Ocean warmth predicts US drought and fire risk

By Tim Radford

LONDON, 29 July, 2017 – There is now a new way to forecast western US drought and fire risk, notably in Arizona and California. It’s simple: test the temperature of the oceans.

If the Atlantic is warm while the Pacific is relatively cold, then the risk of prolonged drought and wildfire conditions in California and on the other side of the Rockies becomes higher. It’s a natural consequence of oceanic cycles but, scientists warn, global warming as a consequence of human action can also make such droughts more severe.

Research like this matters because it identifies yet another working part in the global machinery of climate. Changes in ocean temperature drive vast and long-distance atmospheric changes that send the moisture-laden winds away from the thirsty soils.

The implication is that sustained drought, followed by raging wildfires in tinderbox forests, does not simply represent a bad run of the climate dice. Long-term natural forces are at work. And if climate scientists and meteorologists know in advance that drought is more likely, they can give farmers and growers and city authorities some useful warning.

“We were able to show that without anthropogenic effects, the droughts in the southwestern United States would have been less severe”

Scientists from Utah in the US, South Korea, Hawaii, the UK and California report in the journal Science Reports that they have combined complex ocean and atmosphere observations to develop what they call a “multi-year dynamical prediction system” that could advise on the probabilities of wildfire and drought at least 10 months in advance.

Such droughts hit Texas and Mexico in 2010-2011, the Great Plains in 2012 and California from 2011 to 2014: in California alone the bill for drought and fire reached at least $2.2bn and cost 17,000 jobs. So an earth system model that could give advance notice would be of huge value.

“Our results document that a combination of processes is at work. Through an ensemble modelling approach, we were able to show that without anthropogenic effects, the droughts in the southwestern United States would have been less severe,” said Axel Timmermann, who directs a centre for climate physics at Pusan National University in South Korea.

“By prescribing the effects of man-made climate change and observed global ocean temperatures, our model can reproduce the observed shifts in weather patterns and wildfire occurrences.”

Professor Timmermann, while at the University of Hawaii, identified unprecedented levels of ocean warming in 2014. But he is not the first to have cautiously identified a global warming component in the recent Californian drought.

Emerging pattern

No single climate event can be attributed to climate change as a consequence of the profligate combustion of fossil fuels and the build-up of greenhouse gases in the planetary atmosphere, but in a procession of such events, researchers have begun to see climate change as a contributing factor.

And once soil moisture evaporates and ground cover becomes parched, the risk of fire amplifies. Researchers have warned that global warming must make fire risk ever greater, particularly in the US southwest, even though many blazes begin to race through the dry forests as a consequence of human action.

And it now seems that the risks of such droughts can be read in advance in the details of temperature differences in two oceans. The models offered a forecast time of between 10 and 23 months for wildfire, and 10 to 45 months for drought. The next step is to test such a mechanism for forecasting fire and drought in other vulnerable parts of the world: the Mediterranean, or Australia.

“Of course, we cannot predict individual rainstorms in California and their local impacts months or seasons ahead,” said Lowell Stott of the University of Southern California in Los Angeles, and a co-author.

“But we can use our climate computer model to determine whether on average the next year will have drier or wetter soils or more or less wildfires. Our yearly forecasts are far better than chance.” – Climate News Network