El Nino events significantly increase fire incidence and associated carbon emissions in pan-tropical forests, according to a paper published online this week in Nature Climate Change. The study finds that El Nino events reduce precipitation and water storage across pan-tropical forests, facilitating increased fire ignition and spread. These fires follow a predictable seasonal progression across tropical continents, which may help with fire-risk forecasting.

El Nino Southern Oscillation (ENSO) is a periodic variation in the coupled ocean-atmosphere system, with sea-surface temperature conditions - warm El Nino and cold La Nina - in the tropical eastern Pacific recurring every 2-7 years. ENSO has a pronounced influence on year-to-year variations in climate.

Yang Chen and colleagues identify the climatic conditions associated with burned area and fire emissions using satellite data from 1997-2016, covering six El Nino and six La Nina events. They find El-Nino-associated reductions in precipitation and water storage increase fire emissions in pan-tropical forests by 133% (on average) compared to La Nina events. The authors show that fires peak in equatorial Asia from August to October, while El Nino strengthens, before moving on to south-east Asia and northern South America from January to April of the following year, then Central America from March to May, and finally the southern Amazon from July to October.

The predictable cascade of fire described by the authors reveals an important lag in the Earth system response to ENSO, which could help improve seasonal fire-risk forecasts and explain some of the increased growth rate in atmospheric CO2 concentration during El Nino events.