Destruction of tropical forests worldwide increased last year, led again by Brazil, which was responsible for more than a third of the total, and where deforestation of the Amazon through clear-cutting appears to be on the rise under the pro-development policies of the country’s president.

The worldwide total loss of old-growth, or primary, tropical forest — 9.3 million acres, an area nearly the size of Switzerland — was about 3 percent higher than 2018 and the third largest since 2002. Only 2016 and 2017 were worse, when heat and drought led to record fires and deforestation, especially in Brazil.

"The level of forest loss we saw in 2019 is unacceptable," said Frances Seymour, a fellow with the environmental research group World Resources Institute, which released the deforestation data through its Global Forest Watch program. "We seem to be going in the wrong direction."

"There has been so much international effort and rhetoric around reducing deforestation, and companies and governments making all these commitments that they are going to reduce by half their tropical forest loss by 2020," said Mikaela Weisse, who manages the Global Forest Watch program. "The fact that it’s been so stubbornly persistent is what’s worrying to us."

Global Forest Watch researchers estimated that the loss of primary tropical forest in 2019 resulted in the release of more than 2 billion tons of carbon dioxide, or more than the emissions from all on-road vehicles in the United States in a typical year.

Ms. Seymour said the outlook for 2020 is not good as the coronavirus pandemic continues.

Restrictions on mobility and looming budget cuts as a result of the economic fallout from the global crisis may hamper efforts to enforce anti-deforestation laws, she said. "Bad actors will try to take advantage with more illegal logging, mining, clearing and poaching."

Global Forest Watch uses data from researchers at the University of Maryland who have developed machine-learning software to analyze satellite imagery for loss of tree cover. Overall in the tropics, that loss amounted to nearly 30 million acres last year. Since 2000, the world has lost about 10 percent of its tropical tree cover.

Other analyses of deforestation come up with different numbers. Two United Nations agencies, in their most recent report on the subject, issued last month, said deforestation worldwide averaged about 25 million acres a year since 2015. Their analysis relies on reporting from each country.

Much of the tree cover loss that the Maryland researchers’ data reveals occurs tree plantations or other areas that are not old-growth forests. The scientists then do additional analysis to determine the loss from those old-growth forests, which are important for storing carbon dioxide from the atmosphere and for maintaining biodiversity, and can take decades to recover once destroyed.

That destruction can occur in several ways: clear-cutting for agriculture, ranching, mining or other uses and for accompanying roads and other infrastructure; selective logging; or through fires that are set as part of land-clearing efforts but can spread out of control.

Brazil and many other tropical countries experience those kinds of fires every year. Brazil had a high number in 2019, especially in August. The blazes, which were widely reported on social media drew widespread condemnation from environmental groups and world leaders who have been critical of the Brazilian president, Jair Bolsonaro.

Mr. Bolsonaro, who took office at the beginning of 2019, has aggressively pursued development in the Amazon, including mining and large-scale agriculture, and has begun dismantling programs that protect Indigenous lands.

Ms. Weisse said that the fires actually contributed relatively little to Brazil’s total primary forest loss of about 3.4 million acres in 2019, an amount only slightly higher than 2018’s total.

Many of the fires occurred on lands that had been previously deforested and were being burned in preparation for planting or ranching, she said. Only about one-fifth of fires burned in primary forest.

Instead, data from the Brazilian government’s forest-monitoring programs and other projects showed an increase in clear-cutting of primary forests for agriculture, Ms. Weisse said. "Even though the overall primary forest trend is only a small increase, we think that deforestation is getting worse," she said.

In neighboring Bolivia, fires were a major cause of what was a significant increase in deforestation last year. The country’s primary forest loss of 720,000 acres was nearly double the total from 2018. Bolivia now ranks fourth in deforestation globally behind Brazil, the Democratic Republic of Congo and Indonesia.

However, there were encouraging signs that efforts to reduce deforestation had some results in 2019. Indonesia provided a rare bit of good news, with primary forest loss declining for the third year in row. The decrease, by 5 percent from 2018, to about 800,000 acres, came despite extensive fires in the country last fall.

Widespread fires earlier in the decade had caused extensive deforestation and hazardous air pollution that reached neighboring countries. Since then, the Indonesian government, under international pressure, has established policies that include a moratorium on land clearing for certain activities, ramped up enforcement of illegal forest cutting, and coordinated efforts to limit the spread of fires.

The data shows that while there were a significant number of fires last year in Indonesia, most were on land that had already been degraded in the past, which was also the case in Brazil.

Elsewhere, Colombia also showed improvement, with a decline in deforestation similar to the level of 2016. A peace agreement that year between the government and a leftist guerrilla movement that had strictly enforced limits on logging in areas under its control had led to a power vacuum in those areas, allowing illegal logging to proliferate. Deforestation in 2017 and 2018 soared.

In Central Africa, the Democratic Republic of Congo showed little sign of progress. Annual loss of primary forest has more than doubled since 2012, and although the 2019 total was slightly lower than the year before, it was higher than 2017. "We’re seeing sustained amounts of loss," said Elizabeth Goldman, a research manager for Global Forest Watch. While most of the deforestation appears to be linked to subsistence farming, there are signs that some may be related to large-scale commercial agriculture or mining.

But in West Africa, both Ghana and Ivory Coast showed significant declines in primary forest loss, the data showed. Ghana’s total of about 14,000 acres was its lowest since 2014; Ivory Coast had its lowest total since 2005, at 29,000 acres.

Deforestation in both countries has largely been spurred by increasing cocoa production for world markets. The governments of both countries, and large cocoa and chocolate producers, had agreed on initiatives to reduce or end deforestation. The decline is a sign that these efforts might be working, Ms. Weisse said, although "it’s a little early to say too much yet, because it’s just one year."