The global weather cycle known as El Niño leads to a significant increase in bushfires and subsequent carbon emissions in much of pan-tropical Asia and South America, while eventually reducing the number of fires in northern Australia, according to a paper published this week in the journal Nature Climate Change.
The report says fires follow a predictable seasonal progression across tropical continents, which may help with fire-risk forecasting.
It found that they peaked in equatorial Asia early in the cycle, known as the El Niño/Southern Oscillation (ENSO), when El Niño was strengthening, from August to October. Then in the following year this increase in fire activity moved to south-east Asia and northern South America, from January to April, central America from March to May, and the southern Amazon from July to October.
In northern Australia, in the second year after an El Niño, the reduction in fuel availability caused by dry conditions resulted in far fewer fires during September and October.
A team of researchers led by Yang Chen from the University of California, Irvine, in the US, used satellite data to measure burnt areas and the resultant emissions, incorporating six El Niño and six La Niña events from 1997 to 2016. They found that, on average, reduced rainfall and water storage led to increased emissions attributable to fires in pan-tropical forests by 133% during and following El Niño, as compared with La Niña.
El Niño and La Niña – in Spanish, respectively, the little boy and the little girl – have perhaps the strongest influence on year-to-year climate variability in Australia. They are associated with sustained periods lasting months of warming (El Niño) or cooling (La Niña) in the central and eastern tropical Pacific.
An El Niño occurs when sea surface temperatures in the central and eastern tropical Pacific Ocean become substantially warmer than average, and this causes a shift in atmospheric circulation. Typically, the equatorial trade winds blow from east to west across the Pacific Ocean. El Niño events are associated with a weakening, or even reversal, of the prevailing trade winds.
Warming of ocean temperatures in the central and eastern Pacific causes this area to become more favourable for tropical rainfall and cloud development. As a result, the heavy rainfall that usually occurs to the north of Australia moves to the central and eastern parts of the Pacific basin.
This ENSO cycle recurs every two to seven years, the report notes, adding that it is “synchronised with the annual seasonal cycle, which contributes to the predictability of ENSO impacts”.
The changes to rainfall distribution caused by ENSO have a bigger impact on fire activity around the world than other climate factors, Chen’s team found. The effects have been observed in many parts of the world, including Australia, Indonesia, Africa, Canada, and the south-eastern
US. The report cites a strong El Niño event in 1997-1998 during which “widespread increases in fire emissions” occurred across multiple continents and caused “substantial environmental damage and socioeconomic loss”.
The study analysed information on fire emissions and burned-area estimates from the fourth edition – the latest available – of the Global Fire Emissions Database.
Researchers compared the advent of fires with rainfall, water storage, and fire-related emissions in eight tropical regions, and explored the differences for a 19-month ENSO interval that extended from June to December of the following year.
Fires released more emissions during and after El Niño compared with La Niña. But even though the differences in sea-surface temperatures that are the hallmark of the event were greatest from June to April, significant differences in emissions and burned areas, both positive and negative, persisted well beyond the end of the event.
This provides evidence that ecosystems respond slowly to climate factors such as ENSO, the report says, with consequences for fire ignition, spread and termination that emerge long after the event has dissipated.
Recognising this cause-and-effect patterns adds “a high degree of predictability” that may make it easier to manage ecosystems in a sustainable manner.