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.
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