a civilization that developed advanced technology but didn’t develop the wisdom to use it wisely.

POWELL: What about the impact of global warming on agriculture? As the climate changes, will people have enough to eat?

Bill Easterling: One of the most remarkable achievements of the 20th century was the way we were able to increase the global food supply in pace with unprecedented population growth. We will have to raise the food supply another two times to feed all of the people that we think will be alive by the latter third of the 21st century. We have reason to be somewhat sanguine about doing it if climate stays more or less the same, but how will we do it with the climate change? Based on our simulations and on 25 years of research, what bothers us most is that in the tropics, where the majority of poor people live today, crops are currently raised at temperatures pretty close to their photosynthetic optimums.

POWELL: Meaning that higher temperatures will make it difficult for us to produce the amount of food we need?

Easterling: If you go any higher, yields begin to fall. On the other hand, in the midlatitudes—in the temperate zones where we live and where many of the grain belts of Europe and North America are located—a little bit of warming in some cases is not a bad thing, at least not at first. These are regions where crops are currently cold-limited. In other words, if you warm the temperatures, you actually might get a little bit of additional yield. So you’ve already begun to set up a kind of haves and have-nots, an imbalance where the poorest people in the world who vitally depend on agriculture as a development tool, in addition to providing food security, are now being even further disadvantaged. But there really aren’t many winners in the long run because even if the higher latitudes are given an advantage, they still are faced with moving food across large distances and making sure that it’s done in such a way that the farming systems in the receiving countries are not put out of business because of the inundation of free or subsidized food.

POWELL: Climate change is such a huge issue that people tend to feel paralyzed by it. Stephen, you’ve framed it in a helpful way as a problem of risk management. What does that mean?

Stephen Schneider: I often testify before Congress and talk to the media, and they always ask the same question: Is the science settled enough for us to have policy? Do we know enough to spend money fixing this? But science, and especially system science, is very complicated. Now, in any system that’s complicated there are some components that are well established. In other words, they’re relatively settled. We know that the world is now 0.75 degrees Celsius warmer than it was a century and a half ago. We know that the ice sheets are decreasing. But then there are other components with competing potential outcomes—for instance, will a change of three degrees make crop yields go up or down?

POWELL: So how should we separate out the well-established parts, and how do we evaluate the ones that are not so certain?

Schneider: When you’re covering climate change, you don’t get somebody from a deep ecology group to tell you we’re near the end of the world and then somebody from the Competitive Enterprise Institute who’s going to tell you carbon dioxide is a fertilizer while forgetting about ocean acidification. If you do that, the two lowest-probability outcomes get most of the time in the media and you get this dumbed-down debate. It’s bipolar, and that’s not how system science works. There are multiple potential outcomes. What we do is whittle out the relative likelihood of each of these outcomes so we can make a value judgment about whether or not the risks are adequate to move forward. Risk is what can happen, multiplied by the probability of its happening. That’s what we call an objective or scientific assessment. We try to make the risk aspects clear and then leave the risk management where it properly belongs, which is out among the public and in the political world.

POWELL: In the news, climate change is often described in terms of legislation and treaties. So I was surprised, Ken, when I heard you call it a “hardware problem.” Can you explain that?

Caldeira: Many people have been looking at this as a problem of getting international cooperation and international agreements. We’ve had the Kyoto Protocol and the Copenhagen Round Table coming up. But when China builds a power plant that’s going to spew carbon dioxide into the environment for at least the next 75 years, what’s important is that we build it correctly. To me the risks are so clear. Economists estimate that transforming our economy into having an energy system that does not emit carbon dioxide into the atmosphere might cost 2 percent of our wealth each year. Now, I can go to any group of people and say, “Let’s pretend we already have energy systems based on solar, wind, and other sources that don’t emit greenhouse gases.” Then I say, “You can make 2 percent more money each year, but in return for being 2 percent richer we’re going to have to melt the ice caps and acidify the oceans and shift weather patterns. Now, would you trade all that environmental risk in order to be 2 percent richer?” I’ve asked this of climate skeptics. Even they say, “Well, if we already had this carbon-neutral energy system, I would go with it.” That makes me think it’s not the cost of transforming our energy system—it’s that we don’t have the cooperation we need to start doing the job. If we wait until we have international cooperation, it will be too late. What we need is leadership that will say, “We have to stop building devices that emit carbon dioxide into the atmosphere.”

POWELL: What does “leadership” mean in this context?

Caldeira: We need to say we can’t afford to have gasoline cars that emit carbon dioxide—so can we have an electric car system with swappable batteries? We can’t afford to build a coal-fired power plant with CO2 coming out—so can we develop carbon capture and storage technologies, or should we be looking at solar-thermal? We have to start building the new energy system, and we need the political leadership to say we’re going to start doing this. We need research and development to come up with the technology because it’s not all on the shelf. I think if the United States started doing this in a serious way, Europe would follow. If the United States and Europe did this, I think it would not be long before China and India joined us.

POWELL: Snap poll: Do you consider yourselves fundamentally optimistic or pessimistic about whether we have the technological and political will to fix this problem?

Bell: I’m an optimist. Oh, we are changing the planet. We may have already changed the ice sheets to a point that some parts of them may go, but we have the ability to stop changing it more and to adapt to what we have already done.

Caldeira: I’m also optimistic about our abilities. Unfortunately I’m pessimistic about our wisdom. We have the capability to do amazing things in a short amount of time, but it takes a political decision with follow-through. I’m not confident we’ll get it.

Easterling: I temper my optimism by saying that we’re probably looking at an adaptive challenge. That is, we’re going to have to adapt to a certain amount of warming no matter what, even if we were to bring global emissions of greenhouse gases back to year-2000 levels, and that adaptation would be draconian if we were to do it all at once.

Schneider: The first time I was asked that question in a public place was sometime in the 1970s in front of a congressional committee. My answer was a little bit like Ken’s. I said, “I’m technologically optimistic and politically bleak.” That proved to be a pretty good forecast for the next 35 years. But now I’m getting more optimistic because there’s getting to be some alignment of the stars between Congress and the White House. The tough problem is going to be China, India, Indonesia, Brazil, Mexico, and the countries that are even poorer than that. Can you imagine telling this current Congress that we need half a trillion dollars’ worth of technology transfers to help developing countries go through the transition?

POWELL: That raises a good question: How much money do we need to spend on addressing global warming?

Schneider: I think we’ll need $500 billion to get going. Over time, it will be trillions.

Caldeira: But we’re talking about something that’s small compared with the money we invest in medical care. It’s smaller than what we invest in the military budget. I think we need to start looking at climate change as the kind of threat that we must spend real money on to address.

POWELL: Have you all adjusted your personal lifestyle in response to climate change? What measures are important? For instance, will driving a hybrid car or turning down your thermostat really be meaningful, or is this just a drop in the ocean?

Bell: Personally, I’ve done all the easy things. We’ve changed our lightbulbs [to compact fluorescents]. We got rid of one of our cars. But the work I do is an energy hog. I travel to events like this to talk about the changing climate. I go to Antarctica and I burn a lot of carbon to do what I do. So professionally I’m a pig when it comes to carbon. The really hard thing is airline travel. My husband won’t get on a plane for fun anymore. But he’ll go anywhere on a sailboat.

Easterling: I’m the dean of a college at Penn State, and I am notorious for being in a charcoal gray suit riding my bicycle in three inches of snow. I try to set an example, and I haven’t fallen yet.

Caldeira: I’ve done some things, like I now drive a little scooter instead of a car to work, but I really think that this emphasis on the personal carbon footprint plays into the interests of the people who would like to see our current energy system continue. We won’t solve this problem by telling people not to have toast.

Schneider: This is the hypocrisy question. Yeah, Senator Inhofe loves to ask that one of Al Gore, and I get it all the time too. In fact, I ask it of my freshmen and sophomores: “Is your professor a hypocrite?” I live in a green house with twice the legally required insulation and a heat recovery ventilator, and I drive a hybrid and I bike and I live two miles from work, but then there are those 170,000 miles up there on United Airlines, and that is 90-odd percent of my footprint, just as Robin said.

Audience member: What is the most compelling evidence you have that human behavior is actually warming the planet?

Caldeira: To me the most compelling evidence is the fact that the stratosphere—the upper atmosphere—is cooling while the lower atmosphere and the land surface are warming. That’s a sign that greenhouse gases are trapping energy and keeping that energy close to the surface of the earth. I mentioned that in ocean acidification, you actually see animals that should make shells unable to make shells anymore. You could demonstrate the same kind of effect in a bell jar in the lab. There is a level of certainty about it.

POWELL: What about you, Bill? You’re looking not at climate records but rather at agriculture. Do you see a real break from the past there, indicating a unique signature of global warming?

Easterling: One of the problems with agriculture is that it’s a highly managed ecosystem. So it’s often tricky to try to separate out the climate change signal from what might be a host of other things relating to how we manage crops and livestock. But we have seen an increase in the length of the frost-free season. We have seen changes in the incidences and the life cycles of critical agricultural pests, which can be explained only by a general warming. Of course, this is all circumstantial. What made all this come into sharp focus for me was not what we were observing but what we were able to simulate on a computer. Over the past 10 to 15 years, we have been running experiments with very complex and increasingly reliable global climate models. When we entered into the computer all the various things that forced the climate to change, we were able to faithfully reproduce the temperature record of the past 100 years globally. When you take out the component of human-generated carbon dioxide, the models don’t work at all. There are all these people who say, “Well, what about the sun? Why don’t they think about solar variability?” Of course we think about the sun. The models think about all these things, but the models work only if you put all the components in, and one of the big components is us.

POWELL: How you deal with skeptics, both in Congress and in the public, who always seem to have a contrary statistic?

Schneider: First, with regard to your due diligence as a publisher, why hasn’t DISCOVER published a compelling account of the other side? Because there isn’t any. That’s a pretty good reason. There are a lot of things in that speculative and competing explanations category, but there is no preponderance, and that is what is compelling to me. For example, take the evidence that Robin cited. If you were a cynic and you asked about the probability of the ice sheet in the north going up, it’s 50 percent. Going down? Fifty percent. And the South Pole going up? Fifty percent. Going down? Fifty percent. Probability they are both going together? Twenty-five percent. What’s the probability of the stratosphere cooling while the earth gets warmer? Again, assuming we knew nothing, 50 percent. Troposphere warming? Fifty. The probability that one will go up while the other goes down? Twenty-five percent. Same thing for other patterns, like the way high-latitude continents are warming more than low-latitude ones are. With any single line of evidence, you can say, “Oh, well, there’s still a 25 percent chance it’s random,” but what happens when you put all these events together? The probability of all these events’ lining up the same way is pretty darn low unless we are dealing with global warming.

Caldeira: Climate science has reached the point that plate tectonics reached 30 years ago. It is the basic view of the vast majority of working scientists that human-induced climate change is real. There is a real diversity of informed opinion on how important climate change is going to be to various things that affect humans, and there is a diversity of opinion on how to address this problem, but the debate over human-induced climate change is over.

Audience member: I work in a hard-rock mining industry, and the majority of my colleagues tell me I’m crazy when I talk about climate change. Where are some good sources of information that rationally discuss all of these different naysayers’ theories?

Caldeira: One useful Web site is

Schneider: I have a contrarian section on my Web site, which is We’re about to triple it because we had to deal with those famous climate professors, you know, professors Limbaugh and Crichton. [Laughter] They have a standard technique, doing much the same thing that the American Tobacco Institute did for a long time, which is to cite the three studies that were equivocal and ignore the 33 studies that were definitive. They use the argument that we still do not, to this day, understand the detailed biological connections between smoking and cancer, but the evidence and data are so overwhelming you’d have to be nuts not to act on it—unless you’re in the business.

Caldeira: There was a climate contrarian who testified before the Senate last week. He made the claim that climate scientists were some kind of club and they all made money by somehow supporting each other’s findings. The reality of science is that a scientific career is made by showing that all the people around you believe something that’s not true. If a scientist could provide evidence that the climate theory is incorrect and that global warming is not a product of human activities, he or she would be held up as the Darwin or the Einstein of climate science. We’re highly incentivized to show that all our colleagues are wrong. If we could come up with good evidence that they’re wrong, we would be out there publishing it. The evidence just doesn’t exist.

Easterling: Even science is not value neutral. We make decisions and we have values in our judgments, but at the end of the day we have a code of ethics that says we look at the data and, using our prior knowledge, we make our best judgment. That’s all it is.

Robin Bell A senior research scientist at Columbia University’s Lamont-Doherty Earth Observatory, Bell has coordinated eight major Antarctic expeditions. She studies the mechanisms of ice sheet collapse, the origins of subglacial lakes, and their hidden ecosystems.

Ken Caldera A professor at Stanford and staff member in the department of global ecology at the Carnegie Institution of Washington, Caldeira works at the nexus of climate, the carbon cycle, and energy. He has studied issues such as ocean acidification, intentional intervention in climate systems, mass-extinction events in the earth’s geologic history, and the scale of change needed to address our present carbon-driven climate problems.

Bill Easterling Dean of the College of Earth and Mineral Sciences at Pennsylvania State University, Easterling studies global warming and its potential effects on the world’s food supply. He has served on the National Research Council and the National Science Foundation and was a lead author on the Intergovernmental Panel on Climate Change (IPCC) team.

Stephen Schneider A senior fellow at the Woods Institute for the Environment at Stanford University, Schneider assesses ecological and economic impacts of human-induced climate change to identify potential political and technological solutions. He has been a principal member of the Intergovernmental Panel on Climate Change (IPCC) since 1988. In 2007 he joined four generations of IPCC authors, including Easterling, in receiving a Nobel Prize for their groundbreaking work.

Corey Powell

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