Climate change: why is El Niño “acting up”?

El Niño, unsea­son­ably high tem­per­a­tures, marine heat­waves, man-made cli­mate change… These phe­nom­e­na, which have been wide­ly pub­li­cised in recent months, can be con­fus­ing. Some­times their effects are sim­i­lar. Some­times there is a link between them. But beware: although they coex­ist at the moment, their pace, dura­tion and ori­gin are very different.

An El Niño of mod­er­ate inten­si­ty has been under­way since this sum­mer¹. The phe­nom­e­non had not occurred for 7 years and hit the head­lines because of ini­tial fore­casts – now down­grad­ed – sug­gest­ing the pos­si­bil­i­ty of an intense El Niño. El Niño is a nat­ur­al oscil­la­tion in cli­mate. Typ­i­cal­ly last­ing between 6 and 18 months, the phe­nom­e­non occurs every 2 to 7 years. This cycle alter­nates with neu­tral phas­es and La Niña phas­es, the oppo­site of El Niño. “We don’t real­ly know what trig­gers an El Niño,” says Lau­ri­ane Bat­té. “How­ev­er, the phe­nom­e­non and its con­se­quences are now very well documented.”

How can we iden­ti­fy an El Niño? First of all, there is a slow­down in the trade winds, which are the inter-trop­i­cal winds blow­ing from east to west. Unusu­al gales from the west occur in the Pacif­ic, and the cen­tral and east­ern equa­to­r­i­al Pacif­ic warms up. Cur­rent­ly, the sur­face tem­per­a­ture in the cen­tral equa­to­r­i­al Pacif­ic is 1.5°C above nor­mal. “Cer­tain pre­con­di­tions for the onset of El Niño make it pos­si­ble to antic­i­pate its occur­rence sev­er­al months in advance.” Explains Juli­ette Mignot “How­ev­er, its inten­si­ty is linked to the unusu­al west­er­ly gales: these are fair­ly unpre­dictable, which com­pli­cates cli­mate projections.”

The cur­rent scale of sea sur­face tem­per­a­ture anom­alies is such that it can only be a sig­na­ture of glob­al warming

What are the con­se­quences? Although the phe­nom­e­non is localised in the trop­i­cal Pacif­ic, it trig­gers cli­mat­ic tele­con­nec­tions across many regions of the globe²: the cli­mate varies up to sev­er­al thou­sand kilo­me­tres away from El Niño. Rain­fall increas­es in cer­tain regions of South Amer­i­ca and Cen­tral Asia, in the south­ern Unit­ed States and in the Horn of Africa³. Con­verse­ly, dry spells occur in Aus­tralia, Indone­sia and parts of south­ern Asia, Cen­tral Amer­i­ca and north­ern South Amer­i­ca. Tem­per­a­tures are ris­ing in some regions, and cyclonic activ­i­ty is also chang­ing: major cyclones are occur­ring in the Pacif­ic, Poly­ne­sia is more exposed to the risk, while the risk is reduced in the Atlantic basin. There is lit­tle impact on the Euro­pean climate.

From one year to the next, the glob­al cli­mate varies accord­ing to the pres­ence of El Niño or La Niña. But these phe­nom­e­na are not the only nat­ur­al mod­u­la­tions of the cli­mate sys­tem. Oth­ers are well known: the North Atlantic Oscil­la­tion, the South­ern Annu­lar Mode, and the Indi­an Ocean Dipole. Like El Niño, the Indi­an Ocean Dipole will con­tin­ue until at least Decem­ber⁴. “Along with El Niño, these vari­a­tions have the great­est impact on sea­son­al vari­a­tions in cli­mate from one year to the next,” explains Lau­ri­ane Bat­té. Oth­ers exist, on much larg­er time scales: the Atlantic mul­ti­decadal vari­abil­i­ty can per­sist in the same phase for 50 to 70 years!

On top of these phe­nom­e­na, we have the effect of human activ­i­ties on the cli­mate. But be warned: this is a change of spa­tial and tem­po­ral scale. This con­cerns the whole plan­et and is caus­ing a steady increase in the glob­al tem­per­a­ture – of the atmos­phere and the oceans – since the indus­tri­al era. Glob­al warm­ing linked to human activ­i­ties now stands at +1.1°C com­pared with the pre-indus­tri­al era⁵. And since 2012, the glob­al sea sur­face tem­per­a­ture has been sys­tem­at­i­cal­ly high­er than the aver­age for the years 1982 to 2011⁶.  “The cur­rent scale of sea sur­face tem­per­a­ture anom­alies is such that it can only be a symp­tom of glob­al warm­ing,” explains Lau­ri­ane Bat­té. “Nat­ur­al oscil­la­tions are localised, but in this case we’re see­ing warm­ing across the globe.”

As a result, cli­mate mea­sure­ments – ocean and atmos­pher­ic tem­per­a­tures – com­bine all these fac­tors. Long-term glob­al warm­ing linked to human activ­i­ties, short-term El Niño phe­nom­e­na and the ongo­ing Indi­an Ocean dipole.… “A very intense El Niño can increase the aver­age glob­al tem­per­a­ture by more than 1°C,” explains Juli­ette Mignot. “This year, the aver­age glob­al tem­per­a­ture will there­fore be high­er, but that doesn’t mean that the warm­ing caused by human activ­i­ty is accel­er­at­ing! This will be a sig­na­ture of El Niño, com­bined with glob­al warm­ing of anthro­pogenic origin.”

To fur­ther com­pli­cate the pic­ture, cli­mate change linked to human activ­i­ties is also mod­i­fy­ing nat­ur­al oscil­la­tions! For exam­ple, it is very like­ly that green­house gas emis­sions and/or ozone deple­tion con­tributed to a pos­i­tive phase in a cli­mat­ic oscil­la­tion cen­tred in Aus­tralia (the South­ern Annu­lar Mode) between the 1970s and 2000⁷. As for the Atlantic Mul­ti-Decadal Oscil­la­tion, in a cold phase between the 1960s and 1990s, it seems that its dura­tion and inten­si­ty were influ­enced by aerosol emis­sions linked to human and vol­canic activ­i­ties. “These exam­ples show that we are not just see­ing the effects of cli­mate change due to human activ­i­ties added to nat­ur­al vari­a­tions in the cli­mate, but that there is a real inter­ac­tion between the two,” adds Juli­ette Mignot. Lau­ri­ane Bat­té con­tin­ues: “As far as El Niño is con­cerned, on the oth­er hand, there is still a great deal of uncer­tain­ty about the impact of cli­mate change on the phe­nom­e­non.” On a day-to-day basis, the weath­er is the result of a com­bi­na­tion of fac­tors gov­erned by nat­ur­al vari­a­tions but also by human activ­i­ties. When faced with cli­mate-relat­ed infor­ma­tion, it’s vital to bear in mind the time and space scales involved.

Anaïs Marechal