Permafrost

"It's very difficult to look at trends in air temperature, because it's so variable," Romanovsky explained after we were back in the truck, bouncing along toward Deadhorse. It turned out that he had brought the Tostitos to stave off not hunger but fatigue--the crunching, he said, kept him awake--and by now the enormous bag was more than half empty. "So one year you have around Fairbanks a mean annual temperature of zero"--thirty-two degrees Fahrenheit--"and you say, 'Oh yeah, it's warming,' and other years you have mean annual temperature of minus six"--twenty-one degrees Fahrenheit--"and everybody says, "Where? Where is your global warming?' In the air temperature, the signal is very small compared to noise. What permafrost does is it works as low-pass filter. That's why we can see trends much easier in permafrost temperatures than we can see them in atmosphere." In most parts of Alaska, the permafrost has warmed by three degrees since the early 1980s. In some parts of the state, it has warmed by nearly six degrees.

--Elizabeth Kolbert, Field Notes from a Catastrophe