So how do “they” know that the climate change we’re seeing is caused by people?

There is no big “aha” smoking gun here, which is why the Nobel for the IPCC’s scientists is so appropriate. Our understanding is based on a long and laborious accumulation of little bits of science. Let me cite a single example, just out in Geophysical Research Letters. It’s “Contributions of natural and anthropogenic forcing to changes in temperature extremes over the United States,” by Gerald Meehl and colleagues. They looked at the record of temperature extremes – things like length of growing season, which is a function of time between last frost of spring and first frost of fall. It’s a record that shows clear changes. But how to explain those changes? We know there are various things that can “force” climate – solar variability or volcanoes, for example, or changes in trace gases in the atmosphere emitted by humans. Meehl and his colleagues looked at the possibility that known natural forcings all by themselves could explain what we’ve seen. Then they looked at the anthropogenic forcings. The only reasonable explanation, according to their data, is greenhouse gases emitted by humans:

The natural forcings experiments (including only solar and volcanoes) show little change in these extremes indices for the latter part of the 20th century. This indicates that the recent observed changes in temperature extremes over the U.S. have been mostly due to changes in anthropogenic forcings associated with increases of GHGs.

What the New York Times’ Andrew Revkin calls “the tyranny of the news peg” means there will likely be no big headlines for the new Meehl et al. paper. It’s one tiny piece of the puzzle. But it serves as a nice example of the enormous body of work out there for which the IPCC has been honored.