Temperature Records

Groups of scientists from several major institutions — NASA's Goddard Institute for Space Studies (GISS), NOAA's National Climatic Data Center (NCDC), the Japanese Meteorological Agency and the Met Office Hadley Centre in the United Kingdom — tally data collected by temperature monitoring stations spread around the world and make an announcement about whether the previous year was a comparatively warm or cool year.

Multiple institutions monitor global surface temperatures. Despite subtle differences in the ways the scientists perform their analyses, these four widely referenced records show remarkable agreement. (Image credit: NASA Earth Observatory/Robert Simmon)

There are large areas in the Arctic without weather stations. NASA GISS approaches the problem by filling in gaps with data from the nearest land stations. The Climatic Research Unit at the University of East Anglia, which works jointly with the Met Office Hadley Centre, leaves much of the region out of its global temperature analysis. (Image credit: NASA Earth Observatory/Robert Simmon)

In 2010, global temperatures continued to rise. A new analysis from the Goddard Institute for Space Studies shows that 2010 tied with 2005 as the warmest year on record, and was part of the warmest decade on record. The thin line represents annual data, while the thick line is a five-year running mean. (Image credit: NASA Earth Observatory/Robert Simmon)

NASA's announcement this year — that 2010 ties 2005 as the warmest year in the 131-year instrumental record — made headlines. But, how much does the ranking of a single year matter?
Not all that much, emphasizes James Hansen, the director of NASA's Goddard Institute for Space Studies (GISS) in New York City. In the GISS analysis, for example, 2010 differed from 2005 by less than 0.01°C (0.018°F), a difference so small that the temperatures of these two years are indistinguishable, given the uncertainty of the calculation.

Meanwhile, the third warmest year — 2009 — is so close to 1998, 2002, 2003, 2006, and 2007, with the maximum difference between the years being a mere 0.03°C, that all six years are virtually tied.

Even for a near record-breaking year like 2010 the broader context is more important than a single year. "Certainly, it is interesting that 2010 was so warm despite the presence of a La Niña and a remarkably inactive sun, two factors that have a cooling influence on the planet, but far more important than any particular year's ranking are the decadal trends," Hansen said.

One of the problems with focusing on annual rankings, rather than the longer trend, is that the rankings of individual years often differ in the most closely watched temperature analyses — from GISS, NCDC, and the Met Office — a situation that can generate confusion.

For example, while GISS previously ranked 2005 warmest, the Met Office listed 1998 warmest. The discrepancy helped fuel the misperception that findings from the three groups vary sharply or contain large amounts of uncertainty. It also fueled the misperception that global warming stopped in 1998.

"In reality, nothing could be further from the truth," said Hansen. Global temperatures have continued to rise steadily.

"The three official records vary slightly because of subtle differences in the way we analyze the data, but they agree extraordinarily well," said Reto Ruedy, one of Hansen's colleagues at GISS who helps analyze global surface temperatures.

All three records show peaks and valleys that vary in virtual sync with each other since 1880. All three show particularly rapid warming in the last few decades. And all three show the last decade is the warmest in the instrumental record.