If you happen to follow the public debate on climate change, you might we aware that, in some circles, 2008 is being viewed as the definitive end of global warming. After all, it marks a decade where temperatures have trended downwards, and an especially cold start to the year was heralded as "wiping out a century of warming." So, it might come as a surprise to learn that, now that the year's numbers have been crunched, NASA's Goddard Institute and the UK's Climactic Research Unit rank 2008 as the 9th and 10th warmest year (respectively) in the 150 years or so humanity has been keeping careful track of these things.

The facts are that 2008 was cooler than the last few years, but warmer than most in recent history, which lends itself to spin based on the predilections of the person talking about it. But some of that spin specifically plays upon the widespread innumeracy of the public, which isn't well prepared to separate trends from short-term variability, or recognize when certain figures are selectively chosen. We'll try to separate out some of these in a way that will hopefully help readers make a bit of sense out of the conflicting noise.

Parsing 2008's temperature

Why was 2008 colder than the past several years? The short answer is La Ni?a. The tropical Pacific Ocean undergoes a cyclical change in surface temperatures called the El Ni?o-Southern Oscillation, which has a dramatic impact on global temperatures. (NOAA has a tutorial and an FAQ about the ENSO, for the curious.) I've taken an image from a Goddard Institute analysis and modified it to highlight this impact.

Blue bars, which are roughly centered on the cool La Ni?a events, are often associated with a drop in global temperatures. Warmer, El Ni?o events tend to correspond to warmer periods in the global temperature record. This relationship isn't absolute, as the ENSO isn't the only thing influencing short-term variations—the drop in the early 1990s is thought to be the product of the massive eruption of the Pinatubo volcano, for example. If 2008 was chilly, there's a good chance that the La Ni?a event was a major contributor to that.

The ENSO and global temperatures

The other thing that should be apparent from this photo is that the data are noisy—they vary rapidly between many of the adjacent three-month periods shown in the upper portion of the graph. That makes drawing any significant conclusions based on a short time window—such as when a single month's readings were used to conclude that the earth was "wiping out a century of warming"—a very dangerous thing. As should be clear from the information above, temperatures rose enough later in the year to turn it into one of the warmest on record.

The other thing that should be apparent from the graph is that temperatures swing both ways. If we were to conclude that the trend towards a warmer globe had ended back in February, what should we have concluded in the late 1990s, when a strong El Ni?o event shot the temperatures up to previously unprecedented levels? It certainly wasn't taken as evidence that the seas were on a trajectory to boil at the time.

Pick your trends carefully

Those exceptional temperatures are the product of an exceptional year, as the CRU has 1998 it as the warmest year on record, and Goddard places it in a statistical dead heat with the warmest. I first became aware of its significance two years ago—I know it was two years ago, because I was told that global warming had stopped eight years earlier. Now, those statements have been replaced with ones indicating that the earth has been cooling for a decade. One of the most significant things these statements have in common is their starting date: 1998. It's instructional to perform a similarly superficial analysis with the surrounding years.

Looking at the 1990s until now, using the deviation from the 1950-1980 mean in C

All the numbers indicate how much warmer that year was than the mean temperature from the 1950-1980 period. The data should make clear that, to get a drop in temperatures using that measure, you need to pick 1998 as your starting point; neither of the two neighboring years will work.

CRU (top) and Goddard data, showing smoothed trends. (Click for larger version)

You'd get a different result using CRU data (which rates 1998 as warmer, and recent years as a bit cooler), but the message would be the same: the years you choose for your analysis can determine your trend. In fact, a quick look at the chart above shows that you could pick a decade to support any conclusion you want. In fact, the start of the instrument era is, in effect, an arbitrary year, as is the present. So, how can we tell anything?

To deal with this, both the CRU and Goddard apply smoothing to their data; Goddard provides a moving five-year average, CRU more dramatic smoothing, as shown below. Both of these make the overall temperature trends clear by limiting the ability of the variability in the underlying data to obscure it.

In both cases, the temperatures of 2008 combine with several years of largely similar data to create a slight drop in the trend. Drops of a similar magnitude appear in the past, meaning that the current one is consistent with an overall upward trend in temperatures.

Should the current dip turn into a longer trend, then it would be cause for a significant revision of our understanding of the climate. The clearest indication of whether that's going to happen might be the next time an El Ni?o arrives—assuming Mount Pinatubo doesn't go off again in the meantime.