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Unpacking unpaused global warming – climate models got it right

 

Eyjafjallajokull Volcano Eruption in Iceland

Eyjafjallajokull Volcano Eruption, Iceland. Particulate emissions from volcanoes are among the factors temporarily slowing down global surface warming. Photograph: Arctic-Images/Getty Images

 

Powered by Guardian.co.ukThis article titled “Unpacking unpaused global warming – climate models got it right” was written by Dana Nuccitelli, for theguardian.com on Monday 25th August 2014 13.00 UTC

Although the global climate has continued to build up heat at an incredibly rapid rate, there has been a keen focus among climate contrarians and in the media on the slowdown of the warming at the Earth’s surface. The slowdown is in fact a double cherry pick – it focuses only on the 2% of global warming that heats the atmosphere (over 90% heats the oceans), and it only considers the past 10–15 years. Nevertheless, because there was so much attention paid to the surface warming slowdown, the latest IPCC report addressed it specifically, saying,

The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence). There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years (e.g., 1998 to 2012).

From 1998 through 2012, the Met Office estimated that global surface temperatures had warmed by about 0.06°C, whereas the average climate model projection put the value at closer to 0.3°C. This apparent discrepancy only represented a tiny fraction of overall global warming, and over a short enough period of time that the internal noise of the climate system could be having a significant influence, but it was nevertheless a challenge for climate scientists to explain the precise causes of the difference.

However, research published since the release of the IPCC report has gone a long way to explain the discrepancy, and it turns out the climate models are doing a pretty good job after all.

Measurement Biases Exaggerated the Discrepancy

One important paper published late last year by Kevin Cowtan and Robert Way found that the discrepancy wasn’t nearly as large as believed. Climate scientists have known that the surface temperature record is incomplete because of gaps between instrument stations, especially in the Arctic. The Arctic happens to be the fastest-warming region on Earth, and so if it’s excluded from the surface temperature record, the record will have a cool bias.

Cowtan & Way took two separate clever approaches to address this issue, using statistics and satellite measurements to fill in the gaps. Both methods yielded similar results, showing that the Met Office estimates were accurate up to 1998, but underestimated the warming since 1998 by more than half, due in large part to the particularly rapid warming in the Arctic during that time.

The Cowtan & Way estimates put the global surface warming from 1998 through 2012 at about 0.17°C, explaining nearly half of the apparent difference between measurements and the average climate model projections. But the other half still needed to be explained.

Cooling Influences from Oceans, Volcanoes, and the Sun

A great many studies published over the past two years have shown that since 1998, more heat has been funneled into the deep oceans, leaving less to warm the atmosphere. Kevin Trenberth and colleagues produced one of the best data sets measuring ocean temperatures, finding an unprecedented warming of the deep oceans since 1998. A new paper shows that while oceans around the world are warming, most of the deep layer warming is happening in the Atlantic and Southern oceans.

A recent study led by Matthew England showed that unprecedented trade wind strength has invigorated the circulation of the Pacific Ocean, causing more heat from the surface to be mixed down into deeper ocean layers, while bringing cooler waters to the surface. Papers by Kosaka & Xie and James Risbey and colleagues showed that when climate models accurately reflect ocean cycles, their global surface temperature simulations are also accurate.

At the same time, solar activity has declined and volcanic activity risen since 1998. Volcanoes pump aerosol particulates into the atmosphere that cause short-term cooling, and a decline in the amount of solar energy reaching the Earth’s surface also has a cooling effect.

 

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