2008 cooler than last year? Yes. Globe still warming? Yes.

NASA's Goddard Institute for Space Studies has released its preliminary temperature plot for 2008 (December's not over yet). As the chart (above) shows, overall temps in the past almost-a-year took a dip, with 2008 the coolest year of the past eight.

Time to fire up the Hummer? Not just yet. See how the lines jig and jag while moving in an overall upward direction? This indicates that while there's been a pretty steady cycle of up-and-down variability in comparative temperatures from year to year, unfortunately there's been no stop in the planet's overall warming trend.

Time will tell, of course...but according to GISS, 2008 was the ninth warmest year since record-keeping began in 1880, and the nine warmest years on record have all occured between 1998 and now.

This year's cooling is ascribed to a strong La Niña in the Pacific -- a known natural oscillation of tropical temperatures, with El Niño years being the opposite phase of the cycle.

Gavin Schmidt of the Goddard Institute explains the oscillation -- and why not to read too much into a cooler 2008 -- on Real Climate,

So what to make of the latest year's data? First off, we expect that there will be oscillations in the global mean temperature. No climate model has ever shown a year-on-year increase in temperatures because of the currently expected amount of global warming.

A big factor in those oscillations is ENSO [El Niño - Southern Oscillation] - whether there is a a warm El Niño event, or a cool La Niña event makes an appreciable difference in the global mean anomalies - about 0.1 to 0.2ºC for significant events. There was a significant La Niña at the beginning of this year (and that is fully included in the D[ecember]-N[ovember] annual mean), and that undoubtedly played a role in this year's relative coolness. It's worth pointing out that 2000 also had a similarly sized La Niña but was notably cooler than this last year.

While ENSO is one factor in the annual variability, it is not the only one. There are both other sources of internal variability and external forcings. The other internal variations can be a little difficult to characterise ... but the external (natural) forcings are a little easier. The two main ones are volcanic variability and solar forcing. There have been no climatically significant volcanoes since 1991, and so that is not a factor. However, we are at a solar minimum. The impacts of the solar cycle on the surface temperature record are somewhat disputed, but it might be as large as 0.1ºC from solar min to solar max, with a lag of a year or two. Thus for 2008, one might expect a deviation below trend (the difference between mean solar and solar min, and expecting the impact to not yet be fully felt) of up to 0.05ºC. Not a very big signal, and not one that would shift the rankings significantly.

There were a number of rather overheated claims earlier this year that 'all the global warming had been erased' by the La Niña-related anomaly. This was always ridiculous, and now that most of that anomaly has passed, we aren't holding our breath waiting for the 'global warming is now back' headlines from the same sources.