- · Quirin Schiermeier
16 April 2013
Article tools
“This would be a fine country if it only had
water,” observes a settler looking at the barren west Texas plains. “So would
Hell,” replies a despairing farmer.
That old Texas joke probably
originated in the 1950s, when the state was baked by its most relentless
drought in recorded history. Last year, rain kept clear of the region again,
and scientists predict that the entire North American southwest will become
increasingly drought-prone as climate change proceeds
Reliable forecasts of future
‘megadroughts’ would be a boon to farmers and water managers. But results
presented last week at the annual assembly of the European Geosciences Union in
Vienna suggest that such forecasts are still beyond the reach of current
climate models.
Sloan Coats of Columbia University’s
Lamont-Doherty Earth Observatory in Palisades, New York, and his colleagues
tested whether a state-of-the-art climate model could simulate the droughts
known to have occurred in the southwest during the past millennium. The model
incorporated realistic numbers for factors that affect temperature and
rainfall, such as atmospheric carbon dioxide levels, changes in solar radiation
and ash from volcanic eruptions.
It also incorporated changes in the
El Niño/Southern Oscillation (ENSO), a recurring temperature anomaly in the
tropical Pacific that greatly affects weather in the western United States and
many other parts of the world. (The warm phase — El Niño — often brings
torrential rain and flooding; the cold phase — La Niña — tends to bring
drought.) The team then compared the results of its simulations with data from
the North American Drought Atlas, a detailed compilation of droughts based on
the thickness of tree rings.
The results were puzzling. Although
the simulation produced a number of pronounced droughts lasting several decades
each, these did not match the timing of known megadroughts. In fact, drought
occurrences were no more in agreement when the model was fed realistic values
for variables that influence rainfall than when it ran control simulations in
which the values were unrealistically held constant. “The model seems to miss
some of the dynamics that drive large droughts,” says study participant Jason
Smerdon, a researcher at Lamont-Doherty who studies historical climate
patterns.
Other climate models tested by the
team fared no better, he says. In particular, the models failed to reproduce a
series of multi-decadal droughts that occurred in the southwest during the
Medieval Climate Anomaly, a period between AD 900 and 1200 when global
temperatures were about as high as they are today.
“The model seems to miss some of the
dynamics that drive large droughts.”
The problem may lie in the models’
inability to reproduce the cycling between the ENSO’s El Niño and
La Niña phases, especially given that many scientists think that
La Niña is the major driver of drought in the southwest. The ENSO “behaves
much messier in the real world than in climate models”, says Jessica Tierney, a
climate scientist at the Woods Hole Oceanographic Institution in Massachusetts
who has investigated the role of the ENSO in East African rainfall variability. “We’re not sure how it has varied
in the past, and we don’t know how it might change in response to climate
change. This is really one of the big uncertainties we’re facing.”
In addition to their failure to
reproduce El Niño and La Niña, existing models do not fully capture other
factors that influence rainfall, such as clouds and vegetation. But Smerdon
adds that the atmospheric and oceanic dynamics that inhibit rainfall and favour
prolonged drought may be essentially random and so almost unpredictable.
Last week’s findings highlight the
broader challenge of predicting how precipitation patterns will change as the
global climate warms. Models are often at odds over the very direction of
regional changes. For example, different projections prepared for the Colorado
Water Conservation Board disagree on whether mean precipitation in the state
will increase or decrease by 2050 (ref.).
But the uncertainties don’t change
the larger picture, scientists say. “Climate models are not perfect, but they
do the big things really well,” says Tierney. “We can be pretty confident that
the southwest will warm and that water will become scarcer.”
References
No comments:
Post a Comment