Â鶹´«Ã½

Claims that global warming has slowed down over the past decade were partly based on faulty data. Instead, the rate of global warming was underestimated because of a new way of measuring sea-surface temperatures, suggests a new study.

Since the 1970s average global temperatures have , but over the past decade they seemed to rise by only 0.09 °C, an apparent slowdown of 0.07 °C. John Kennedy and colleagues at the UK Met Office have now found that the real slowdown was smaller.

Over the past decade, sea-surface temperature has mostly been measured by thermometers on buoys, whereas previously it was measured aboard ships. Ship measurements tend to be too high because the water warms up as it is taken on board.

So although the newer buoy measurements are more accurate, the switch in method has erroneously shown .

“Compared with ships, buoys show cooler temperatures,” says Vicky Pope at the Met Office. “You have to be careful of false signals.”

Record for 2010?

Kennedy says the underestimation of the change in sea-surface temperature could account for up to 0.03 °C of the apparent slowdown in global temperatures. The correction could mean that 2010 will be the warmest year on record, surpassing 1998 and 2005.

Part of the apparent slowdown in temperature rise does appear to be genuine, however. Earlier this year, of the National Oceanic and Atmospheric Administration in Boulder, Colorado, and colleagues , weakening the greenhouse effect and taking 0.04 °C off the temperature in the past decade.

Taken together, the mismatch in sea-surface temperature data and the fall in water vapour levels account for almost all of the slowdown that earlier studies had suggested.

From now on climate measurements from the buoys will be “corrected” so that they can be compared with the decades of data from ship measurements, and vice versa.

It’s not the first time measurements have had to be corrected: a change in how ships measured sea temperatures caused the apparent cooling in the 1940s.

Journal reference: , in press