New Research Sheds Light on Changing Nature of Droughts

Summary

New research pub­lished in the International Journal of Climatology focuses on under­stand­ing the fac­tors behind pro­longed droughts in Italy, which are becom­ing more intense and last­ing longer due to ris­ing tem­per­a­tures. The study found that wide­spread multi-year droughts in Italy are trig­gered by spe­cific atmos­pheric cir­cu­la­tion pat­terns, such as the North Atlantic Oscillation, and that reli­able pre­dic­tions for these events are still a chal­lenge, high­light­ing the impor­tance of inno­v­a­tive water resource man­age­ment and the need for more trust­wor­thy sea­sonal fore­cast mod­els.

New research explores the dynam­ics behind droughts per­sist­ing in spe­cific areas for extended peri­ods.

The study, pub­lished by the International Journal of Climatology, focused on Italy to under­stand the fac­tors trig­ger­ing multi-year drought phe­nom­ena.

“Even if these phe­nom­ena can be con­sid­ered cycli­cal and typ­i­cal in the region, they now tend to last longer, their inten­sity is increas­ing and their impacts are broader,” Salvatore Pascale, a researcher at the University of Bologna’s depart­ment of physics and astron­omy, told Olive Oil Times.

The sci­en­tists defined a wide­spread multi-year drought as an event last­ing more than 12 months, often extend­ing over two or three years.

To mea­sure these con­di­tions, the authors relied on the stan­dard­ized pre­cip­i­ta­tion evap­o­tran­spi­ra­tion index (SPEI), which con­sid­ers pre­cip­i­ta­tion and mois­ture loss due to evap­o­tran­spi­ra­tion. Evapotranspiration occurs when water evap­o­rates from water bod­ies and soil and is tran­spired by plants.

A wide­spread multi-year drought begins when it affects at least 30 per­cent of Italian ter­ri­tory (SPEI +1) and ends when more than 70 per­cent returns to near-nor­mal con­di­tions (SPEI ‑1).

The study focused on events over the last 123 years in Italy.

“We found that such events fre­quently impact more than 30 per­cent of the ter­ri­tory,” Pascale said. ​“In most cases, these droughts affected nearly the entire coun­try, reach­ing about 60 to 70 per­cent of Italy dur­ing their peak.” 

Even the most recent intense wide­spread multi-year drought involved large areas of Italy. ​“One might assume the warmer south is most affected, but that’s not the case,” Pascale said.

When droughts per­sist, they worsen con­di­tions in areas already impacted by accel­er­at­ing evap­o­tran­spi­ra­tion.

“Temperature is the pri­mary fac­tor influ­enc­ing this process,” Pascale said. ​“Other fac­tors like wind also mat­ter, but tem­per­a­ture is dom­i­nant. Surface tem­per­a­tures are ris­ing due to global warm­ing, caus­ing a decline in avail­able sur­face water and soil mois­ture.”

The loss of soil mois­ture directly impacts agri­cul­ture and soil fer­til­ity.

“Considering soil mois­ture loss alone, droughts are clearly becom­ing more severe, with longer-last­ing con­se­quences,” Pascale said.

The researchers empha­sized how tem­per­a­ture has become as crit­i­cal as rain­fall in shap­ing drought dynam­ics. While rain­fall vol­umes have not decreased, the capac­ity of soil and veg­e­ta­tion to retain water has sig­nif­i­cantly declined due to ris­ing evap­o­ra­tive demand.

Currently, rain­fall vol­umes have a lim­ited impact in a rapidly chang­ing sce­nario.

“We observe wors­en­ing extreme events becom­ing more intense and fre­quent,” said Pascale, spec­i­fy­ing that over­all rain­fall vol­umes remain rel­a­tively sta­ble.

“Instead, tem­per­a­tures con­tinue ris­ing, accel­er­at­ing the deple­tion of sur­face water,” he added.

In Italy, higher tem­per­a­tures also shorten the snow sea­son. Alpine snow accu­mu­la­tion is reduced and melts ear­lier than in the past.

“The moun­tain water we his­tor­i­cally relied upon in sum­mer is increas­ingly unavail­able. The buffer­ing role of snow and water stor­age his­tor­i­cally played by Italy’s moun­tains is dimin­ish­ing,” Pascale said.

Reduced snow also means crit­i­cal rivers for agri­cul­ture, such as the Po River, may no longer pro­vide suf­fi­cient water to regions heav­ily depen­dent on farm­ing.

“The aggres­sive­ness of the wide­spread multi-year drought event from 2021 to 2023 was aston­ish­ing. The Po River reached lev­els never recorded in the last 200 years,” Pascale said.

The study high­lights that recent drought sever­ity results from per­sis­tent weather pat­terns main­tain­ing high-pres­sure sys­tems over Europe, block­ing pre­cip­i­ta­tion.

Widespread multi-year drought events in Italy orig­i­nate from spe­cific atmos­pheric cir­cu­la­tion pat­terns, such as the North Atlantic Oscillation (NAO).

“The per­sis­tence of the pos­i­tive phase of the NAO results in less rain­fall over the Mediterranean, as Atlantic storm sys­tems are pushed toward the north­east, toward Scotland and Scandinavia.”

“Some of these con­di­tions per­sist over Europe for extended peri­ods, some­times recur­ring mul­ti­ple times within two or three years,” he added.

In such sce­nar­ios, wide­spread multi-year droughts occur pre­dom­i­nantly under anti­cy­clonic con­di­tions, char­ac­ter­ized by higher-than-aver­age tem­per­a­tures and reduced rain­fall.

“These phe­nom­ena reflect nor­mal atmos­pheric vari­abil­ity and are not dri­ven by global warm­ing,” Pascale said. ​“Understanding if and how global warm­ing influ­ences atmos­pheric cir­cu­la­tion dynam­ics is very chal­leng­ing; cli­mate mod­els on this topic remain unre­li­able.”

“Global warm­ing means the cli­mate retains more energy, caus­ing tem­per­a­tures to rise, a ther­mo­dy­namic real­ity,” Pascale added. 

“Rainfall, how­ever, depends not only on ther­mo­dy­nam­ics but also on dynam­ics such as wind pat­terns,” he con­tin­ued. ​“Rainfall forms when winds con­verge, accu­mu­lat­ing humid­ity. That accu­mu­lated water must then be released as pre­cip­i­ta­tion.” 

According to the researcher, vol­umes depen­dent on wind and cir­cu­la­tion dynam­ics are more com­plex than tem­per­a­ture-dri­ven changes.

“That means we need sig­nif­i­cantly more time and fur­ther warm­ing to observe clear trends,” Pascale said.

The authors cross-checked data from two dis­tinct cli­mate datasets to strengthen their find­ings and uti­lized pre­cip­i­ta­tion-only and com­bined indices.

This approach helps address regional data dis­crep­an­cies and enhances con­fi­dence in the study’s con­clu­sions.

What researchers do know for sure, how­ever, is that droughts will become more severe, with increas­ingly harsh impacts on affected areas.

“This means wise, inno­v­a­tive and dynamic man­age­ment of water resources becomes cru­cial. Not only for pop­u­la­tions and agri­cul­ture but also for indus­try, which con­sumes approx­i­mately 30 per­cent of potable water,” Pascale said.

Climatologists are not yet capa­ble of reli­ably pre­dict­ing wide­spread multi-year drought events. However, exten­sive research is under­way to develop trust­wor­thy sea­sonal fore­cast mod­els.

“We fore­see a future where sea­sonal fore­casts become reli­able enough to pre­dict when wide­spread multi-year droughts will start,” Pascale said. ​“We have short-term fore­casts, which we use daily, and long-term cli­mate pro­jec­tions span­ning decades or even cen­turies.” 

“In between are sea­sonal fore­casts, such as pro­jec­tions made in April for the upcom­ing sum­mer,” he added. ​“These fore­casts account for pre­dictable and slowly evolv­ing cli­mate fac­tors, like El Niño events, trig­gered by warm­ing Pacific Ocean waters.”

The goal is fore­cast­ing weather three to six months in advance.

“However, these mod­els are not yet reli­able, with high lev­els of uncer­tainty. The good news is that exten­sive research is under­way, high­light­ing their poten­tial impor­tance in our rapidly chang­ing cli­mate,” Pascale said.

Predicting wide­spread multi-year drought would enhance the abil­ity of coun­tries to adapt.

“Adaptation is essen­tial because we are already expe­ri­enc­ing cli­mate change. In many sec­tors, adap­ta­tion is our only viable response, neces­si­tat­ing mea­sures to mit­i­gate drought impacts,” Pascale said.

“But adap­ta­tion does­n’t solve the root prob­lem. True mit­i­ga­tion of global warm­ing requires dra­mat­i­cally reduc­ing or even elim­i­nat­ing green­house gas emis­sions,” he added.

“Should human­ity ever achieve this goal, it will still take con­sid­er­able time. Meanwhile, adap­ta­tion, along­side devel­op­ing more reli­able sea­sonal fore­casts, remains our best strat­egy,” Pascale con­cluded.