Satellite data shows nearly 4 million hectares of tree cover disappeared, an increase from last year and the third-largest loss this century.
Some experts find hopeful notes among the bad news, however. While Brazil's forest losses have increased under right-wing President Jair Bolosonaro, policies to curb deforestation appear to be working in Indonesia, Colombia and West Africa.
The destruction of mature tropical forests is a massive hit to biodiversity and is responsible for about 8% of global carbon dioxide emissions, according to the World Resources Institute, the research and advocacy group that oversees Global Forest Watch.
Since forests are massive sponges of carbon dioxide, reversing their loss would play an outsize role in fighting climate change.
The United Nations set a goal of ending deforestation by 2020, "but we seem to be going in the wrong direction," WRI Distinguished Senior Fellow Frances Seymour said.
According to satellite imagery analyzed by the University of Maryland and WRI, Brazil alone lost 1.4 million hectares of mature forest in 2019, more than one-third of the world total and nearly three times more than the country with the next-largest loss, the Democratic Republic of Congo.
Not counting record-breaking forest fires in 2016 and 2017, it's the largest loss since 2006.
Brazil had been a source of optimism until recently. Conservation policies under former President Luiz Inácio Lula da Silva curbed deforestation rates from 2004 to 2015.
It was "one of the great conservation successes of this millennium," said Robert Heilmayr, a University of California, Santa Barbara environmental economics assistant professor, who was not involved with the WRI research.
"I think that gave rise to this hope that if we can harness the policies that worked in one place and just deploy them around the world, we're going to see an end to deforestation globally," Heilmayr said.
But the latest data shows "we still have a long ways to go," he said.
President Bolsonaro has encouraged development in the Amazon rainforest and loosened enforcement of environmental laws. His administration is backing a law that would increase access to protected indigenous lands for mining and supports legislation that environmental groups say would legalize land grabs.
"You're starting to see the enforcement of the laws that are on the books back off," Heilmayr said, "and I think that's creating an opening for more aggressive deforestation."
On the other hand, the loss of mature forests in Indonesia declined in 2019 for the third straight year.
"I'm continuing to be pleasantly surprised that there's a decrease" in Indonesia, said Greg Asner, director of the Center for Global Discovery and Conservation Science at Arizona State University, who was not involved with the research.
While Indonesia lost the third-largest area of mature forest after Brazil and the Democratic Republic of Congo, it's the smallest loss since the early 2000s.
The country has made permanent a 2011 moratorium on logging and land-clearing for oil palm plantations, which had been a major driver of deforestation. The government has stepped up fire prevention and enforcement of existing forest laws.
Colombia also saw a steep drop in the loss of primary forest last year, after two years of increases. Deforestation had spiked after a peace agreement ended decades of civil war and freed up land previously occupied by the Revolutionary Armed Forces of Colombia.
The country has set deforestation and reforestation goals and has sent the police and military to fight deforestation in its national parks.
It's not clear if the trend will hold. Global Forest Watch's early-warning system has logged an increase in alerts this year.
Chocolate cuts its losses
More tentative good news comes from West Africa. Cote d'Ivoire and Ghana, the two countries with the largest increase in mature forest loss in 2018, both cut those losses in half last year.
The chocolate industry has pledged to reduce deforestation for cocoa cultivation, a major crop in West Africa, and the governments have signed forest carbon deals with the World Bank.
These programs may be responsible, but WRI says it's too soon to tell if the impact will last.
The cooperation of palm oil companies has been a big part of Indonesia's decline in deforestation, UCSB's Heilmayr said.
"When we see those two components, government and the international markets that provide the strongest incentive for deforestation, working together and in harmony with each other to disincentivize further deforestation, that's where we generally see the biggest success," he noted.
"The 2019 data corroborates what we already know," WRI's Seymour added. "If governments put into place good policies and enforce the law, forest loss goes down. But if governments relax restrictions … forest loss goes up."
Seymour is concerned that fallout from the COVID-19 pandemic could push global forest losses up this year.
"We do have historical precedents," WRI's Seymour said. Poverty and a lack of enforcement drove up deforestation after the Asian financial crisis in the late 1990s, she noted.
And with attention turned elsewhere, Heilmayr said, "I worry that this is a moment where the governments that want to enable additional land grabbing, that want to enable agricultural expansion, may turn away from enforcing the laws that already exist."