For months, forest fires have been burning out of control across Indonesia, exacerbated by particularly dry El Niño conditions. The vast majority of these fires, which have been especially severe in Sumatra and Kalimantan, are set illegally by farmers and plantation companies to clear land. The consequences are disastrous: In 1997, the last major El Niño year, fires in Indonesia released roughly 13 to 40 percent of all global fossil fuel emissions that year and caused thousands of deaths.

Conditions are no better today. The World Resources Institute has reported that, since September, the daily emissions generated by Indonesia’s forest fires exceed the average daily emissions from all economic activity in the United States. As of this month, over 200,000 Indonesians in seven provinces have suffered respiratory illness; at least six people have died. President Widodo now faces growing domestic pressure to stop the fires and the illegal deforestation causing them.

Mr. Widodo has deployed over 25,000 personnel to control the fires and instructed the government to prosecute those who set them. But he understands that this is not enough. Mr. Obama should build on this momentum and help him implement measures now being discussed in Indonesia: a moratorium on all conversion of peatlands; re-wetting of degraded peatlands; a robust plan to monitor illegal land clearing; and a pledge to condition provinces’ access to funding on low deforestation rates. These strategies have worked in Brazil. If Mr. Widodo were to commit to these measures as part of a joint announcement with Mr. Obama that included a U.S. pledge of aid, he would have the political capital to follow through.

There is significant private sector interest in reducing deforestation. Industry leaders have grown fearful that ties to deforestation will trigger an international backlash. Since September 2014, five of the country’s largest palm oil producers — representing the vast majority of Indonesia’s palm oil market — have agreed to purge illegally forested palm oil from their supply chains, but they say that this will be difficult without better government policies. Absent such policies, the country’s most significant industry could suffer major financial losses — or, more likely, its leaders will abandon their environmental commitments.

Private foundations in America and Europe have also offered funding and technical assistance to help Indonesia’s small farmers increase productivity without resorting to illegal land clearing. Foreign governments have pledged support as well. Norway in 2010 dedicated a $1 billion pay-for-performance fund to combat deforestation, but much of it remains unspent for lack of progress.