July 29, 2017, by Tim Radford
Ocean cycles help to determine
US drought and fire risk in several western states, with global warming adding
to their severity.
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.
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
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