Is there evidence of global cooling?

Rong Fu
Rong Fu Photo: Sasha Haagensen

Myth 1: What global warming? Earth has actually been cooling since 1998.

Some people skeptical of global warming claim that Earth's global surface temperatures have been falling or have leveled off since 1998. They point to data now several years out of date from U.K. researchers that put 1998 as the warmest year on record. They also point to an unusually cool summer in North America in 2009 followed by an abnormally cold winter across all of the northern hemisphere. People who had to shovel record snows from their driveways or live without power during ferocious snowstorms in the northeastern United States began to doubt decades of scientific evidence on global warming.

The scientific data does not support the claim Earth has been cooling since 1998 and in fact strongly shows a warming trend.

First, it's important to note that many climate scientists don't think 1998 was even the warmest year on record. Scientists at NASA's Goddard Institute for Space Studies (GISS) have determined that 2005 was actually the warmest and that 1998 is in a statistical tie for second place with four other years: 2002, 2003, 2006 and 2007. The difference between their analysis and that of the U.K. researchers is complicated, but basically boils down to the way each handles missing weather data in the historical record. In the GISS analysis, five years since 1998 were as warm or warmer, clearly not a sign of global cooling.

Rong Fu, an expert in climate observations, also noted that you can't tell anything about climate from one year. For example, 1998 was an unusually warm year because of a strong El Niño.

"That was an anomalous year," said Fu. "It doesn't represent the state of the climate for the 1990s."

Likewise, 2008 was a cooler year, relative to the rest of its decade, because of La Niña, the flipside of El Niño. And 2009, despite regional chills, was still one of the hottest years on record from a global perspective. To understand how climate is really changing, Fu says, you have to smooth out those big year-to-year swings by taking a running average. In other words, you average each year's temperature with the temperatures from at least a few years before and a few years after.

Graph showing the global land-ocean temperature index
When scientists take a five-year running average of global surface temperatures (red line), Earth has clearly been warming for several decades. Image: NASA's Goddard Institute for Space Studies

Scientists at GISS compile just such a graph for the Global Land-Ocean Temperature Index. The red line, which indicates a five-year running average, shows a clear and strong warming trend from 1998 through the middle of the past decade (as far as a five-year running average can go because each year relies on data from five years into the future). The warming trend is nothing new. Scientists trace it back several decades before 1998.

Another key in establishing long term trends is to show that it is independent of specific start and end dates. In this case, if you start with the year 1997 or the year 1999 and then run forward to the present, the apparent cooling trend evaporates and you're left with a clear warming trend.

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