Dr. Ralph Cicerone, chancellor of the University of California, Irvine, chaired a National Academy of Sciences committee that prepared and last month released a report for the White House on global warming. Cicerone founded UCI’s Department of Earth System Sciences, which counts among its faculty Dr. F. Sherwood Rowland, who won a Nobel Prize for discovering ozone depletion and who also served on the NAS committee. Cicerone, Rowland and other UCI scientists often are called on to advise officials, agencies and corporations on environmental science. The release of the NAS report generated headlines worldwide, coming on the heels of President Bush’s rejection of the Kyoto Protocol on controlling greenhouse gas emissions. This is excerpted from a speech by Cicerone to a regional symposium of the American Academy for the Advancement of Science at UCI on June 18.
The subject of global warming is a difficult one and in a way it exemplifies a lot of scientific topics today: increasing complexity, increasing specialization, with some impact on society and the rest of us, and increasing difficulty in explaining to people who are not experts. So it is a real challenge.
Let me tell you some of the questions that we received from the White House, which were intelligent. Some of them sound a little bit old hat, things that probably most of you know. On the other hand, if you come into a new administration without having thought about these things before, it’s understandable why you might ask questions that seem as if we have known the answers for awhile. In many cases, the answers are not as clear as we would like them to be.
“What is the range of natural variability in climate?” You’ll see as I go through these that some of them are more or less straightforward scientific questions and others have a kind of political edge on them. There are people who are willing to agree that climate is changing right now, but they will assert that it is entirely natural. So this question came partly from people who are asking, well is it? Isn’t it completely natural, everything that we’re seeing right now?
The ones about greenhouses were relatively easy to answer. “Are they increasing? What are the rates? Are they due to human activities and so forth? Are the greenhouse gases causing climate change?”
There was one that the committee and I felt that we really could not answer: “Has science determined whether there’s a safe level of concentration of greenhouse gases?”
We visited the White House 10 days ago and gave them our early view of this report. It turned out that the reason they asked this question was in 1992 there was an international meeting in Rio de Janeiro where the former President Bush, signed on to a framework for future agreements about climate change, and one of the goals was to prevent the accumulation of greenhouse gases from ever reaching what would be considered a dangerous level. So this question was basically, well, what’s a dangerous level? What’s a safe level?
Some Answers
As I say, most of these questions are very intelligent. We tried to give the best, most direct answers we could on all questions, starting with the simplest one and then showing how as we get into the more and more interesting aspects, how many assumptions are involved.
A little bit of background,the Earth sitting in space receives whatever energy it has to work with from the sun, mostly in the form of visible light. We receive at the outer edges of the Earth’s atmosphere 342 watts per square meter. This figure of 342 watts per square meter is an average over the course of the year from the sun, and about one-third of that power is directly reflected back to space by the white tops of the clouds, by snow and ice on the surface of the Earth. The remaining 237 watts per square meter are absorbed by either the dark surfaces of the Earth or the blue-green waters of the ocean or soot in the atmosphere.
From the numbers, we can calculate how warm the Earth should be. You can solve the equation because everything is known, and you end up calculating brilliantly that the Earth is at minus 18 degrees Centigrade, which is sub-freezing. I can do the same calculation for Mars and get the answer dead on, but for Earth and Venus it doesn’t work. This is indirect evidence that there’s something here called the greenhouse effect.
Now, there are present in the atmosphere a number of gases that we’ve come to call greenhouse gases. Most of them are natural, but the issue that is in front of us now is that on top of the natural backgrounds of these gases we’ve added more of them due to human activities.
What the data shows is an increase in carbon dioxide, the main greenhouse gas, starting from about 330 parts per million in the early ’70s to about 370 parts per million now. If you go back to 1957, it was about 310 parts per million.
I’m not going to present to you today all of the evidence that the increases are due to human activities, but there is very compelling evidence that that is the case. One kind of evidence comes from ice cores. These ice-core data actually go back now nearly 400,000 years. The carbon dioxide concentrations we have now have never occurred before. In previous ice ages, the amount of carbon dioxide was about 180 parts per million. Whenever the world has warmed up after an ice age, carbon dioxide has gone up to about 280 parts per million.
Then there are temperatures measured in the last 120 years all around the Earth from ships and land weather stations, by good thermometers. In the last perhaps 60 or 70 years, there’s been evidence of a warming at the surface of the Earth. But what you also see is that it was not uniform. From 1940 to ’75, in fact, the planet cooled down a little bit, especially in the Northern Hemisphere, but then starting around 1980, we’ve had a rapid warming.
So what do you make of this? It’s real. Everyone believes it is real. These temperatures are the best data that we have. But how can we connect them or not connect them with human activity? Well, in our report, what we said is the weight of evidence has swung towards the belief that this effect over the last 20 years is due to human activities for several reasons. First of all, it matches the rise in greenhouse gases, the rapid rise in the last 20 or 30 years.
But Earth is known to change. Climate change is always occurring. We can’t really be sure. However, let’s look at the possible explanations.
One is that there’s some natural oscillation of the climate system that we don’t understand. There are always things that we don’t understand. It’s not very satisfying, but it’s a real possibility.
One specific suggestion is that the sun has varied with time in the past 100 or 200 years. But this is a 20-year period and it is the only 20-year period when humans have measured the sun carefully enough to be able to address this question. When astronomers put the data together, matching up the different instruments on Earth and satellites, you do not see evidence of an increase of the sun’s output. Neither do you see any evidence that the sun is getting colder.
I can go back to my ifs ands and buts once again, but taking the minimum number of assumptions, the scientific evidence has swung very sharply toward the fact that human-generated greenhouse gases are what’s causing the warming over the last 20 years.
So we mentioned all of the uncertainties, but we also have tried to be as useful as we could, to be as clear as we could about minimum things that we know. We said some powerful things in the first several sentences of our report.
“Greenhouse gases are accumulating in the Earth’s atmosphere as a result of human activities, causing surface air temperatures and sub-surface ocean temperatures to rise.”
There are no weasel words there. That’s pretty strong. Go to the next sentence and you’ll start to see where we build in more uncertainty.
“The changes observed over the last several decades are likely mostly due to human activities. But we cannot rule out that some significant part of these changes are also a reflection of natural variability. Human-induced warming and associated sea-level rises are expected to continue throughout the 21st century.”
We are very sure that the planet is going to continue to warm for the next 50 years no matter what we do, because there’s already increased warming built into the system, what we call unrealized heating. The planet is not completely in an energy balance yet, and for the next 40 or 50 years, even if we were to freeze the atmosphere’s chemical composition and not allow any more changes in greenhouse gases, the planet will continue to display higher temperatures. On top of that, we have new warming that will occur due to any further increase in the greenhouse gases.
Predicting the future is difficult. The problem is, it’s not only the climate models that are in question, but human activities themselves. What are we going to do about energy, with agricultural sources of methane, with future chemicals that are unanticipated, with sulfate particles? The range of uncertainty is large. We could have a warming by 2050 that is maybe 1.25 degrees or it could go all the way out to 2.5 degrees Centigrade, about 4 degrees Fahrenheit.
The Hard Questions
This gets us to policy questions. Developing countries want to have a higher standard of living, even as their populations grow. The industrialized countries have changed the atmosphere so far. We are responsible for nearly all of the changes that have happened so far, but in the future, more than 1 billion people in India will start consuming more energy, and 1.25 billion in China will obtain a higher standard of living, more cars, larger homes and so forth. So how do you go about dealing with this? Who should take the brunt of the action right now, whether or not there’s pain involved? Who should take the first responsibility or should everybody share equally? Should it be on a per capita basis, per dollar basis or what? And who’s going to answer these questions?
But the way these agreements happen will be very, very human. People will get together. They’ll have all-night sessions. They’ll bargain. They’ll philosophize. I don’t know how this is going to go. You can tell that our White House is in some disarray in responding to the situation. They didn’t understand how strong the world reaction was going to be when President Bush announced that the United States was not going to adhere to the Kyoto Protocol, which was a way to get started. Now that we’ve seen that reaction, I think they’re reconsidering their options and this is why they asked the National Academy of Sciences at least for an assessment on the scientific side of it.
