(CNN)The world's rainforests are under siege.
Climate change has driven the average temperature of the Amazon rainforest up by as much as 1.5 degrees Celsius. As a result of higher temperatures and deforestation, drought has surged. In 2019 alone, tens of thousands of wildfires destroyed 2.24 million acres.
Humans have wiped out or degraded nearly two-thirds of the world's tropical rainforests, according to an analysis by the non-profit Rainforest Foundation Norway. Some researchers worry that rainforests will soon reach a tipping point — a point of no return in which the once-lush forests transition to arid regions.
On World Rainforest Day, here are six facts about some of the most critical places for life on Earth.
Rainforests are Earth's air conditioners
Rainforests account for around 20 percent of all the photosynthesis that takes place on land. Because of that, they are a major sink for carbon dioxide.
Rainforests are often described as the "lungs" of the planet, but they are better described as Earth's air conditioning — rainforests remove greenhouse gas from the atmosphere, which prevents it from warming the planet even more.
Some have suggested that as humans emit more carbon dioxide, plants will compensate by absorbing more. But a 2015 study found that the Amazon rainforest is absorbing less carbon dioxide as more is emitted.
Rainforests cover just 6 percent of land, but contain 50 percent of Earth's biodiversity
They are also some of the most biodiverse places on Earth.
Scientists at the Field Museum in Chicago estimate that the Amazon rainforest alone housed around 16,000 tree species. Millions of species of plants, animals and insects live in tropical rainforests, and researchers continue to find new species every day.
Some of the plant species discovered in rainforests have played a key role in the development of medicine. A significant proportion of modern pharmaceuticals — including things like novocaine and quinine — came from native rainforest plants.
More than 30 million people live in the Amazon
The world's largest rainforest spans eight countries and covers 40 percent of South America — an area nearly two-thirds the size of the US. The Amazon is larger than the next two largest rainforests combined, according to Rhett Butler, a rainforest researcher and the CEO of Mongabay.
The majority of the Amazon is in Brazil. Critics say the administration of President Jair Bolsonaro is not doing enough to fight deforestation. The administration's 2022 target still allows for about 16 percent more deforestation than in the year before Bolsonaro took office, CNN reported in April.
Tree clearing is on the rise
Tree clearing has slashed the size of the Amazon rainforest by 15 percent since the 1970s, according to an analysis published in Nature Magazine. More than 19 percent has been cut down in Brazil. In 2019 — as the Amazon experienced one of its worst wildfire years on record — deforestation in Brazil shot up by 30 percent.
Around 70 percent of the Amazon's deforestation is due to cattle ranching, according to Butler.
Rainforests lose moisture when trees are cut down, which in turn leads to hotter temperatures and more drought.
Wildfires destroyed 2.24 million acres of Amazon rainforest in 2019
It was one of the Amazon's worst wildfire years. At one point, an area the size of one-and-a-half soccer fields was being destroyed every minute of every day, according to Brazil's National Institute for Space Research.
Farmers and cattle ranchers have long used fire to clear land and make it ready for use, and were likely behind the unusually large number fires burning in the Amazon, according to Christian Poirier, the program director of non-profit organization Amazon Watch.
What is World Rainforest Day?
World Rainforest Day was created in 2017 by Rainforest Partnership, a non-governmental organization based in Austin, Texas, as a way to promote action that preserves the vital resources that rainforests provide.
The day is honored annually on June 22.