The rain forest is more like the planet’s air conditioner, says scientist Dan Nepstad of the Earth Innovation Institute. If we cut off the source, things could get really hot and dry.

Like you, we’ve seen these stories before. But this is not just about destruction and guilt (though there may be some of that). Let’s start with the basics.

A fifth of Brazil’s rain forest is already gone completely.

That might explain why certain places in Brazil, like the coastal state of Sao Paulo, are experiencing drought.

Imagine a flying river. In a healthy ecosystem, the forest recycles about half of the precipitation that falls into it. It evaporates back out and travels through the air, raining throughout Brazil. But with fewer trees to recycle water, the river in the sky dries up.

So how bad is it? Geologist Ben van der Pluijm of the University of Michigan puts it this way:

“The Earth is not in danger.”

“It’s humanity that’s in danger,” he qualifies.

Great. In other words, the world will keep turning, with or without the AC. For humanity to stay cool, though, Brazil must do something that no other developed country has pulled off:

Stop deforesting.

Who’s doing it? And how? It’s maybe not what you’d expect.

Maria Leonilda Mattara is a subsistence farmer in Rondonia. She calls her purple home “the house at the end of the world.”

Half of all deforestation in Brazil takes place incrementally, on small plots like hers.

It has happened a lot like the land rush in the American West did. Over the past few decades, the poor and dispossessed of other Brazilian states have been encouraged to move in. Trees quickly gave way to farms and cattle ranches. Mattara is only in the latest wave of settlers.

She is trying to eke out a living with her daughter and two granddaughters, clearing the land with the hope of turning it into pasture and raising cattle. That’s where the money is. Unfortunately, though, she says, the soil isn’t very good here, where rain forest once stood.

That’s why Mattara burns trees. She says the ash will make the land fertile again.

“You either burn or you starve,” she says.

It looks crazy when you think, “Oh, yeah, rain forest was here.” But actually, the same thing happened in the United States.

So who can blame Brazil?

Its economy had been weak for so long and the forest looked green — as in money — much like it had for the U.S.

In the 1960s, the Brazilian government started investing in roads and infrastructure. Suddenly the Amazon was a lucrative resource.

The forest shrank.

But look what else happened:

The Amazon may seem like a weird place to find cows. In fact, more than two-thirds of Brazil’s deforested land is used for cattle ranching.

The number of cattle in the Amazon has tripled since 1989, according to the Brazilian Institute of Geography and Statistics.

Here’s another interesting fact: The U.S. Department of Agriculture forecasts that Brazil will produce more soybeans than ever in 2015-16, second only to the United States. The U.S. consumes most of its own soy, but Brazil will be the largest exporter of soy in the world — to countries like China.

And of course this is also about lumber.

Tropical hardwood is great for your patio deck. Decades ago it was used widely in public spaces, so you’ll find planks of it in the Brooklyn Bridge and Coney Island’s boardwalk.

The U.S. is still the biggest importer of Amazonian wood. Just last year it imported $282 million worth of tropical hardwoods, according to the U.S. Department of Agriculture, and about a third of that came from Brazil’s Amazon.

The U.S. has banned the import of illegally forested lumber. Of course, that’s really hard to track.

A nonprofit research institution called Imazon estimates that between August 2011 and July 2012, 78 percent of the timber logged in the state of Para (where most lumber comes from) was illegal.

Meanwhile, countries like the U.S. have also publicly acknowledged the environmental risks — and have begun pressuring the Brazilian government to regulate deforestation.

And look at this:

That would be like driving along, looking out the window where forest once stood, and seeing … nothing.

For an hour.

But we said this wouldn’t be all destruction. So is anything being done about it?

Well, on the ground in Rondonia, vigilante environmentalists like Elizeu Bercacola (center) are taking things into their own hands. They call themselves “guardians of the forest.”

They live sustainably on protected reserves, tapping trees for rubber. This is the kind of healthy, productive tree they look for.

It’s also what illegal loggers want to take down. At the mill, the wood is worth about $650 per cubic yard.

So Bercacola and his crew locate the camps of illegal loggers …

And set them on fire when no one’s around.

Here’s the thing: On the ground, no one seems happy that the forest is disappearing — but no one really seems to know how to stop it.

“To cut down a tree is like cutting out a piece of us,” a rubber tapper named Giselda Pilker tells us between tears. “No one does anything to save us. We people of the forest are peaceful; we don’t want this war.

“So many have died to defend what you see here.”

Sixteen rubber tappers in this area alone have been murdered in the past decade. One corpse was found stuffed into a wild animal’s burrow. Brazil is one of the most dangerous places to be an environmentalist.

It’s watching. Not so much for vigilantes, but for illegal forest degradation — fires and excessive logging — which can be observed via satellite. If detected, a small infraction can lead to a huge penalty.

Mattara, the subsistence farmer in Rondonia, says she owes about $18,000 in fines. She makes $125 a month. All the fines stack up in paper towers.

And the trees stack up as logs.

The government is still trying to figure out how to preserve the Amazon. At the U.N. Climate Change Conference, which convenes Nov. 30, countries will try to agree on how to prevent climate change. Brazil is an important player in these talks.

Brazil’s president has already made a promise: to end illegal deforestation and reduce greenhouse gases by 43 percent — by 2030. It’s the first major developing country to promise absolute reduction of greenhouse gases.

But (and of course there’s a but) —

Brazil’s Congress won’t make that easy. A few years ago, it passed a bill all but dismantling the existing Forest Code protections, despite the president’s counterefforts.

For Ivo Cassol, the current senator from Rondonia, it comes down to money. He’s one of the wealthiest senators in Brazil and has been elected to key posts for more than two decades. One public prosecutor in Rondonia calls him that state’s “founding father of deforestation.”

So where does that leave us?

Some experts we interviewed seemed cautiously optimistic that, with the right combination of government intervention and economic incentive, the worst can be avoided. But is avoiding the worst the best we can hope for?

The task of the U.N. conference this December is to figure this out.

No pressure. ∎