El Nino not responsible for US heatwave

New Scientist Environment Blog, Catherine Brahic: …. A new study shows that human emissions of greenhouse gases made it 15 times more likely that the US would see record-breaking temperatures in 2006. In the event, temperatures were not the hottest, but the second-hottest since records began in 1895. The hottest year ever was 1998, which was also marked by a powerful El Niño. Scientists have widely attributed the record-breaking temperatures to El Niño.

So when data revealed that 2006, also an El Niño year, was the second-warmest year ever, Martin Hoerling at NOAA in Colorado and his colleagues decided it was time to find out if this was mere coincidence, or if El Niño was responsible for the warmth. Looking at data from 10 El Niño events since 1965, they found that El Niño tends to cool US temperatures slightly - not warm them.

The team have also used computer models to check this effect. They simulated US climate with and without El Niño and again found a slight cooling when El Niño was "switched on". Further computer models were used to look at the effect of greenhouse gases and aerosols on US temperatures and showed that they tend to warm US temperatures.

Finally, the El Niño and the greenhouse gas simulations were compared with what actually happened in 2006. The researchers found that the El Niño simulations were inconsistent with the idea that El Niño cause the 2006 temperatures. The greenhouse gas models, however, matched the actual records well. Hoerlin's team estimates that there is a 16% chance that 2007 will break the record set in 1998. The results will be published in Geophysical Research Letters on 5 September.

You might also be interested to know that this study is similar to one that was done in 2004 by Peter Stott of UK Hadley Centre for Climate Prediction and Research. Stott showed that human greenhouse-gas emissions at least doubled the likelihood that Europe would suffer a heatwave in 2003 (Nature, vol 432 p 610). During the heatwave, 35,000 people are thought to have died because of extreme temperatures.