The world’s old-growth rainforests are shrinking at an alarming rate, with enough trees lost last year to cover all of Belgium or two Connecticuts, a new report shows.

Tropical rainforests are found mainly in Equatorial countries, but they store vast amounts of carbon dioxide, so keeping them intact is crucial to fighting global climate change. In addition, they are home to a broad range of species, including orangutans, mountain gorillas and tigers.

Once cut down, such forests may never return to their original state, according to a study published Thursday by Global Forest Watch.

Using data from the University of Maryland, the group found that some 3.6 million hectares (8.9 million acres) of primary tropical forest disappeared last year. While that was below the peaks in 2016 and 2017, when fires helped push forest loss to record levels, it was still the third-highest annual loss since records began in 2001. What’s more, the moving three-year average for last year was the highest ever recorded.

“Old growth, or ‘primary’ tropical rainforests, are a crucially important forest ecosystem, containing trees that can be hundreds or even thousand years old,” Global Forest Watch said. “They store more carbon than other forests and are irreplaceable when it comes to sustaining biodiversity.”

The biggest loss of primary rainforest land last year was seen in Brazil, where some 1.35 million hectares disappeared, followed by the Democratic Republic of the Congo, Indonesia, Colombia and Bolivia.

Global deforestation is being driven primarily by agriculture, mining, infrastructure development and fires that have intensified, thanks to global warming, according to the WWF.

When looking at the loss as a percentage of total primary forest, the biggest decrease in trees was seen in Madagascar and Ivory Coast, indicating a shift in countries with the greatest loss rates, compared with 17 years ago.

“In 2002, just two countries — Brazil and Indonesia — made up 71 percent of tropical primary forest loss,” Global Forest Watch said. “More recent data shows that the frontiers of primary forest loss are starting to shift. Brazil and Indonesia only accounted for 46 percent of primary rainforest loss on 2018, while countries like Colombia, Ivory Coast, Ghana, and the Democratic Republic of the Congo saw loss rates rise considerably.”

The total loss of tree cover in the tropics last year amounted to 12 million hectares, which was the fourth-highest annual loss since 2001, according to the report.

In Indonesia, however, the loss of forests dropped to its lowest level since 2003, “continuing a hopeful decline that started in 2017,” Global Forest Watch said. Indonesia has been pushing to preserve wide swathes of peat areas, or carbon-rich organic soils.