Introduction

“There IS a problem with global warming,” one headline recently quipped, “it stopped in 1998.” Making the same argument about declining temperatures since 1998, another headline proclaimed, “Alarmists still heated even as world cools.”

These articles suggest that climate scientists have misled the public. While climatologists warn that greenhouse gas emissions from burning fossil fuels are warming our planet, some people argue that Earth is actually cooling: they point to declining temperatures since 1998 as “proof” that global warming has stopped.

But this argument about global cooling begs an important question: Why measure temperatures since 1998? Why not 1997 or 1999?

Global temperature anomalies from 1992 through 2008 as estimated by the Hadley Centre. Temperatures are shown as the difference from the 1961-1990 average. The colored lines drawn on the graph illustrate how temperatures for 1997, 1998, and 1999 compare to 2008.

The answer might lie in the fact that 1998 was arguably the warmest year on record. “The reason we had such a warm 1998 is that we had a very, very strong El Niño, probably one of the strongest El Niños of the twentieth century,” explains David Easterling, Chief of the Scientific Services Division at NOAA’s National Climatic Data Center. “If you have a very strong El Niño—an extensive swath of the ocean’s surface with much-warmer-than-average temperatures—that tends to warm the globally averaged air surface temperature.”

If global temperatures from subsequent years are compared to those of 1998, warming since that time is certainly difficult to detect. Easterling himself pointed out in a 2009 paper that a linear trend line from 1998 to 2008 shows no statistically significant trend up or down. While a straight line drawn from 1998 to 2008 does indicate that 2008 was cooler, a line from 1999 to 2008 shows that the planet warmed, and a line from 1997 to 2008 is almost flat. But Easterling doesn’t advocate starting the measurements from either of those years. Instead, he suggests looking at the bigger picture.

David Easterling of NOAA’s National Climatic Data Center.

“The bottom line is that current temperatures are way above the long-term average,” Easterling says. The United Kingdom’s Met Office and the World Meteorological Organization concur; in December 2009, the organizations described the first decade of the twenty-first century as by far the warmest on record.

Average global temperature anomalies by decade, estimated by the Hadley Centre, shown as the difference from the 1961 – 1990 average. Vertical lines indicate the upper and lower 95% uncertainty ranges for each decade.

The cooling argument is challenged even further in that not every estimate pegs 1998 as the warmest year. One group of climate researchers instead finds 2005 to be the warmest year. The difference doesn’t result from error or whim, but from the inherent difficulty in measuring global temperatures.