The Amazon has been burning for weeks, creating clouds of smoke so intense that they plunged São Paulo, Brazil, into darkness this week. You can see the smoke blanket the country from space.

Tropical rainforests are some of the wettest ecosystems on earth, so how did the largest one—the Amazon—catch fire? According to experts, the answer lies not in temperatures or wind patterns in the Amazon. Instead, the root cause of the blazes is human activity.

“Humans are driving these fires, either in a very direct sense or a global sense by changing the ecosystem so much,” Ruth DeFries, an ecology professor at Columbia University, said.

Fire is not a natural part of the Amazonian ecosystem, as it is in places such as the American West. According to DeFries, the Amazon, like all tropical rainforests, is usually too humid to sustain fires for long. However, humans have completely changed that ecosystem through deforestation and deliberately setting fires, which in turn makes the region even more susceptible to fires. It's a vicious cycle.

Fires are one of the major drivers of deforestation, DeFries said, because they're the cheapest, easiest, and fastest way to clear debris. Many farmers or landowners use fire to clear their land, but those fires can quickly run wild if conditions are dry. Peak fire season in the Amazon runs August through October, as it’s the driest time of year.

“The important thing to know about the Amazon is that few fires occur there naturally,” said Mikaela Weisse, who tracks deforestation and fires as part of the World Resources Institute’s Global Forest Watch. “Pretty much everything is started by humans.”

The Global Forest Watch team is seeing fires crop up on indigenous land in the Amazon. According to Weisse, there’s recently been increased deforestation in the Karipuna and Ituna Itata reserves in northern Brazil—the latter of which is home to an uncontacted tribe.

Last week, indigenous women marched to protest far-right Brazilian president Jair Bolsonaro’s policies supporting deforestation, which threatens their land. Amazon Watch, a non-profit advocating for indigenous rights in the Amazon, said in a press release that, under this presidency, farmers and ranchers feel emboldened to set fires to expand their land in the rainforest.

According to Brazilian newspaper Brasil de Fato, ranchers and farmers in Pará state held a "day of fire" recently in which they coordinated a massive burn-off of trees to show support for the government's position. According to a local publication based in Pará, the burn-off took place on August 10.

Forest fires and deforestation are twin crises, each worsening the other and both feeding global climate change. “One way that the deforestation and the fires impacts us is the feedback loop that helps drive climate change,” DeFries said.

Trees hold a lot of water, which is why the Amazon ecosystem is so wet. The more trees you lose, the drier the forest gets, and those dry conditions make way for more fires. That vicious cycle has a broader effect: releasing the carbon that’s stored in those trees.

“The Amazon is a major bank of carbon, when trees gets burned and carbon is released into the atmosphere, that exacerbates our global warming," DeFries said.

Climate change makes conditions even drier, leading to yet more fires and deforestation. Some scientists fear that the Amazon is reaching a tipping point, and could become a much drier, more open ecosystem in the future.

While this loop seems hard to break, Brazil has actually done it before. In the early 2000s, deforestation was rampant in Brazil, and so were intense fires in the Amazon. Then, after 2004, deforestation rates fell rapidly, largely thanks to conservationist policies.

Since Bolsonaro took power, deforestation has surged. Many blame Bolsonaro’s pro-agribusiness policies and failure to stop illegal logging for clearing the way for this extreme forest loss. He also recently fired the head of the agency monitoring deforestation who warned against rapid deforestation for economic gain.

On Wednesday, Bolsonaro blamed NGOs for starting the forest fires to make his administration look bad. He has no evidence to back these claims up.

“He can deny all he wants and blame it on NGOs, but that’s not going to stop the smoke going to São Paulo,” DeFries said.