WHAT HAS BEEN CAUS­ING 2018’S SUM­MER HEAT­WAVE?

Focus-Science and Technology - - DISCOVERIES -

This sum­mer has seen ex­treme weather right across the North­ern Hemi­sphere, seem­ingly far be­yond what has been seen in pre­vi­ous years. Wide­spread heat­waves have been observed on ev­ery con­ti­nent, with weather records be­ing bro­ken left, right and cen­tre. On 27 June, Oman re­ported a night-time tem­per­a­ture that never dropped be­low 42.6°C, a world record for the high­est min­i­mum tem­per­a­ture within a 24-hour pe­riod. Across the Red Sea, in the Sa­hara Desert, a new con­ti­nen­tal record max­i­mum day­time tem­per­a­ture of 51.3°C was observed. Else­where on the planet, lo­cal tem­per­a­ture records have been bro­ken in re­gions as di­verse as the Arc­tic Cir­cle, the US, Ja­pan, Greece and the UK.

In many places the heat has been made worse by a lack of rain, which, if present, takes some of the en­ergy from the Sun in the form of evap­o­ra­tion, thereby leav­ing less of the ‘felt heat’ in the sur­round­ing area (a form of heat me­te­o­rol­o­gists call la­tent heat.). Satel­lite im­ages of Great Bri­tain show a clear and strik­ing brown­ing of the en­tire coun­try for this sum­mer com­pared with last, and hosepipe bans have been put in place in some coun­ties to con­serve water. In­evitably, hand-in-hand with hot, dry conditions come wild­fires, and much of the hemi­sphere ex­pe­ri­enced wide­spread loss of forests, other veg­e­ta­tion, and hu­man lives. Nowhere more so than Greece, where wild­fires were vis­i­ble from space, with strong winds com­pound­ing the out­break, spread­ing the fires faster and dis­pers­ing the ash to the sur­round­ing re­gions, lead­ing to Hol­ly­wood­style apoc­a­lyp­tic scenes of rag­ing fires and ash-cov­ered streets be­low an omi­nous red sky.

But in this world tainted by hu­manin­duced cli­mate change, are these ex­traor­di­nary weather events re­ally a sur­prise? Some cau­tion is re­quired here, be­cause while it’s true that Earth’s land has warmed by 1.6°C since prein­dus­trial times, cli­mate and weather pat­terns other than global warm­ing can play crit­i­cal roles in all types of ex­treme weather, in­clud­ing those seen this sum­mer.

El Niño, a well-known global cli­mate pat­tern that’s as­so­ci­ated with cen­tral Pa­cific ocean tem­per­a­tures, causes even warmer heat­waves, and in­deed led to 2016 be­ing the warm­est year on record. But this sum­mer El Niño has been in a neu­tral phase, mean­ing that the wide­spread ex­treme heat oc­curred with­out the help of this nat­u­ral mode of vari­abil­ity – mak­ing the heat and wild­fires even more ex­traor­di­nary.

An­other factor in cli­mate vari­abil­ity is the jet stream, which is re­spon­si­ble for ex­treme weather in the mid­lat­i­tudes. The high-in­ten­sity winds of the jet stream cir­cum­nav­i­gate the globe at around 10km above sea level and fa­cil­i­tate the move­ment of at­mo­spheric waves, sim­i­lar to the waves we ob­serve on the beach but far larger in scale. Much like waves on a beach, these at­mo­spheric waves can break, which is what we saw over north­ern Europe and Ja­pan, cre­at­ing weather pat­terns known as at­mo­spheric blocks – re­gions of high pres­sure. But the Euro­pean block­ing this sum­mer was spe­cial: re­lent­lessly static, and al­most as if it was nailed in place over Scan­di­navia. The con­se­quences? A com­plete block­ing of any cooler and un­sta­ble weather com­ing from the west, along with the cre­ation of cloud­free re­gions over north­ern Europe and the UK, leav­ing the land at the mercy of di­rect sun­light.

When it comes to these cli­mate pat­terns, it’s of­ten about what side of the jet stream you are on, so while the UK has been ex­pe­ri­enc­ing months of sought-af­ter beach weather, Ice­land for in­stance has been ex­pe­ri­enc­ing a dreary, wet cou­ple of months. Un­der­stand­ing how cli­mate change may al­ter the ex­act po­si­tion of these pat­terns is there­fore of high pri­or­ity, but also prov­ing to be par­tic­u­larly prob­lem­atic. The con­sen­sus is that sum­mer block­ing conditions are un­likely to in­crease in duration, and in­deed may de­crease at low north­ern lat­i­tudes, as the block­ing sys­tems mi­grate pole­wards due to cli­mate change.

The fu­ture may have a whole bunch of un­cer­tain cir­cu­la­tion pat­terns in stores for us, but you can be sure that these pat­terns will be su­per­im­posed on a back­ground of much warmer air, mak­ing it ex­tremely likely that heat waves and wild­fires like this year will be­come the norm in the decades ahead. In­deed, if we don’t act to sta­bilise our cli­mate now, a typical weather re­port in 50 years’ time may read, ‘Conditions this year are rel­a­tively cool, with tem­per­a­tures and wild­fires akin to those of 2018’. Let’s not wait to see what an ex­treme sum­mer looks like in that world.

Dann Mitchell is a cli­mate sci­en­tist based at the Uni­ver­sity of Bris­tol.

BE­LOW: Much of Great Bri­tain has turned brown due to lack of rain, as seen in this satel­lite im­age taken by NASA at the end of July

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