Calmer breezes across the world are rais­ing con­cerns among cli­mate sci­en­tists. MICHAEL LUCY ex­plains.

Cosmos - - Digest -

THE WIND ISN’T what it used to be. Sci­en­tists say sur­face wind speeds across the planet have fallen by as much as 25% since the 1970s. The eerie phe­nom­e­non – dubbed ‘still­ing’ – is be­lieved to be a con­se­quence of global warm­ing, and may im­pact every­thing from agri­cul­ture to the live­abil­ity of our cities. It has taken more than a decade for sci­en­tists to get a han­dle on still­ing, a term coined by Aus­tralian Na­tional Univer­sity eco­hy­drol­o­gist Michael Rod­er­ick in 2007.

Rod­er­ick had spent years study­ing a 50-year de­cline across Europe and North Amer­ica of a cli­mate met­ric called pan evap­o­ra­tion. It mea­sures the rate at which wa­ter evap­o­rates from a dish left out­side. With his col­league bio­physi­cist Gra­ham Far­quhar, he found the cause: the sun­light had dimmed due to air pol­lu­tion. Less light equals slower evap­o­ra­tion.

In 2002, af­ter pub­lish­ing the ex­pla­na­tion in the jour­nal Science, Rod­er­ick re­ceived a query from Roger Beale, the head of Aus­tralia’s fed­eral depart­ment for the en­vi­ron­ment. Was pan evap­o­ra­tion also de­clin­ing in Aus­tralia? “To my em­bar­rass­ment,” Rod­er­ick re­calls, “I had to say I didn’t know, be­cause I’d never looked.”

Two years later, he had an an­swer: the pan evap­o­ra­tion rate was also fall­ing in Aus­tralia. It was puz­zling, how­ever, as air pol­lu­tion lev­els on the con­ti­nent were lower than those of Europe or North Amer­ica.

Rod­er­ick went back to ba­sics. The rate of evap­o­ra­tion de­pends on four fac­tors: air tem­per­a­ture, hu­mid­ity, the amount of so­lar ra­di­a­tion and wind speed. Af­ter an­other three years of comb­ing through me­te­o­ro­log­i­cal records, he had pinned down the cul­prit: “To my ab­so­lute sur­prise, we found the main rea­son for the drop in Aus­tralia was less wind – and by a lot.”

Rod­er­ick un­earthed other stud­ies from around the world with sim­i­lar find­ings, but till then no one had joined the dots.

He teamed up with Tim Mcvicar, a hy­drol­o­gist at Aus­tralia’s na­tional science agency, the CSIRO, who was look­ing for global wind pat­terns and their ef­fects on evap­o­ra­tion. In 2012 this team – led by Mcvicar – compiled re­sults from al­most 150 regional stud­ies to show still­ing was tak­ing place across much of the world.

In Aus­tralia in the 1970s, av­er­age wind speed a cou­ple of me­tres above the ground was 2.2 me­tres per sec­ond: in 2017 it was 1.6 me­tres per sec­ond.

Over land­masses from as far north as Sval­bard, 1,050 km from the North Pole, to as far south as the coast of Antarc­tica, “ob­ser­va­tions show that wind is still­ing”, Mcvicar says.

Con­versely, the wind is get­ting faster around the poles and in cer­tain coastal ar­eas. In a per­plex­ing twist, ocean winds also ap­pear to be accelerating.

Sev­eral ex­pla­na­tions have been pro­posed for the still­ing.

Robert Vau­tard, who stud­ies cli­mate change at France’s Na­tional Cen­tre for Sci­en­tific Re­search, has a be­nign an­swer for some of the change: more veg­e­ta­tion, spurred by ris­ing tem­per­a­tures and car­bon diox­ide lev­els. It in­creases ‘sur­face rough­ness’, which slows the wind.

The planet’s ris­ing tem­per­a­tures are an­other likely cul­prit.

One pro­jected con­se­quence of global warm­ing is ex­pan­sion of the ‘Hadley cell’, a planet-girdling dou­ble dough­nut of at­mo­spheric cir­cu­la­tion in which warm air rises near the equa­tor, loops to­wards the poles, cools and falls to the sur­face at

around 30 or 40 de­grees lat­i­tude, then heads back to its ori­gin. This cir­cu­la­tion, com­bined with the Co­ri­o­lis ef­fect of Earth’s ro­ta­tion, causes the con­sis­tent east­erly trade winds found in the trop­ics and the pre­vail­ing west­er­lies of the mid­dle lat­i­tudes. An ex­pand­ing Hadley cell means many common storm tracks are slip­ping to­wards the poles, tak­ing their high winds – and as­so­ci­ated rain­fall – away from the tem­per­ate re­gions.

Rod­er­ick takes a more tele­scopic view: air move­ments are pow­ered by dif­fer­ences in tem­per­a­ture at dif­fer­ent places. The big­ger the dif­fer­ence be­tween warm and cold air, the stronger the wind. One ef­fect of global warm­ing is to flat­ten those dif­fer­ences. The poles are warm­ing faster than the equa­tor, win­ters are warm­ing faster than sum­mers, and nights warm­ing faster than days. “Every­thing be­comes more uni­form,” Rod­er­ick says.

What does the drop in wind speed mean? The de­crease in evap­o­ra­tion has im­me­di­ate im­pli­ca­tions for the pre­ci­sion cal­cu­la­tions used in mod­ern ir­ri­ga­tion, and more com­plex ef­fects on rain­fall pat­terns.

While less evap­o­ra­tion may be good for some plants in arid ar­eas, still­ing may make oth­ers less able to dis­perse wind-blown seed to suit­able new habi­tats, and hence less re­silient to cli­mate change.

Less wind could also hurt city-dwellers. In what may be a taste of things to come, the win­ter of 2016/17 saw Europe be­calmed, lead­ing to smog so bad that Paris banned cars for six days, and the city of Skala in Poland briefly over­took Bei­jing atop the world’s air-pol­lu­tion ta­bles.

Po­ten­tial ef­fects on wind power are an­other area of con­cern, though there does not ap­pear to be any­thing to worry about in the short term. Still­ing has so far been de­tected only at heights up to 10 me­tres, while tur­bines har­vest their en­ergy 50 to 150 me­tres above the ground.

“We cer­tainly haven’t seen any­thing that looks like still­ing,” says Keith Ay­otte, the chief sci­en­tist of Aus­tralian wind power out­fit Wind­lab, who mon­i­tors more than 100 sites across the world where the com­pany has tur­bines.

Though these higher-al­ti­tude winds will change over the 21st cen­tury, Vau­tard has used cli­mate sim­u­la­tions to project the ef­fect on to­tal wind power avail­able across Europe is un­likely to be more than 5%. One dif­fi­culty with pre­dic­tion is a lack of ob­ser­va­tions. As Mcvicar notes, ac­cu­rate and con­sis­tent mea­sure­ments only ex­ist “for the past 40 or 50 years”.

Ce­sar Azorin-molina, a cli­mate sci­en­tist at the Univer­sity of Gothen­burg in Swe­den, has em­barked on an EU­funded archival project with the ef­fort­ful acro­nym STILL­ING: “To­wards im­proved un­der­stand­ing of the world­wide de­cline of wind speed in a cli­mate change sce­nario.”

His mission is “res­cu­ing historical wind speed data” like log­books from Ponta Del­gada in the Azores and Blue Hill Ob­ser­va­tory in the US that go back more than a cen­tury. The age of anemome­ters – the de­vices that mea­sure wind speed – can af­fect read­ings but, by com­pil­ing a sin­gle set of qual­ity-con­trolled data, Azorin-molina hopes to de­ter­mine whether still­ing is purely a re­cent phe­nom­e­non or if sim­i­lar de­clines have hap­pened in the past.

For Mcvicar, the still­ing of the planet’s winds is a re­minder that global warm­ing has mul­ti­ple and un­pre­dictable flow-on ef­fects. “We’re deal­ing with cli­mate change, not just ris­ing tem­per­a­tures.”

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