The difficulties spread beyond Sumatra. During 2013, much of Singapore and southern Malaysia were shrouded in thick smog caused by the dense smoke billowing across the Strait of Malacca from Sumatran forest fires. Schools in Malaysia and Singapore had to close, and people with respiratory ailments and heart conditions had to stay indoors. This was no one-off incident. Large fires spark up on Sumatra most years, destroying forest, pumping vast quantities of carbon dioxide into the atmosphere, and smothering countries downwind in a toxic haze.

Many of the wildfires on Sumatra are triggered by "slash and burn" deforestation going out of control. But the ease with which fires start appears to be increasing, and many speculate that climate change is also playing a role. To investigate, Kartika Lestari from the University of Tokyo in Japan and colleagues used sea-surface temperature to drive two atmospheric general circulation models – one with and one without an anthropogenic warming component.

Focusing on the period between 1960 and 2011, the researchers showed that climate change has increased the frequency of drought in this region by around 20%. "We show that a weakening of the 'Walker circulation' [the easterly trade winds that move water and warm air to the west in the tropics] is associated with tropical ocean warming, which leads to a robust decrease of precipitation over Indonesia, and consequently the increase of drought frequency in Indonesia, including Sumatra," said Lestari, whose findings are published in Environmental Research Letters (ERL).

Looking ahead, Lestari and colleagues analysed Coupled Model Intercomparison Project Phase 5 model ensembles. They observed an increased drying trend over Indonesia, amplified by enhanced surface ocean warming in the central equatorial Pacific. The ensembles indicate that the reduction in rainfall will lead to a further 25% increase in severe drought events in the region by 2050, compared with now. "Without wiser reforms in land use and large caveats, this suggests that there will be more severe fires in Sumatra," said Lestari.

There is little that can be done to stop the droughts, but there is still time to take action and reduce the incidence of wildfires. "Biomass burning could be reduced by introducing stricter government policies and enforcing the law to limit logging by using combustion," said Lestari. Of course, it is not just Sumatra that will benefit from bringing the wildfires under control: the surrounding countries are keen to solve the problem too, ensuring that everyone can breathe easily.

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