To be honest, I've never been terribly interested in the long-running pseudo-debate over whether global warming "stopped" in 1998. If you don't know what I'm talking about, here's the dime version: 1998 was an exceptionally hot El Niño year, we're all agreed. Global temperatures have still been trending upward over the past decade, and the ten warmest years on record have all occurred since 1997, but it's quite true that there hasn't been a hotter year than 1998 yet. (OK, actually, if you use NASA's data, 2005 was the hottest year on record, but never mind.) But honestly, who cares? Here's the long view in two colorful graphs:

The normal thing to do would be to gawk at those charts, understand that 1998 was an outlier year, and conclude that global average temperatures are on the upswing. But occasionally you get conservatives like George Will who write columns implying that global warming has somehow ceased since 1998 and therefore it's not a problem. Kevin Drum deals well with this surprisingly persistent brand of innumeracy.

Anyway, denier babble aside, there is a rather interesting question as to why 1998 was so exceptionally hot, and why temperatures haven't yet returned to that peak. For that, we'll have to take leave of The Washington Post's op-ed page and consult real scientists. There's a new study (sub-only) this week in Geophysical Research Letters, by Judith Lean of the U.S. Naval Research Laboratory and David Rind of NASA’s Goddard Institute, that tries to nail down precisely why temperatures have stabilized, more or less, since 1998. The answer, alas, isn't very heartening. Here's the Guardian's write-up:

Their work is the first to assess the combined impact on global temperature of four factors: human influences such as CO2 and aerosol emissions; heating from the sun; volcanic activity and the El Niño southern oscillation, the phenomenon by which the Pacific Ocean flips between warmer and cooler states every few years.

The analysis shows the relative stability in global temperatures in the last seven years is explained primarily by the decline in incoming sunlight associated with the downward phase of the 11-year solar cycle, together with a lack of strong El Niño events. These trends have masked the warming caused by CO2 and other greenhouse gases.

As solar activity picks up again in the coming years, the research suggests, temperatures will shoot up at 150% of the rate predicted by the UN's Intergovernmental Panel on Climate Change. Lean and Rind's research also sheds light on the extreme average temperature in 1998. The paper confirms that the temperature spike that year was caused primarily by a very strong El Niño episode. A future episode could be expected to create a spike of equivalent magnitude on top of an even higher baseline, thus shattering the 1998 record.

Makes sense: Even though carbon-dioxide is steadily forcing temperatures upward, those effects can be temporarily slowed by weather events and solar activity—but only temporarily. The authors expect global temperatures to rise over the next five years by 0.15 ±0.03 °C, barring anything unforeseen like an El Niño warm spell, and then keep increasing thereafter, as the IPCC forecasts. Umm... except that's not good, because climatologists just declared that a new El Niño is indeed kicking off in the second half of 2009. Explains why, back in January, NASA announced that "it still seems likely that a new global temperature record will be set within the next 1-2 years."

If those forecasts pan out, the only bright spot will be George Will having to dredge up a new and no doubt amusing talking point. But that's not really much consolation, since the bad news... well, the bad news has been pretty well spelled out by now. If you want reasons to fret about El Niño, do read up on coral bleaching, which promises to be particularly nasty this go-round.