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