1 March, 2016
In
the years before the Syrian conflict erupted, the region’s worst
drought on record set in across the Levant, destroying crops and
restricting water supplies in the already water-stressed region. A
new study shows that that drought, from 1998-2012, wasn’t just the
most severe in a century of record-keeping—it was the Levant’s
most severe drought in at least 500 years and likely more than 900
years.
The
study,
published in the Journal
of Geophysical Research: Atmospheres, is
the first to quantitatively evaluate droughts across the
Mediterranean region during the past 900 years at a high level of
detail. It doesn’t identify a cause of the recent Syrian drought,
but it does provide independent support for studies that have
suggested global warming may already be having an effect there, said
lead author Benjamin
Cook,
a climate scientist at Columbia University’s Lamont-Doherty Earth
Observatory and NASA Goddard Institute for Space Studies.
“We
can now say with some degree of confidence that what we’re seeing
in that part of the Mediterranean is likely separable from natural
variability,” Cook said. “If climate change is having an impact
and is making droughts worse, then we should see this in the record
over several centuries—and we do.”
Cook
and his colleagues are among the first scientists to analyze data
from the 2015 Old
World Drought Atlas,
which maps dryness and wetness year-by-year across the Mediterranean
over the past millennium based on tree ring records. The results
provide new insight into climate patterns that can help scientists
develop better drought risk assessments and improve understanding
both of the bounds of natural climate variability and what may be the
emerging influence of greenhouse gases.
Drying
in the Levant
Climate
models suggest that the Levant is likely to grow drier as global
temperatures rise. Some studies suggest that may already be
happening. A 2015
study that
linked the recent Syrian drought to the start of the civil war there
determined that increasing levels of greenhouse gases from human
activities like the burning of fossil fuels had likely increased the
probability of such a severe and persistent drought in the region,
making it two to three times more likely.
“After
the release of the Old World Drought Atlas last year, we have been
eagerly awaiting research like this that places recent droughts in
the context of variability over the past millennium, far longer than
the century or so that can be analyzed with weather station records,”
said Richard Seager, a climate scientist at Lamont and co-author of
the 2015 Syria drought study.
“Cook
and colleagues’ result is consistent with human-driven climate
change and drying of the Middle East having contributed to the
severity of the drought. This also raises confidence in climate
models that indicate the eastern Mediterranean will be a hot spot of
aridification due to rising greenhouse gases and that this change is
already underway,” Seager said.
Mediterranean
Climate Patterns
Across
the Mediterranean, the new study shows some basic patterns. Large
droughts tend to affect both the west end of the Mediterranean
and the east end at the same time. North and South tend to be
the opposite: When Turkey and the Anatolian region are wetter than
normal, coastal Libya, Egypt and the Levant tend to be drier than
normal. The study highlights several periods of persistent drought
across the entire Mediterranean region during the Medieval period,
including in the 1100s, 1200s and 1300s, but the scientists found no
evidence of megadroughts of more than 30 years as studies have shown
in North
America during
that time.
Some
unusual patterns also appear that provide fodder for future research.
For example, a nearly two-decade period of excessive rain from 1125
to 1142 in what today would be Spain, Morocco, Algeria, Tunisia,
Italy, the Balkans and Turkey occurred at the same time as extended
drought across much of North America.
That suggests that there could
have been a large-scale shift in atmospheric circulation across the
Atlantic, possibly related to sea surface temperatures, the authors
write.
This
new ability to see how periods of drought and extreme wetness play
out across the Mediterranean basin over a millennium can help provide
better evaluations of climate model simulations, Cook said. Climate
model simulation, in turn, may tell scientists more about the
dynamics and the drivers of different drought events.
“Even
if you don’t care about climate change, our instrumental records
are pretty short, so our understanding of natural variability is
pretty limited by the last 100 years of data. These longer paleo
records allow us to better characterize natural variability to really
better understand, for example, how often droughts of a certain
magnitude would happen even without climate change,” Cook said.
The
Old World Drought Atlas was created from the analysis of thousands of
tree ring samples collected into local chronologies. In the Levant,
tree ring scientists have been able to date existing forests and
timber beams and cedar flooring in ancient buildings going back to
the 1300s. Older chronologies from within a 1,000 kilometer radius
help to fill in details for the region going farther back in time.
Cook’s
co-authors on the new study are Kevin Anchukaitis, Ramzi
Toucahn and David Meko of the University of Arizona, and Edward
Cook,
a research professor at Lamont and co-founder of the Lamont Tree
Ring Lab.
A
new study looks at recent droughts across the Mediterranean in the
context of the past 900 years. The charts highlight dry years
(brown) and wet years (blue) in three regions. The map, from the Old
World Drought Atlas, shows the period from 1980 to 2012. From Cook,
et al., 2016.
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