Is the globe warming or cooling?

— -- As our legions of dedicated USA TODAY commenters enjoy pointing out, every year since 1998 — when the Earth's temperature peaked at a record high — has been cooler than that year. 2008, for example, was the planet's coolest year since 2000. Could this be evidence against global warming?

No, say two scientists in this week's issue of Geophysical Research Letters, a journal of the American Geophysical Union. The scientists, David Easterling of the National Climatic Data Center and Michael Wehner of the Lawrence Berkeley National Laboratory, say that up-and-down temperatures year-to-year don't undermine the overwhelming evidence for global warming.

"The reality of the climate system is that, due to natural climate variability, it is entirely possible to have a period as long as a decade or two of 'cooling' superimposed on the longer-term warming trend due to anthropogenic [human-caused] greenhouse gas forcing," write the authors. "Climate scientists pay little attention to these short-term fluctuations as the short term 'cooling trends' mentioned above are statistically insignificant and fitting trends to such short periods is not very meaningful in the context of long-term climate change."

Further evidence for a warming world is that the 10 warmest years on record have all occurred since 1998 and that even though 2008 was the coolest year since 2000, it was still the 8th-warmest year on record.

According to the 2007 Intergovernmental Panel on Climate Change, the Earth's atmosphere has warmed by about 1 degree Fahrenheit over the past century due to increasing amounts of carbon dioxide from the burning of fossil fuels. Future projections vary as to how much more the Earth will warm, anywhere from 2 to 10 degrees depending on the level of greenhouse gases that continue to be released into the atmosphere.

Statistically, the study authors note, the current run of level temperatures over the last decade falls within the bounds of past decades and climate projections, and that climate models do produce sustained periods of cooling embedded within the longer-term warming.