July 4, 2011

expert reaction to new research on climate change over the decade 1998-2008, as published in PNAS

A paper published in Proceedings of the National Academy of Sciences (PNAS) looked a trends in climate change between 1998-2008 and sought to reconcile this with observed temperatures.

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.”

Prof Piers Forster, Professor of Climate Change at the University of Leeds, said:

“The masking of CO2-induced global warming by short term sulphur emissions is well known. It is believed that the flattening off of global mean temperatures in the 1950s was due to European/US coal burning and just such a mechanism could be operating today from Chinese coal.

“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 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.

“It needs to be emphasized 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 Simon Lewis, Royal Society research fellow at the Earth & Biosphere Institute, University of Leeds, said:

“The paper is easily misinterpreted, as there is still uncertainty about how much the planet has warmed by. Correspondingly there is additional uncertainty about the factors that must be included to accurately predict recent global temperature changes.

“But one thing is certain: while sulphur emissions do have a cooling effect, this is only short-term. Meanwhile, the carbon dioxide emissions from coal use will lead to a long-term planetary warming. The unstated conclusion to be drawn is that there will be substantial warming caused by greenhouse gas emissions.”

Prof Joanna Haigh, Professor of Atmospheric Physics at Imperial College London, said:

“Over a period as short as a decade it is difficult to assess long-term trends in global temperature because of very large natural interannual variability. In the period 1998 to 2008 temperature variations were dominated by the El Niño Southern Oscillation (ENSO) – with large warming events between 1998 and 2000 and mainly cooling events between 2002 and 2005.

“The paper’s authors are making the important point that, underlying these, the warming due to the CO2 released by Chinese industrialisation has been partially masked by cooling due to reflection of solar radiation by associated particle emissions. On longer timescales, with cleaner emissions, the warming effect will be more marked.”

Article 11-02467: Reconciling anthropogenic climate change with observed temperature 1998-2008, Robert Kaufmann et al., Proceedings of the National Academy of Sciences (PNAS), published online on 4 July 2011.