Scientists studying satellite image data from the fires in the Amazon rain forest said that most of the fires are burning on agricultural land where the forest had already been cleared.

Existing forest Deforestation through 2018 Fires in August Brazilian Amazon BRAZIL VEN. COLOMBIA BRAZILIAN AMAZON PERU BRAZIL BOLIVIA Brazilian Amazon BRAZIL VEN. COLOMBIA BRAZILIAN AMAZON BRAZIL PERU BOLIVIA VENEZUELA Brazilian Amazon BRAZIL COLOMBIA BRAZILIAN AMAZON BRAZIL PERU BOLIVIA Brazilian Amazon BRAZIL VENEZUELA COLOMBIA BRAZILIAN AMAZON BRAZIL PERU BOLIVIA Brazilian Amazon BRAZIL VENEZUELA COLOMBIA BRAZILIAN AMAZON PERU BRAZIL BOLIVIA By The New York Times · Sources: Brazil’s National Institute for Space Research; NASA

Most of the fires were likely set by farmers preparing the land for next year’s planting, a common agricultural practice, said the scientists from the University of Maryland.

Satellite images like the one below show smoke plumes from fires emanating from agricultural areas.

Maxar Technologies

The majority of the agricultural land currently in use in Brazil’s Amazon region was created through years of deforestation.

“Most of this is land use that have replaced rain forest,” said Matthew Hansen, who is a co-leader of the Global Land Analysis and Discovery laboratory at the University of Maryland.

“Brazil has turned certain states like Mato Grosso into Iowa,” said Mr. Hanson, referring to the Brazilian state on the southern edge of the Amazon region. “You’ve got rain forest, and then there’s just an ocean of soybean.”

The grid of maps below show the month-by-month pattern of fires across the Amazon rain forest in Brazil each year since 2001. The increase in fires every August to October coincides with the season when farmers begin planting soybean and corn.

Detected fires by month Jan.J Feb.F MarchM AprilA MayM JuneJ JulyJ Aug.A Sept.S Oct.O Nov.N Dec.D 2019 ’19 2018 ’18 2017 ’17 2016 ’16 2015 ’15 2014 ’14 2013 ’13 2012 ’12 2011 ’11 2010 ’10 2009 ’09 2008 ’08 2007 ’07 2006 ’06 2005 ’05 2004 ’04 2003 ’03 2002 ’02 2001 ’01 By The New York Times · Source: NASA

These maps were created using current and historical data from two NASA satellites, Terra and Aqua, which can detect the infrared radiation emitted by fires.

Comparing the area that burned in August this year to an average of the areas burned during the same month in the previous five years illustrate part of the reason why this year’s fires have garnered so much attention.

August 2019 August average 2011–2018 BRAZILIAN AMAZON August 2019 August average, 2011–2018 BRAZILIAN AMAZON August 2019 August average, 2011–2018 BRAZILIAN AMAZON August average 2011–2018 August 2019 BRAZILIAN AMAZON August average 2011–2018 August 2019 BRAZILIAN AMAZON By The New York Times · Source: NASA

Scientists at Brazil’s National Institute of Space Research calculated that there were 35 percent more fires so far this year than in the average of the last eight years.

Cumulative number of fires each year for the entire Amazon region 2003–10 2011–18 2019, through Aug. 22 500,000 detected fires Jan. Aug. Dec. 500,000 detected fires Jan. Aug. Dec. 500,000 detected fires Jan. Aug. Dec. By The New York Times · Source: Global Fire Emissions Database based on NASA Terra and Aqua satellite data. | Note: Counts from 2003 through 2015 are monthly. Counts since 2016 are daily.

“Fires are not a natural phenomenon in these forests,” said Mark Cochrane, an expert on wildfire and ecology at the University of Maryland. “All of the fires in this region are caused by people.”

Mr. Cochrane noted that while a large majority of the fires were on land that had already been cleared, many others were detected burning with particular intensity. He said these were likely deforestation fires, not just fires for clearing previously deforested land.

“When you slash an area, pile it up, let it dry and then burn it, it burns very intensely, and that’s also what puts off a lot of that smoke,” said Mr. Cochrane.

There has been a rise in deforestation in recent years, after a long period of decline.

Annual deforestation in the Brazilian Amazon 1990 1995 2000 2005 2010 2015 sq mi 10,000 8,000 6,000 4,000 2,000 0 1990 1995 2000 2005 2010 2015 sq mi 10,000 8,000 6,000 4,000 2,000 0 1990 1995 2000 2005 2010 2015 sq mi 10,000 8,000 6,000 4,000 2,000 0 1990 1995 2000 2005 2010 2015 sq mi 10,000 8,000 6,000 4,000 2,000 0 By The New York Times · Source: Brazil’s National Institute for Space Research

Brazilian President Jair Bolsonaro, who took office on Jan. 1 this year, has been criticized by environmentalists for policies that have increased deforestation.

A New York Times analysis found that enforcement measures to protect against deforestation by Brazil’s main environmental agency fell by 20 percent during the first six months of this year, compared with the same period in 2018.

After days of growing international outrage over the fires, Mr. Bolsonaro reversed course on Friday and said that he would mobilize the military to enforce environmental laws and to help contain the fires.