Feedback

There's a new report on Arctic temperatures that is not only worrisome, but helps make clear one of the most challenging aspects of the climate change story, specifically the role of feedback.

For example, pseudoskeptics whose primary source of information on climate is Fox News, are forever pointing out, as if it had never occurred to climatologists, that carbon dioxide levels historically follow rising temperatures, instead of the other way around. To the less informed, this pokes a big hole in the whole global warming story.

The thing to know is, the initial temperature rise was quite slight. To explain the dramatic change in temperatures we see in the ice-core records every 100,000 years you have to invoke the opposite causal chain: rising CO2 levels that trigger further warming. The two are intimately linked. You tinker with one, you set off a positive feedback loop. It's not important which comes first. What's important is where things are headed once that the feedback is in operation. It's the same with what's going on in the Arctic right now.

Here's a graph of surface air temperatures between 60° and 90° N latitude, with 0 being the 1961-90 mean:

It's from NOAA's new Arctic Report Card 2008, which reports that "Autumn temperatures are at a record 5° C above normal." That 9° F. And 2007 was the warmest year on record up there. This is due to the incredible shrinking polar ice sheet, which shrank to the smallest area on record last year and came close to doing the same this year.

The disappearing ice can be blamed on complicated ocean currents and pressure systems, which in turn can be linked to global climate change. So it's not the warm air in the Arctic as such that led to the melting ice. But now that there's so much less summer ice, it's getting crazy warm in hyperborean latitudes, and eventually all that heat will begin to play a serious role in further summer melts, and not just on the Arctic Ocean. Greenland's ice cap is also vulnerable, with sea levels rising as a result.

Lose enough ice, push the air temperatures high enough, and eventually we'll head toward a tipping point, beyond which it will be extremely unlikely that things will return to normal. At least, not for a few thousand years. Just as there's a level of atmospheric CO2 beyond which we really don't want to go, so there is an area of ice cover below which we don't want to fall.

The next time anyone tells you that climate change is bunk because temperatures don't rise because of rising CO2 levels, tell them to read up on the concept of climate feedback first. (You could also ask them if they understand the difference between a forcing and a feedback, but they'd probably think you were being an arrogant boor.)