Extreme El Niño events cause short-term CO₂ fluctuations, researchers find

A recent study challenges previous assumptions about the connection between CO₂ in the atmosphere and temperatures in the tropics. Between 1959 and 2011, the CO₂ content in the atmosphere responded twice as strongly to temperatures in the tropics than before.

This has often been attributed to increasing droughts in the tropics and to changes in carbon cycle responses caused by climate change. However, the new study conducted by researchers from the Max Planck Institute for Biogeochemistry and Leipzig University suggests that a small number of particularly strong El Niño events could be responsible for this.

Their findings are published in the journal Science Advances.

Both tropical and non-tropical ecosystems absorb large amounts of carbon that were previously released into the atmosphere through human CO₂ emissions. Globally, land surface ecosystems act as a carbon sink and absorb on average around a third of human CO₂ emissions. These ecosystems are therefore a natural buffer for climate change.

In the 1980s and 1990s, however, researchers observed an increased fluctuation in global carbon storage on land, and it appeared that the CO₂ growth rate was particularly sensitive to temperatures in the tropics. Researchers from Jena and Leipzig found that this “doubling” of sensitivity was caused by the increased occurrence of El Niño events in the 1980s and 1990s compared to 1960–1979.

This also includes the extreme El Niño events of 1982/83 and 1997/98. El Niño events cause severe droughts and heat waves in the tropics, which affect plant growth and thus reduce carbon uptake. In times of El Niño, vegetation even releases large amounts of carbon that would otherwise be sequestered in the soil or forests. This causes the CO₂ content in the atmosphere to increase.

Internal climate variability as the main factor for changes in the carbon cycle

The authors of the study emphasize that this CO₂ increase is due to internal climate variability rather than a systematic change in the carbon cycle caused by climate change. “Our results show that this doubling of sensitivity is not necessarily a sign of a fundamental change in the response of the carbon cycle to climate change,” says Na Li from the Max Planck Institute for Biogeochemistry, first author of the study. Instead, it is caused by the combination of extreme El Niño events and their global impact.

“Through our work, we were also able to show that this phenomenon is related to the ‘slow-in, fast-out’ dynamics of the carbon cycle. This means that carbon is only slowly absorbed by ecosystems, but can be suddenly and quickly released again during extreme weather events such as strong El Niños,” explains Professor Ana Bastos from Leipzig University, senior author of the study.

The results of this study are important because they highlight uncertainties in future climate projections. To date, it was assumed that a heightened sensitivity of the CO₂ increase to temperatures in the tropics is caused by long-term climate-related changes in the carbon cycle, and thus in the global climate system.

However, the study shows that extreme events can cause short-term fluctuations that do not necessarily indicate permanent changes in the carbon cycle.

“These new findings could help to develop more precise climate models and reduce the uncertainties in predicting future climate scenarios,” says junior professor Dr. Sebastian Sippel from Leipzig University.

He also stated that we need to better understand how extreme climate phenomena such as El Niño affect carbon dynamics in order to make more reliable forecasts for the future.