(CNN) The Amazon, the largest rainforest in the world, roughly half the size of the United States, is key to the health of the entire planet. Estimates show that nearly 20% of the oxygen produced by the Earth's land comes from the Amazon rain forest. It also puts an enormous amount of water into the atmosphere at a time when cities are drying up. The Amazon is sucking in carbon and greenhouse gases while slowing the rising temperatures. But it's now burning at a record rate -- with images from space showing the smoke covering much of Brazil.

It's not the only major forest under assault. Nearly half of the world's forests that stood when humans started farming are now gone, and each year an additional 32 million acres are destroyed, according to the nonprofit Rainforest Alliance. The biggest reason is expansion of agriculture into forested areas. In Brazil it's cattle ranching, soy production and logging, according to Nigel Sizer, tropical forest ecologist and chief program officer with the Rainforest Alliance. "It is responsible for 80% to 90% of the loss of tropical forests around the world." Environmental groups say these activities can be slowed or done in a much more sustainable way.

"There has been a lot of analysis and satellite data that shows there is so much land already cleared - a lot abandoned or very poorly used and managed that we could use to grow food on," says Sizer. "We don't need to be clearing new forests to do this in Brazil."

Here's what you can do to help slow forest loss.

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