Drought area changed little over recent decades
- 15 Nov 2012, 11:40
- Freya Roberts
Major droughts in the United
States and
Russia this summer highlighted the risks climate extremes pose
to everyday life, and prompted discussion about links between
drought and climate change.
Past research supports a link between climate change and drought -
suggesting that globally, the area affected by drought has
increased in recent decades as global temperatures have
risen.
But a
new study in Nature challenges this link, suggesting that the
relationship might not be that simple, and that older models may
have overestimated the change in drought over the last 60 years. In
fact, newer models which take a more detailed look at the how
droughts occur suggest the world area in drought may not have
changed much at all.
We take a look at the new research and why overestimating the past
might not affect predictions of how drought may respond to climate
change in the future.
What's changed?
The Intergovernmental Panel on Climate Change (IPCC) concluded in
its Fourth Assessment
Report, published in 2007, that "Globally, the area affected by
drought has likely increased since the 1970s".
In its
2012 Special Report on Extreme Events (SREX), the IPCC was more
cautious about past trends, concluding "there are large
uncertainties regarding global-scale trends in drought".
According to the
new study in Nature, those uncertainties are partly down to the
way simple drought models simulate water evaporation. Earlier
drought models worked out how much water was evaporating based
solely on temperatures. But a number of other factors affect
evaporation in real life, like wind speed, vegetation cover and
aerosols affecting sunlight.
Newer models include more of these factors, giving a more accurate
picture of drought. Using the newer models, the study found there's
been no significant change in the area affected by drought between
1950 and 2008. This led the researchers to conclude that studies
using the earlier models overestimated the global area affected by
drought.
Justin Sheffield, lead author on the study, told Carbon Brief:
"Our findings confirm what the SREX
report recognized: that at the time of its writing (6 months ago)
there was uncertainty about global trends in drought and that the
previous IPCC AR4 conclusion [...] was likely overestimated."
Many uncertainties still exist, even with the new models - the
meteorological data sets used to calculate levels of drought are
incomplete, and the method for using these models differs from one
researcher to the next. That makes quantifying change hard, and
more studies will be needed to add certainty to these
findings.
But this isn't the only research finding little change in droughts
over past decades.
Is "little change" new?
Sheffield and his colleagues aren't the first to find little
change in the global area affected by droughts over past decades.
In 2006, researchers from the Climatic Research Unit (CRU), based
at the University of East Anglia, found similar trends even when
they used their own versions of the older, simpler
models. Their research on European droughts between 1901 and
2002 concluded:
"evidence for widespread and unusual
drying in European regions over the last few decades is not
supported by the [models]"
Similarly, the CRU researchers found no statistically
significant trend in drought trends over North
America during the same time period. Using newer models to
simulate drought, they've also looked at global drought trends. The
findings, yet to be published, suggest drought has not become
unusually strong or widespread in recent decades, agreeing with
their earlier work.
Although they differ on how much old and new models agree, the
research from both CRU and from Sheffield et al. suggest that when
averaged out over the globe, drought has changed very little in
recent decades. But that's not to say it won't in the future.
Does this affect future predictions of
drought?
The finding that some models may have overestimated past drought
is unlikely to affect predictions about future droughts. That's
because working out how climate change might affect drought in the
future is done using different - climate - models. These climate
models are more sophisticated and operate in a different way than
the simple models this study looked at to analyse past
drought.
Lead author Justin Sheffield explained to Carbon Brief:
"[Climate models] respond and feedback
with the projected changes in precipitation, temperature, radiative
forcing etc. in a more realistic manner."
The authors of the new paper specifically warn that trends
identified by the models used to look at past drought shouldn't be
extrapolated into the future, not least because the assumptions
these models make based on temperature could overestimate future
levels of drought.
Climate models however, as the authors explain, are much better
at recreating the links between temperature, precipitation and
evaporation, so offer a better way to try to estimate future
changes.
What this new study does do is inform the way climate
projections are interpreted. At a time when links between climate
change and extreme events are under discussion, this research is a
reminder that drought is affected by a number of factors. And it's
the combination of these factors, rather than simply temperature
rise, that will determine how drought changes in the
future.