Scientists 'underestimated' rate of global warming

Scientists measuring rises in global temperatures underestimated the rate of warming in the past decade, they said today, setting out the “growing” evidence for climate change.

The rate at which temperatures are rising has slowed in the past decade, although there is still a warming trend over the 10 years since 2000 and the decade was the hottest on record, researchers from the UK's Met Office said.

In a report published today, the Met Office said the recent decrease seen in the rate of warming – often seized on by climate sceptics as evidence that man-made global warming is overstated or not happening – was the result of natural variation in the weather.

But they also said a lack of data from the Arctic, where warming has been particularly strong in the last 10 years, and changes to the way sea surface temperatures are measured have led to an underestimate of the rate at which temperatures are rising.

Ahead of the next round of international talks aimed at securing a deal on climate change, the UK Met Office also said the 12 months to the end of September were the second warmest on record – while another analysis by scientists in the US indicate the year was the hottest ever.

Since the 1970s, the long-term rate of global warming has been around 0.16C a decade but that slowed in the last 10 years to between 0.05C – 0.13C depending on which of the three major temperature record series are used.

Vicky Pope, head of climate science advice, said: “The warming trend has decreased slightly. There’s still a warming trend but it’s not as rapid as it was before.

“The question is why has that happened. It’s a question that sceptics often bring up.”

The climate is influenced by natural variability, while other contributing factors to the slowdown in temperature rises could include solar variability, increased aerosol emissions from Asia because of rising industrial activity and changes in the amount of water vapour in the stratosphere.

Dr Pope also said: “We may be underestimating the warming.”

Partly this is due to gaps in the temperature data from the Arctic, where there is evidence warming has been stronger than other parts of the world.

The Met Office does not make estimates for areas where there are gaps in the Arctic data, instead leaving them out, which would leave their overall results for global temperatures on the low side.

And changes to the way sea surface temperatures are measured – with a shift from predominantly ship-based measurements to the use of buoys drifting around the oceans in the past 10 years – led to an underestimate of temperature rises.

Correcting the analysis of the sea surface temperatures could mean global temperatures as a whole could have risen by up to 0.03C above what has already been recorded.

The Met Office also said there were a wide range of indications the climate was changing, in addition to the rises in air temperatures, which were consistent with rising greenhouse gases.

Changes include rising sea levels, an increase in humidity as a warmer atmosphere holds more water, the shrinking of the Arctic sea ice, glaciers and northern hemisphere spring snow cover.

And while the UK is currently experiencing a cold snap and last year had the harshest winter for 30 years, the scientists said the evidence for man-made global warming had grown even stronger in the past year.

Dr Pope said for global warming it was important to look at the global picture - which last year saw many parts of the world experience very warm temperatures even while the UK was gripped by snow and ice.

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