PEKANBARU, the capital of Riau province on the Indonesian island of Sumatra, has been shrouded in an acrid white cloud of smoke so dense that visibility is down to 50 metres (about 160 feet). The air quality is officially described as “dangerous”, and most people are wearing face masks, even indoors. Nearly 50,000 people in Riau have already been treated for respiratory, eye or skin problems. All flights last week were cancelled, and only a few have got through since. The provincial governor has declared a state of emergency. The haze is back, and has arrived earlier this year than usual.

Drive to Pekanbaru from the ferries that dock at a north Sumatran port, and you can see what the cause of the problem is: mile upon mile of smouldering or charred land. Much of it is peat-bog, where fires can burn up to two metres underground and take weeks to end. A few minutes in, the inferno stings the eyes and sets off a hacking cough. These fires have been burning for nearly two months. According to Greenpeace, a pressure group, more than 1,000 fires covering at least 12,000 hectares (30,000 acres) of forest and peatland are burning in Riau. And that is just one province among several with fires blazing.

For Sumatra, it is an annual environmental trauma. But when the haze drifts across the Strait of Malacca to cover Singapore and Malaysia, it is a regional nightmare, too. Some schools began to close in Malaysia this month; the night air in Singapore smelled of bonfires before rain provided some relief. Last year the haze shattered all air-pollution records in both countries, and diplomatic rows broke out with Indonesia. It will take only a slight change in wind direction for Sumatra’s fires to cause an even bigger haze over the region this year.

Everyone knows why the haze happens, yet it keeps on occurring. That angers Indonesians as much as Singaporeans and Malaysians. The fires are usually started deliberately, often to clear land to make way for palm-oil plantations. Indonesia is the world’s biggest producer of palm oil. Successive governments have passed a mountain of laws and regulations outlawing burning, yet enforcement is so poor that people carry on regardless. On March 16th, when he dropped in on Riau on an emergency visit, President Susilo Bambang Yudhoyono said he was “ashamed” of the situation .

One recent fire illustrates many of the problems. Between Pekanbaru and the east coast is Giam Siak Kecil Bukit Batu, a huge UNESCO-listed peatland and forest reserve that is home to the Sumatran tiger, among other species. It has been invaded by over 2,000 people from north Sumatra who are clearing the land by burning about 3,000 hectares. Local village heads are suspected of selling plots of land to the newcomers, aided and abetted by a local military officer previously convicted of illegal logging in western Sumatra. That points to the habitual collusion between the authorities and those illegally clearing land, both of whom profit from the lucrative palm-oil business. In response to this case, the head of the army in Riau, Prihadi Agus Irianto, has said that it is time for the armed forces to acknowledge that officers are involved in illegal logging and burning. On March 12th a former governor of Riau, Rusli Zainal, was found guilty of graft, including issuing logging licences that resulted in forest destruction. He was sentenced to 14 years in prison. Yet, as some Indonesian politicians are keen to point out, the problem is not all down to official corruption. About half of the fires burn on plantations owned by big palm-oil and logging corporations. By law, they are supposed to be responsible for preventing and putting out fires on their concessions, but the regulations are seldom enforced. This year, using satellite imagery, Greenpeace claims to have found fires on land owned by 36 pulp-and-paper companies and 15 palm-oil companies. Many of these are local subsidiaries of Malaysian and Singaporean giants. Yet proving a deliberate intent to start the fires has always been hard. The case against one company, Adei Plantation and Industry, a subsidiary of Kuala Lumpur Kepong, listed in Malaysia, is being watched closely. The firm was one of eight accused last year of starting fires, and now—a first in Riau—a local manager and a director have been brought to trial. If found guilty, they face up to ten years in jail and the company could lose its permit.

The director of a local NGO, Muslim Rasyid of the Network for Riau Forest Rescue, argues that this is a landmark trial, and that if the company is punished, it could act as a deterrent to others. Perhaps, but first the trial has to reach its conclusion. Proceedings have been delayed. The judge, who is based in Jakarta, the capital, could not attend the courtroom last week because planes could not land in the haze.