Buried beneath your feet are trillions of tons of dead plants and algae.

Over millions of years, their carbon-rich bodies have been compressed under so much heat and pressure that they've transformed into energy-rich fossils.

In fact, we call them ‘fossil fuels’ since we burn them to release their energy.

But burning this coal, oil and gas is releasing carbon-dioxide into the atmosphere, where it’s rapidly building up and heating our planet.

So far, we’ve burned our way through some 600 billion tons of this carbon.

That’s about the same weight as 4 Mount Everests.

It’s a lot, but we can easily get our hands on way more, because there are 30 Everests worth of carbon still buried in the ground - enough for us to keep burning at our current rates for another 400 years.

What would it be like to live on our planet if we burned all the fossil fuels?

[OPEN] Today, out of every million molecules floating around our air, 400 are carbon-dioxides.

That may sound like a small amount, but it’s more carbon dioxide than Earth’s atmosphere has seen in the last 3 million years.

And if we burned all the carbon still underground, carbon dioxide levels could reach around 2000 parts per million.

That’s higher than they’ve been for the past 50 million years, which is so far back that Earth didn't even look like it does today.

A future world with this much carbon dioxide in the air would be radically different from today, but the same physical laws would still govern how that world operated.

Like, gravity would still make things fall, and carbon dioxide molecules would still trap heat.

So, we can use these physical laws to sketch out what the world might look like with 5 times as much carbon dioxide as today.

It would be a hot mess.

A really hot mess.

Earth’s average temperature could soar by 11 to 18 degrees fahrenheit.

That’s enough to make New York hotter than present-day Houston, and Moscow hotter than today’s Madrid.

This wouldn’t happen just in places that are already hot, like the Middle East.

The map of hot places on Earth would expand big-time.

Over half the world’s population lives in places likely to overheat.

Yeash.

But intolerable heat is just the start of the story.

Melting ice could raise sea levels by over 100 feet (30m), drowning coastal cities around the world.

Rainfall patterns would change too; some parts of the world could get less than a third of their normal rainfall, and some parts could get twice as much.

Not exactly ideal, if you’re a farmer.

And it’s worth pointing out that once we’ve burned our way through 30 Everests-worth of carbon… there’d probably be even more left, because 30 is a conservative estimate that doesn’t even include fossil fuels that are harder to dig up, like shale gas and methane hydrates.

Once you include those, we may actually have 60 Everests left today.

If we burned all of those, it would raise carbon-dioxide levels higher than they’ve been for the past half a billion years .

Earth is only 4.5 billion years old, so that’s a tenth of our planet’s history.

It’s so long since we’ve had that much carbon dioxide, that it’s hard to say what the world might be like.

But we can be sure that it would be extremely wet, because there’s a serious risk we’d melt all the world’s ice sheets, leading to over 200 feet [60m] of sea level rise.

That’s enough to flood the land where a billion people live today.

So, if we burn all the fossil fuels, a lot of bad things will probably happen.

The alternative is to stop using them before they run out, and when that happens is up to us.

The more we burn, the hotter and more extreme our future becomes, so most of the world’s countries have agreed that we should keep Earth from warming more than 2 degrees Celsius.

That limits us to burning only 3 more Everests of fossil fuels, which means we'll have to keep the vast majority - around 57 Everests worth - buried in the ground.

So, how do we keep all that carbon underground?

The problem is, fossil fuels are cheap and easy to get a hold of, and they’ve made life pretty prosperous today.

One option, then, would be to make them more expensive.

If governments choose to increase the price of fossil fuels to include these future damages to society, they’d quickly become less appealing.

Or, we could keep making the alternatives more appealing.

Renewable power sources like wind and solar don't release carbon dioxide into the air, and we could improve them even more by researching new technologies to boost their efficiency, making them more dependable, and scaling up production to drive down costs.

Or, we could make fossil fuels socially unacceptable - just like big health campaigns have turned smoking unsexy.

Or heck, we could do all these things!

If we want to.

In fact, these changes are already beginning to happen, each in their own way, and around the world we’re slowly starting to move away from coal, oil and gas.

Based on those possible futures we just talked about, that’s a good thing too, because if we DID end up burning all the world’s fossil fuels, we would be at serious risk of becoming fossil fools.