This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.

The results are in for 2007, and it was a warm one. How warm? It depends who you ask. According to NASA, 2007 was the second warmest year on record. NOAA’s analysis put it in fifth place. The University of East Anglia’s Climatic Research Unit (CRU) ranked it eighth.

The results differ because it’s not as straightforward as you might think to take the temperature of the globe. There are many subtleties to consider.

For one thing, how do you deal with that fact that temperature sensors aren’t evenly spaced? Which temperature trend are you measuring - just land, just ocean, or a combination of the two? And how do you handle "urban heat islands" - the tendency of cities to become warmer than nearby rural areas?

Different research groups take different approaches. For example, NOAA’s map of temperature has grids of 1 degree latitude x 1 degree longitude. CRU, in addition to 1 x 1 degree grids, produces grids of 5 degrees latitude x 5 degrees longitude and 0.5 degrees latitude x 0.5 degrees longitude. And it’s not just about the grid. CRU doesn’t extrapolate to include the Arctic, which is experiencing rapid warming; NASA does.

NASA scientists made two other observations to put 2007 in context. First, last year’s warmth was "noteworthy because it occurs at a time when solar irradiance is at a minimum." And second, the Pacific Ocean has shifted into a La Niña phase, when global temperatures are normally cooler than average. So 2007 should have been cooler than average, yet it’s one of the hottest.

Despite the complexities in measurement, scientists agree on two things. No matter which trend you’re looking at or how the data are processed, global warming is unequivocal. And this warming is caused by increasing levels of greenhouse gases due to human activities. It’s past time to cap our emissions!