Does global warming mean that temperatures will always rise, year after year, steadily and without ever dipping? No, of course not. There's always variation. Despite what George Will might think, just because 1998 was the hottest year on record so far, and global average temperatures have flat-lined since, that doesn't mean there's no long-term warming trend. There is. The greenhouse effect is well-established, and climatologists can say with a good degree of confidence that a doubling of carbon in the atmosphere will eventually force a roughly 3C (or 5.4F) global average temperature increase. But that still leaves room for short-term fluctuations.
You'd think this would be obvious, but it's not, which is why David Easterling of the National Climactic Data Center and Michael Wehner of Lawrence Berkeley Lab just wrote a soon-to-be-published paper for Geophysical Research Letters showing that we should expect to see periods—lasting a decade or more—of "cooling" even as temperatures soar upward over the long run. Here's one possible climate simulation for a "business as usual" increase in emissions, where global average temperatures rise 4C (or 7F) by 2100, yet you still have whole decades where surface air temperatures plateau or even drop:
Notice how, in the simulation, temperatures level off between 2016 and 2031, no doubt sending an octogenarian George Will into a furor. Yet the century-long trend is perfectly clear. Last year, Gavin Schmidt of NASA's Goddard Institute explained why global temperatures don't always rise monotonically, due to natural variability. (Average surface air temperatures can dip in certain years, for instance, if more of the Earth's heat is temporarily absorbed by the oceans, as happens during La Ni