The time lag before increasing carbon dioxide levels for El Niños originating in the eastern tropical Pacific is about 8.5 months, the researchers say, whereas the lag for El Niños starting in the central tropical Pacific is about 5.2 months. However, the sensitivity of carbon dioxide rise to tropical near-surface air temperatures is similar for each type of El Niño.

The results should enable scientists to better constrain the effects of El Niños in climate models.

"The time lag can be used to form a reasonable hypothesis about the way vegetation reacts to the El Niño effects," said Petr Chylek of Los Alamos National Laboratory, US.

It is common knowledge that climate is affected by atmospheric carbon dioxide concentrations, but what can be less obvious is how carbon dioxide is affected by the climate. When it comes to El Niños, the important factors are the absorption of carbon dioxide by the oceans and by vegetation.

An El Niño results from a warm ocean, which is less able to absorb carbon dioxide than a cold one; what’s more, an El Niño has the potential to cause droughts in some regions, either inducing wildfires or restricting the growth of vegetation, so limiting its capacity to photosynthesize.

The overall effect is that any El Niño leads to a delayed rise in carbon dioxide – although "most experts are of an opinion that the vegetation effect probably dominates," said Chylek.

The resulting rise in carbon dioxide is usually between one and two parts per million – roughly the same as the overall annual rise in the gas – and is short-lived, lasting about a year. Nevertheless, understanding its dynamics is important for fine-tuning climate models.

Chylek and coworkers analysed historical data on tropical near-surface air temperatures from the University of Alabama, US, and carbon dioxide levels from the Mauna Loa Observatory in Hawaii, for the years 1960–2016. As expected, they found that each peak in temperature was followed by an increase in carbon dioxide, although the time lag was different depending on the El Niño origin.

The researchers suspect the reason El Niños originating in the central tropical Pacific have a smaller time lag is they can be linked to wildfires, whereas El Niños originating in the eastern tropical Pacific can be linked to changes in vegetative respiration. "Future research will confirm or reject our hypothesis," they said.

The team, which published the study in Environmental Research Letters (ERL), now plans to follow the paths taken by El Niños in terms of their changing precipitation, to see from satellite data the effect on vegetation.

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