William Brangham:

Forest fires are common in Brazil this time of year, but their numbers have skyrocketed to more than 75,000 this year. That's up 80 percent from 2018. And the smoke from them has turned Sao Paulo's midday skies into total darkness.

Bolsonaro has said, with no evidence, that nonprofit groups started the fires to undermine him. But scientists point to three main causes all stemming from human activity that are driving this crisis, first, rampant deforestation.

A staggering amount of the Amazon rainforest gets cut down intentionally. In June alone, an area half the size of Rhode Island was lost. Much of the logging is illegal, and driven by rising agriculture in the region. Fire is routinely used by farmers to clear out the brush for farming and grazing.

Droughts have also played a key role. They occur naturally, but scientists say climate change is likely making them worse and more frequent.

The right-wing populist president has relaxed environmental protections and prioritized opening up the rainforest for development. Bolsonaro has called the fires an internal matter for Brazilians to resolve, but international pressure for action is mounting.

French President Emanuel Macron plans to highlight the crisis at this weekend's G7 summit with world leaders in France. And Macron threatened to withdraw French support for a trade deal between the European Union and South American countries, including Brazil, if immediate action is not taken. Ireland quickly followed suit.

For the "PBS NewsHour," I'm William Brangham