William M. Briggs, a statistician, presents (from 2009) “A Citizen’s Guide to Global Warming Evidence”:
Why are you so scared about global warming—er, excuse me—climate change? Why do you believe things are as bad as they are? Why is it you feel that the world is coming to an end unless we do something? [...]
The Earth’s climate has never been static. It has always changed. And nobody—not a soul—knows what an ideal climate is.
AGW is not the only theory of climate change. There are many rival theories, but you have never heard of them. One—or even none—of them might also be true and could be useful in predicting future climates.
The accuracy of historical temperatures is questionable. We do not have direct measurements for most of the Earth’s history, and have to rely on statistics—God help us!—to impute the missing records. This process is fraught with error and uncertainty. Anyway…
Historical temperature changes are not direct evidence of AGW. Because it was cooler, or hotter, in the past is not direct evidence that AGW is true. Any historical temperature observation is consistent with all known rival climate change theories. Thus, past temperatures are, at best, indirect evidence for many different climate change theories, and not just AGW.
Statements of what happens when it is hot outside are not evidence that AGW is true. If you heard that an iceberg melted when it was exposed to hot air, you have learned what you already knew: ice melts when it is hot. Absolutely no observation of any plant, mineral, or animal is direct evidence of AGW. Thus, every horror story you have heard about small fish whose native waters got uncomfortably warm, about a species of grass that was stressed under the harsh sun, or that a small town in Argentina set a record high temperature on Tuesday, or another in Pittsburgh was especially wet one afternoon, and on and on, are not direct evidence that AGW is true. They are only statements of what happens when it gets hot out or when it rains or fails to.
Every statement about what might happen if AGW is true is worthless. Horror stories about the evil, wretched future that awaits us once the “tipping point” has been breached are useless as evidence for AGW. They are empty of any kind of proof. “Studies” that claim future awfulness due to AGW are inappropriately and disingenuously used by scientists (and other forms of life) to hint that AGW is true. This is naughty of them. This behavior is equivalent to the Tokyo scientist who solicits his government for a Godzilla-studies grant because of the havoc the old nuclear fire-breather could cause if he were real. This grant is not evidence of Godzilla’s existence.
The best indirect evidence for AGW is the fit of climate models to historical data. AGW climate models can reproduce some of the historical data in some regions fairly well, but they cannot do so in all times or areas. And many of those rival climate change theories fit the historical data equally well. Thus, the ability to reproduce historical data to an arbitrary level of goodness is not especially strong evidence in favor of AGW.
There does not exist direct evidence for the truth of AGW. The only possible direct evidence would be if the AGW models skillfully predicted future climate data. These skillful predictions would tell us that the theory underlying the models is likely to be true. But no AGW climate models have yet skillfully predicted new data. However, some rival climate change theories have. Thus, according to the best direct evidence available, it is more likely that these rival theories are true than is AGW theory.