China's double-edged climate weapon

Yesterday's publishing of the innocuous-sounding ''Reconciling anthropogenic climate change with observed temperature 1998-2008'', in the journal PNAS, kicked off a veritable storm of comment and debate. After many years of fretting about how an industrializing-China's rising CO2 emissions were hotting-up the globe, the claim that those same smokestacks were helping to offset global warming raises many questions.

First, are the authors correct in saying that global temperatures have not risen between 1998 and 2008? That's not a simple question to answer, as climate change is all about average trends over the long-term. There are natural ups-and-downs associated with repeated short-term climate cycles. 1998 was a strong El Nino year, when temperatures are forced higher, whereas 2008 was a strong La Nina, when there is a natural cooling effect. So the year-range chosen is one where natural cycles are most likely to be masking any underlying warming.

And taking a reading from the globe's thermometer is not simple - measuring stations are spread unevenly across the globe, and different international bodies have different ways of merging them. But when looked at as an average, over a decade or more, all of these do show either a relatively stable temperature, or a slower rate of increase, over the 10 years chosen - but certainly no cooling. However, something does seem to have happened to slow the global warming trend for that period, when compared to the runaway warming of the 1980s and 1990s.

But is it the sulfates that are holding the warming back? 'Sulfate aerosols' is a term used to refer a range of tiny particles of sulfate-rich water and solids that do the opposite of CO2. They reflect the sun's energy back into space, so more sulfates high in the atmosphere can cause a cooling effect. They are produced from SO2 by reactions in the atmosphere - either from gases erupted naturally out of volcanic vents, or from the smokestacks of coal-fired power stations.

Although data on SO2 emissions is poor after 2000, the authors calculated the expected global emissions of SO2 out until 2007 - and found an increase after years of decline. They put this down mainly to China's rising use of coal, to power its relentless economic growth. Looking at all of the various inputs to global climate - including CO2 and SO2 emissions from man, and natural cycles like La Nina/El Nino, as well as changes in the sun - they believe that the rising sulfate emissions is the most likely factor to have cause the global warming slowdown, between 1998 and 2008.

However, there are doubts about the work. The IPCC has classed understanding of how sulfates effect climate as poor, and this paper comes from statisticians, not climate experts. Emitting SO2 locally in Asia doesn't have to cause levels of sulfate particles high in the atmosphere to rise, which is where the cooling effect is most apparent. It is likely some more work is needed to confirm whether this is what has indeed happened, over the years of China's economic boom.

But if the authors are on the right track, is China doing the right thing by cleaning up its coal stations, to stop the SO2 emissions? For forests and local environments blighted by hazy pollution and acid rain, the answer is probably a strong affirmation. Perhaps the more urgent question is - how fast can China slow down its CO2 emissions, before they cause the mercury to blow on the global thermometer?