"Long before it's in the papers"
January 27, 2015


Global warming already causes some droughts, scientists say

Oct. 28, 2011
Courtesy of NOAA
and World Science staff

Win­ter­time droughts are in­creas­ingly com­mon in the Med­i­ter­ra­nean re­gion, partly because of hu­man-caused cli­mate change, ac­cord­ing to a new anal­y­sis by U.S. gov­ern­ment sci­en­tists.

If cor­rect, the find­ings would mean that some of the droughts pre­dicted by mod­els of glob­al warm­ing and its con­se­quenc­es have al­ready oc­curred, some­thing that had not been clear. 

Reds and oranges hig­hlight lands around the Me­di­ter­ra­nean that ex­perienced sig­ni­fi­cantly drier win­ters dur­ing 1971-2010 than a  com­par­i­son per­iod of 1902-2010. (Cre­dit: NOAA)

In the last two dec­ades, 10 of the dri­est 12 win­ters have tak­en place in the lands sur­round­ing the Med­i­ter­ra­nean Sea, the re­search­ers found. “The mag­ni­tude and fre­quen­cy of the dry­ing… is too great to be ex­plained by nat­u­ral vari­abil­ity alone,” said Mar­tin Ho­er­ling of the U.S. Na­tional Oce­an­ic and At­mos­pher­ic Ad­min­istra­t­ion’s Earth Sys­tem Re­search Lab­o­r­a­to­ry in Boul­der, Co­lo., lead au­thor of a pa­per pub­lished on­line in the Jour­nal of Cli­mate this month.

“This is not en­cour­ag­ing news for a re­gion that al­ready ex­pe­ri­ences wa­ter stress, be­cause it im­plies nat­u­ral vari­abil­ity alone is un­likely to re­turn the re­gion’s cli­mate to nor­mal,” added Ho­er­ling, who worked with col­leagues at the Co­op­er­a­tive In­sti­tute for Re­search in En­vi­ron­men­tal Sci­ences in Boul­der, Co­lo.

The researchers published a map of Europe high­light­ing which coun­tries were in­creas­ingly dry in win­ter. A strik­ing aspect of the map was that some of the coun­tries awash in red splotches re­pre­sent­ing dry­ness were the same ones—in­clud­ing Greece, Por­tu­gal, Italy and Spain—whose un­paid debts are cur­rently threa­ten­ing the global fin­ancial sys­tem.

The Med­i­ter­ra­nean re­gion ac­cu­mu­lates most of its pre­cipita­t­ion dur­ing the win­ter. Ho­er­ling’s team iden­ti­fied a pat­tern of in­creas­ing win­tertime dry­ness stretch­ing from Gi­bral­tar to the Mid­dle East. Sci­en­tists used ob­serva­t­ions and cli­mate mod­els to in­ves­t­i­gate sev­er­al pos­si­ble cul­prits, in­clud­ing nat­u­ral vari­abil­ity, a cy­cli­cal cli­mate pat­tern called the North At­lantic Os­cilla­t­ion, and cli­mate change caused by green­house gas­es re­leased in­to the at­mos­phere dur­ing fos­sil fu­el use and oth­er hu­man ac­ti­vi­ties.

Cli­mate change from green­house gas­es ex­plained roughly half the in­creased dry­ness of 1902-2010, the team found. This means oth­er pro­cesses not spe­cif­ic­ally iden­ti­fied would al­so have con­tri­but­ed to in­creas­ing drought fre­quen­cy in the re­gion.

The team al­so found agree­ment be­tween the ob­served in­crease in win­ter droughts and in the pro­jec­tions of cli­mate mod­els that in­clude known in­creases in green­house gas­es. Both ob­serva­t­ions and mod­el sim­ula­t­ions show a sud­den shift to dri­er con­di­tions in the Med­i­ter­ra­nean be­gin­ning in the 1970s. The anal­y­sis be­gan with the year 1902, the first year of a recorded rain­fall dataset.

The Med­i­ter­ra­nean has long been iden­ti­fied as a “hot spot” for sub­stantial im­pact from cli­mate change in the lat­ter dec­ades of this cen­tu­ry be­cause of wa­ter scarcity in the re­gion, a rap­idly in­creas­ing popula­t­ion, and cli­mate mod­eling that pro­jects in­creased risk of drought.

“The ques­tion has been wheth­er this pro­jected dry­ing has al­ready be­gun to oc­cur in win­ter, the most im­por­tant sea­son for wa­ter re­sources,” Ho­er­ling said. “The an­swer is yes.”

In the Med­i­ter­ra­nean, win­ter drought is a “new nor­mal” that could threat­en food se­cur­ity, the re­search­ers said. Lessons learn­ed from stu­dy­ing cli­mate in that re­gion may al­so be rel­e­vant for the U.S. West Coast, they added, which has a si­m­i­lar cli­mate to the Med­i­ter­ra­nean re­gion of Eu­rope and North Af­ri­ca.

* * *

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Wintertime droughts are increasingly common in the Mediterranean region, and human-caused climate change is partly responsible, according to a new analysis by U.S. government scientists. If correct, the findings would mean that some of the droughts predicted by models of global warming and its consequences have already occurred, something that had not been clear. In the last two decades, 10 of the driest 12 winters have taken place in the lands surrounding the Mediterranean Sea, the researchers found. “The magnitude and frequency of the drying… is too great to be explained by natural variability alone,” said Martin Hoerling of the U.S. National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo., lead author of a paper published online in the Journal of Climate this month. “This is not encouraging news for a region that already experiences water stress, because it implies natural variability alone is unlikely to return the region’s climate to normal,” added Hoerling, who worked with colleagues at the Cooperative Institute for Research in Environmental Sciences in Boulder, Colo. The Mediterranean region accumulates most of its precipitation during the winter. Hoerling’s team identified a pattern of increasing wintertime dryness stretching from Gibraltar to the Middle East. Scientists used observations and climate models to investigate several possible culprits, including natural variability, a cyclical climate pattern called the North Atlantic Oscillation, and climate change caused by greenhouse gases released into the atmosphere during fossil fuel use and other human activities. Climate change from greenhouse gases explained roughly half the increased dryness of 1902-2010, the team found. This means other processes not specifically identified would also have contributed to increasing drought frequency in the region. The team also found agreement between the observed increase in winter droughts and in the projections of climate models that include known increases in greenhouse gases. Both observations and model simulations show a sudden shift to drier conditions in the Mediterranean beginning in the 1970s. The analysis began with the year 1902, the first year of a recorded rainfall dataset. The Mediterranean has long been identified as a “hot spot” for substantial impact from climate change in the latter decades of this century because of water scarcity in the region, a rapidly increasing population, and climate modeling that projects increased risk of drought. “The question has been whether this projected drying has already begun to occur in winter, the most important season for water resources,” Hoerling said. “The answer is yes.” In the Mediterranean, winter drought is a “new normal” that could threaten food security, the researchers said. Lessons learned from studying climate in that region may also be relevant for the U.S. West Coast, they added, which has a similar climate to the Mediterranean region of Europe and North Africa.