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November 26, 2010 – Original source: The Australian

The recent slowdown in the rate of global warming may be an illusion caused by errors in the way the global temperature is measured, according to a Met Office study.

The findings undermine the arguments of climate skeptics who use the reported reduction in the warming rate since 1998 to oppose cutting man-made emissions.

The Met Office, Britain’s national weather service, said that this year, which has so far been the second warmest on record behind 1998 according to existing measurements, could turn out to be the warmest once errors have been removed.

The long-term rate of global warming was about 0.16C a decade in the 1970s, 1980s and 1990s but it slowed in the past 10 years to between 0.05C and 0.13C, depending on which of three major temperature records is used. The Met Office said that changes in the way ocean temperatures were measured had resulted in an under-estimate of about 0.03C in recent years.

The number of drifting buoys that collect temperature readings has increased in the past 10 years and the number of ship measurements has fallen.

By comparing simultaneous and neighboring measurements from the two, the Met Office has established that the buoys tend to report a cooler temperature than the ships.

A Met Office spokesman said: “Over time the warming trend is underestimated. The correction will increase the trend in the last 10 years to 0.08-0.16C per decade, closer to the long-term trend of 0.16C per decade.”

The Met Office said it had probably also underestimated the level of warming in the Arctic region because it contained relatively few temperature stations. The Arctic is warming at a faster rate than the global average because as ice melts it exposes darker water that absorbs rather than reflects sunlight.

The Met Office does not make estimates for Arctic areas where there are no temperature readings but simply excludes them, resulting in an underestimate of the temperature change across the region.

Vicky Pope, head of climate science advice at the Met Office, said: “We are certainly underestimating the warming in the Arctic but we don’t know by how much. Work is under way to provide a better estimate in the future.”

She said that even without the errors in the measurements the reported reduction in the warming trend could be explained by natural variability in the Earth’s climate system.

Other factors contributing to the slowdown in the warming rate could include changes in solar activity, increased aerosol emissions from Asia because of rising industrial activity, and a reduction in the amount of water vapor in the stratosphere. Aerosols cool the climate by reflecting sunlight, and water vapor is a greenhouse gas.

The Met Office also said that there were a wide range of indications that the climate was changing, in addition to the rises in air temperatures. The changes included an increase in humidity because a warmer atmosphere holds more water, rising sea levels, the shrinking of the Arctic sea ice, and glaciers and spring snow cover in the northern hemisphere.

Dr Pope said that the evidence for man-made global warming had grown stronger in the past year. She said that it was important to look beyond the present cold snap in Britain and last year’s harsh winter in Europe and consider the global picture.

Many parts of the world had experienced very warm temperatures last year although Britain was gripped by snow and ice in the coldest winter for 30 years.

“We are starting to see changes in the climate even in the UK which we can link to global warming,” she said. “We’re seeing more heatwaves and seeing fewer of these cold winters.”

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