The rapid growth in coal-fired power stations in China helped to cool the planet over the past decade and may explain why global warming appeared to stop, a study has found.
Sulphur emissions from burning coal reflect sunlight by increasing clouds and haze. China has been opening one coal-fired plant a week and its consumption of coal more than doubled between 2003 and 2007.
The cooling effect from emitting so much sulphur pollution was sufficient to offset some of the warming effect of rising carbon dioxide emissions, according to the study by scientists at Harvard and Boston Universities in the US.
The findings may strengthen the case for deliberately injecting sulphur particles into the atmosphere to mitigate the impact of greenhouse gases. This is one of several proposed methods of so-called geo-engineering, or deliberately altering the climate to minimise the increase in temperature.
However, unlike carbon dioxide, which lingers in the atmosphere for decades, sulphur particles drop out quickly and their cooling effect ends within months of the emissions ceasing.
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If China’s consumption of coal continued to expand at the present rate, the cooling effect from sulphur emissions would eventually be negated by the warming effect from the CO2.
The study provides an answer to climate sceptics, who argue that the failure to explain the absence of warming since 1998 suggests that the risk from CO2 has been exaggerated.
The Met Office temperature record shows the hottest year globally was 1998, though last year was the second hottest.
Robert Kaufmann of Boston University, the lead author of the study published in Proceedings of the National Academy of Science, said: “Human activities do two things: they cool the planet and they warm the planet. People normally just focus on the warming effect of CO2, but during the Chinese economic expansion there was a huge increase in sulphur emissions.”
Dr Kaufmann said the big increase in industrial activity after the Second World War, powered by burning coal, had resulted in a similar slowdown in global warming because of the shielding effect of sulphur emissions.
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The warming trend resumed in the 1970s when efforts to reduce air pollution and acid rain resulted in a decline in sulphur emissions.
China is now following the West by installing equipment to remove the sulphur from power station emissions.
Professor Piers Forster, Professor of Climate Change at the University of Leeds, said sulphur emissions were only one of the factors which may have contributed to the absence of warming in the past decade.
“Other natural fluctuations in the Sun’s output, volcanoes and water vapour have also been proposed for causing the non-warming noughties, and all may have contributed to an extent.
“The magnitude of the masking effect [of sulphur] is difficult to gauge as other short-term emissions, such as soot, warm the climate; and the effect of these short-term emissions on clouds is difficult to quantify. This makes it difficult to gauge the climate impact of adding scrubbers to Chinese power stations.
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“It needs to be emphasised that any masking is short-lived and the increased CO2 from the very same coal will remain in the atmosphere for many decades and dominate the long-term warming.”
Dr Peter Stott, head of climate monitoring and attribution at the Met Office, said: “The ‘noughties’ decade of 2000-2009 was significantly the warmest in the instrumental record, more than 0.15C warmer than the Nineties decade, part of a long-term warming pattern dominated by the effects of greenhouse gas emissions.
“Global temperatures fluctuate from year to year and this paper has provided some evidence that increased sulphur emissions from China could have contributed to some lessening of the rate of warming over a few years starting with the major El Nino of 1998 (when warm waters spreading to the Eastern tropical Pacific ocean caused a temporary warming of surface temperatures).
“But there is still much work to be done to unravel all the factors that can cause short-term fluctuations in temperature, including due to internal variability of the climate system, and to improve predictions of likely temperatures over the next seasons and decades.
“What is clear is that long-term climate change is dominated by rapidly increasing concentrations of greenhouse gases and the long-term warming will continue unless emissions are reduced.”