The Global Warming Debate

DC The earth’s atmosphere creates a natural greenhouse effect. How is this connected to the threat of global warming?

SB The worry is that carbon dioxide and other greenhouse gases produced by human activity enhance the natural greenhouse effect. The idea is that if more carbon dioxide accumulates in the atmosphere, then the average temperature of the earth should rise. The argument is that carbon dioxide acts to hold some of the heat that would otherwise escape to space; because the system is holding a little more heat, the temperature of the earth should increase. The thinking on this goes back about 100 years.

DC Is there any question that humans have contributed to the carbon dioxide increase?

SB There is no question in my mind. We have measurements showing that the amount of carbon dioxide in the air has been increasing over the last several decades because we have been adding it faster than the ordinary, natural sinks [dissipaters] can pull it out. But along with this, the environment has responded in ways not often reported. For instance, measurements of tree growth have shown increases that I and others believe are due in part to carbon dioxide increase in the air; photosynthesis may be pulling it out very quickly, but perhaps not quickly enough on a worldwide scale.

“The environment has responded to increased carbon dioxide in ways not often reported.”

DC The data indicate that humans contribute 5.5 gigatons of carbon to the atmosphere each year through carbon dioxide emissions. This is certainly a large number when viewed in isolation, but the atmosphere itself carries 750 gigatons of carbon, so what we are contributing as a percentage of the whole would seem to be minuscule.

SB Right, it’s small, but the question is, is it large enough to make a difference in the climate system? The key relationship for the climate system is that the measured amount of carbon dioxide increase should have added a bit of energy. The question is, does the climate system respond by raising the temperature, or are there other processes that can shift or move that energy to other places, from which it is then shed back to space? That’s the key question, and that’s why computer simulations are run to ask about the impact of adding a small bit of energy. It’s not the carbon dioxide per se but the energy that’s retained. Climate simulations try to follow that energy.

DC So the computer simulations are mathematical models that take as many factors as possible into account and make a prediction about the future climate?

SB Exactly. We know there are many millions of parameters that are relevant to the climate system; we know that they have many interactions among each other. We also know that we don’t have good measurements of many of those parameters; we know we don’t have good ideas on the interactions of those parameters. Thus we don’t know how good the predictions are.

DC Is the validity of the models checked by whether they are actually predicting reality, like how well a weather model predicts what will happen tomorrow?

SB Yes, the term is “skill.” What is the model’s skill in predicting, say, recent past climate? What is the skill in predicting ice ages, the major shifts in and out of the periods of glaciation that the earth experiences on a 100,000-year time scale? What is the skill in predicting changes in sea ice at high latitudes, because that is a very critical property in the climate model. Any major phenomenon in the climate system at this point, however, is not well skilled by the climate models.

DC Because there are so many parameters, so many factors?

SB So many parameters and interactions, yes. And even though there are so many that we don’t have a computer big enough to hold them all for computation, the bigger problem is that we know we don’t understand many of those interactions. For example, we all heard in 1997–98 about a phenomenon in the Pacific Ocean called El Niño, which is a warming of the Pacific Ocean that happens every several years and that in turn warms the air. During the ’97–98 El Niño the air temperature rose quite dramatically—about 0.5 degrees Celsius on a global average. That lasted for a few months and then dropped dramatically back down as the ocean currents and air currents shifted. What drives this El Niño? Can any of the big general circulation models reproduce an El Niño? The answer is no.

DC The sun’s changing output of energy is also a variable. What are you finding with your research on the sun’s variability?

SB I got into this research by thinking I could pinpoint how the sun changes, and that those changes might have an impact on climate on earth because we see change on earth that is pegged to the particular changes that the sun makes. My idea was to subtract the changes that the sun might make so as to get a better estimate of the human effect on temperature. We’re finding that the surface temperature seems to follow the changes in the solar output extremely well: the surface temperature changes, especially early in the 20^th century. In other words, changes that occurred before most of the greenhouse gases from human activity were put into the air seem to link extremely well to the brightening of the sun that occurred early in the 20^th century.

DC Have you been able to test your hypothesis against the historical record—test its “skill,” so to speak, in relation to the “little ice age,” for instance?

SB Yes, we have a sketch of what the climate of the earth has been doing over the past thousand years, and we also have fairly good measurements of what the sun has been doing.

DC Where are those climate and solar records found?

SB Sun information affects isotopes both in trees (through carbon 14 in tree growth) and in ice cores (beryllium 10). We don’t have quantitative records of the global average temperature for climate going back a thousand years or so, however. We have data that show environmental changes—they’re called “proxy data”—but not actual, accurate global temperatures. We do have, area by area, evidence of regional changes in climate, and we can ask, Was it warmer a thousand years ago? Was it cooler? What was the trend of temperatures? But we can’t know what the precise numerical temperature was.

DC What story do these records tell?

SB Between about A.D. 800 to 1200, a period of warmth of 50 years or more was seen over most of the world where the proxy data exist; in some places the period of warmth was warmer than in those same locations today. There followed in many of those localities a period of cold for at least 50 years sometime between A.D. 1400 and about 1800 or 1900. The warm and cold periods did not occur everywhere simultaneously but within those broader periods. Then another warming trend followed, starting early in the 20^th century. All of that change predates the major buildup of greenhouse gases in the air. Now, interestingly the sun’s output, determined independently by records from tree rings and polar ice cores, also follows the broad sweep of those environmental data. That is, the sun seems to have been active a thousand years ago, and then decayed into a period of relative inactivity, culminating in the 17^th century with a period of very low activity, and that was the coldest century of the last thousand years. The sun became active again in the 20^th century.

DC If there are all these variables interacting over long periods of time, what can we really know about global warming?

SB We know that the climate models, the computer simulations of global climate, can’t be used to make reliable predictions. We know the 20^th century is warmer than the past few centuries of the little ice age, the unusual cold period. There has been global warming! We know that 20^th-century warming began early in the century before most of the human-made greenhouse gases had entered the air. We know the ecosystem has responded to that warmth, as would be expected no matter the cause of the warming trend. But looking at warmth and concluding it’s man-made is a logical error. Some portion of the recent global warming trend may be human-caused, but the trend is more modest than first thought. We know that.

DC But everybody doesn’t know that. What the average citizen reads implies that scientists really do understand how it all works. How does one come to the reality of what scientists can really tell you and what is unknown about human influence on climate?

SB We’re all hypersensitized to the conclusion that all the recent warming trend is man-made. It’s possible, but not scientifically accurate. This means policymakers may have to make a decision based on something other than science. That’s okay. My passion is for people to understand what it is the science says.

DC The big jump, then, is to say that warmth is caused by man. Is that the problem?

SB Yes, there’s warmth, but, more important, what is the cause of it? There’s another piece that’s critically important in the model predictions. All computer models say that when the greenhouse gases are added to the air, the low layer of air from the surface to 5 miles up warms; it must warm. The warmth from that layer in turn warms the surface temperature. The logic, then, is quite clear. To brand warming in the 20^th century as human-caused, there must be a warming in the low layer of air up to 5 miles and at the surface. Both levels must warm. You can’t discount that lower troposphere, that low layer of air up to 5 miles, and it has been measured.

DC What are the lower atmosphere data telling you?

SB NASA satellites have been measuring it since 1979, and the warming trend from human greenhouse gases should be much steeper in the troposphere than it is at the surface; we know the surface temperature has warmed, but has this low layer of air warmed at least as much, if not more? The answer is no. That layer of air has warmed, but much less than the computer forecasts say. Now the argument—and it’s been a fierce argument—has focused on there perhaps being something wrong with that record of satellite measurements. But independent, reliable sets of measurements have been made by balloons that carry instruments directly up into that layer. They confirm that this layer of air has warmed less than expected. Thus the human-made signal cannot be as large as forecast.

DC This is not the logic that is usually reported.

SB It is an argument not seen much in public print. I often see talk about the surface temperature, but one can add the bounty of information about this key layer of air that must warm. Once added, the air measurements suggest that the earth, the climate system, must then be shedding much extra energy from carbon dioxide increasing in the air back to space. Otherwise this key layer of air would warm.

DC There must be other feedback systems at work here.

SB Yes. I think the argument would be—given something like an El Niño—that if the earth didn’t have in place feedback systems, then the climate system would have gone out of bounds many times long in the past. It hasn’t, so it has some self-adjusting mechanisms; we just aren’t clever enough yet to have uncovered the physics of them.

DC Why is it important to determine cause?

SB Determining the cause helps determine what trajectory you want to take on making a decision. How quickly do we need to make a decision? How dramatic does this have to be? Is there time to collect more information? Is there time to make a decision that’s economically better?

DC But many advocates say we need to make the decisions now because if we wait until we really know, until all of the discrepancies and details are worked out, it’ll be too late.

SB I disagree with that for two reasons. First of all, even if we take the computer simulations that project large temperature change in the next 100 years and ask whether waiting a decade, or two decades, or even three decades before instituting sharp cuts in greenhouse emissions will make any difference in the predicted change, the models say that the temperature rises anyway. In other words, there’s no significant difference by waiting 10, 20, 30 years; the extra warmth 100 years ahead would be little. Now, from the economics point of view, experts tell me that waiting to make the dramatic cuts will be economically better because you make them late when you can afford them, and you make them dramatically then. Cuts are more effectively and easily made later because technology progresses to the point where one relies less on fossil fuels, so you can make the cuts more cheaply later on than you can make them now.

DC So the idea of dealing with fossil fuels and carbon dioxide and cutting our use of those today has no environmental basis—no global warming basis?

SB That’s right, on the basis of the carbon dioxide and greenhouse gases per se. With regard to other emissions, there are matters to be concerned about and ways to control pollutants.

DC How is what you are saying related to the Intergovernmental Panel on Climate Change (IPCC) report and its use as the basis of the call for the reduction of greenhouse gas emissions?

SB I recommend the science report as a good snapshot of what the state of science is, and the chapters detail elaborate uncertainties. The National Academy of Sciences report ratified that there are many uncertainties in these models. I don’t agree, however, with how the policymakers summarized what was in the science report, in terms of uncertainties.

DC You are a signer of the Oregon Petition. How does it relate to the IPCC, and what does it say?

SB It’s a privately funded petition—no foundation, no industry, and no government funding—asking scientists to weigh in on the matter, because we’ve read over and over again that all scientists agree that global warming is human-caused, except for a few detractors. So the Oregon Petition tried to get a sense of the thinking of scientists and engineers in the U.S. I think that to date over 17,000 American scientists and engineers have signed it. It says that there is no evidence for catastrophic human-made warming, and I think that probably reflects the state of climate science. We have no reliable evidence that the buildup of carbon dioxide in the air has caused (even though the climate models say it should have caused) substantial warming, and therefore the extrapolations into the future have to be discounted down. There is no evidence for the catastrophes that fill newspaper headlines.