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Carbon Brief Staff

Carbon Brief Staff

05.07.2011 | 2:00pm
ScienceSulfur emissions may have slowed temperature rise
SCIENCE | July 5. 2011. 14:00
Sulfur emissions may have slowed temperature rise

A paper published in the Proceedings of the National Academy of Sciences is attracting attention in the media and across the blogosphere today.

Despite the clear long-term trend in global temperatures showing the planet is warming, a slowing in the long term warming trend over the past decade has led some to conclude that “global warming has stopped”. While Met Office data shows that this claim can’t be stood up, with temperature rise from 1995 to 2010  statistically significant, it may be that this new work offers an explanation for why warming has slowed in recent years.

Entitled “Reconciling anthropogenic climate change with observed temperature 1998-2008”, the new research uses computer modelling to assess the reasons for the slow-down, and concludes that rapid growth in sulfur emissions from Chinese coal-fired power stations may be offsetting some of the warming effect of rising greenhouse gases.

Here’s the short summary of the paper (the abstract), with our emphasis:

“Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations. As such, we find that recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors with well known warming and cooling effects.”

Richard Black explains the context of this diccussion for the BBC:

“Mainstream climate scientists have traditionally answered the “no warming since 1998” claim in two ways. One is by pointing out that 1998 saw the strongest El Nino conditions on record, which transfer heat from the oceans to the atmosphere, warming the planet. So while you may not see a temperature rise if you start the series in 1998, you do see one if you begin with 1997 or 1999.

The second answer is to point out that temperatures will naturally vary from year to year, and to point to the consistent upward trend seen when long-term average temperatures are used rather than annual figures.”

Although the new paper acknowledges the that the slow-down could be a result of natural variability in the climate system, the authors also explored other factors which might affect temperature. They used data on both man-made and natural drivers of global surface temperature to simulate global surface temperature between 1999 and 2008 with a computer model.

Their conclusion:

“Results indicate that net anthropogenic forcing rises slower than previous decades because the cooling effects of sulfur emissions grow in tandem with the warming effects greenhouse gas concentrations. This slow-down, along with declining solar insolation and a change from El Nino to La Nina conditions, enables the model to simulate the lack of warming after 1998”

So when their computer model factored in greenhouse gas emissions, sulfur dioxide emissions and natural climate cycles, it simulated what temperatures have done over the past 15 years.

This work suggests that rising sulfur emissions have been offsetting the impact of rising greenhouse gases. Sulfur dioxide is an aerosol which cools the planet by reflecting some of the sun’s energy back into space.

Burning coal is a prime cause of sulfur emissions, and as Richard Black outlines, figures from the US Energy Information Administration (EIA) show that the rate at which coal is used has sharply accelerated since 2003, particularly in China, where electricity-generating capacity rose from just over 10 gigawatts (GW) in 2002 to over 80GW in 2006. (A large coal plant has about 1GW capacity).

The Guardian also suggests that

“The effect also explains the lack of global temperature rise seen between 1940 and 1970: the effect of the sulfur emissions from increased coal burning outpaced that of carbon emissions, until acid rain controls were introduced, after which temperature rose quickly.”

So what does this mean for climate policy? The researchers were fairly clear that it doesn’t suggest cutting CO2 emissions is any less important to limit climate change, with lead author Professor Robert Kaufman telling the Guardian:

“If anything the paper suggests that reductions in carbon emissions will be more important as China installs scrubbers [on its coal-fired power stations], which reduce sulfur emissions. This, and solar insolation increasing as part of the normal solar cycle, [will mean] temperature is likely to increase faster.”



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