Persuading a Climate Skeptic

Dilbert cartoonist Scott Adams is at it again. Fresh from congratulating himself for predicting Donald Trump's victory, Adams is applying his "persuasion filter" to climate science. His premise is simple.

I can’t tell which argument is right. I’m not smart enough to evaluate this sort of topic. But if we are looking at the persuasion dimension alone, one of these is far stronger persuasion than the other.

Adams then provides two links—one from climate scientists explaining the challenges in building models, and one from a denier questioning the validity of these models. If, like Adams, you're more interested in the emotional appeal of the arguments, of course the denier will sound more persuasive. Not being bound by facts, deniers have more freedom to phrase their arguments in a way that will appeal to their audience.

And regardless of persuasion techniques, the best case for the reality of anthropogenic global warming (AGW) is not based on models but on measurements. We see an actual increase in average global temperatures as measured by land-based stations. (Satellite measurements of the stratosphere actually show a slight cooling. This may be a major reason many of the AGW skeptics—e.g., Lindzen, Christy, Spencer—happen to be scientists who work with satellite data.)

While land-based measurements show an increase in overall average temperature, we also have measurements of solar output, which currently is decreasing and appears to be headed in the next 15 years or so toward its lowest level in centuries. So the increasing land temperatures are not due to more heat entering the atmosphere from outside—which would partly explain why the stratosphere isn't warming.

Since the 1970s, increasingly precise measurements of the earth's axial tilt and orbital eccentricity have given us a good understanding of the Milankovitch cycles, which show the earth is receiving a smaller percentage of solar irradiation. (That is, a decreaseing percentage of the already decreasing output is even being absorbed by the earth.)

And then there's measurements of albedo, the amount of sunlight bounced back off the earth or its clouds into space. These measurements have been taken by satellites for 40 years, but so far show no signs of significant change one direction or the other. What we've seen is that clouds move around but the overall global albedo hasn't changed.

So that's three natural forcings, two of which show a cooling effect and one which shows stasis.

On the other hand, measurements around the world show increased CO₂ in both the atmosphere and the oceans. And it's not just any carbon. There are several isotopes of carbon. ¹²C is the most common, with ¹³C and ¹⁴C showing up in small but measurable amounts. Plants are more likely to absorb ¹²C, not just because it is more abundant, but because it is lighter and easier for them to absorb. So carbon found in plants—and in animals that eat plants (and animals that eat plant-eaters)—has an even higher ratio of ¹²C than the atmosphere in general. And when these plants (and animals) were buried and fossilized, the carbon inside them kept that higher ratio of ¹²C.

Atmospheric CO₂ measurements show an increase in ¹²C relative to the other isotopes, which indicates that the carbon being added to the atmosphere is derived from those fossils.

We also have rough measurements of the levels of CO₂ actually pumped into the air through the burning of fossil fuels. As the global rate of fossil fuel burning has increased, the total atmospheric CO₂ has begun to climb faster. So we have two lines of independent evidence the increase in atmospheric CO₂ levels is due to fossil fuels. And that's just the beginning.

When scientists study one factor, they're going to see different things depending on whether they are measuring natural forcings or human activity. Scientists studying the earth's natural climate cycles will see us in a cooling trend, but scientists studying human contributions will see the warming effects of fossil fuels. And that is probably the best argument for why the current 40+ year warming trend is caused by human activity.

But it's not likely to persuade those who simply don't want to know.