The extinction crisis is far worse than you think.

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In all of Earth's history, there have been five mass extinction events. You can see them charted here.

Now, we’re on the verge of the sixth extinction. And three-quarters of all species could vanish.

Imagine three out of four species that were common are gone.

Anthony Barnosky, executive director of Jasper Ridge Biological Preserve, Stanford University

This is the first time humans have caused anything like this. Species are going extinct at a rate that's roughly 100 times higher than normal.

If poaching rates continue, experts fear African elephants could disappear within 20 years.

Orangutans are on the brink of extinction.

And the monarch migration is in jeopardy.

About half the life forms people are aware of have disappeared.

Paul Ehrlich, Bing Professor of Population Studies, Stanford University

Why is this happening?

CNN reporters explored five causes.

First cause:Climate change

We’re heating up the planet by burning fossil fuels and chopping down rainforests.

Coral are among the species suffering. Warm, acidic oceans cause coral to bleach, or turn white.

Scientists fear reefs as we know them could disappear by 2050 if we don’t switch to cleaner energy sources.

That has real consequences for people like Lydia.

She lives on a remote island off of Madagascar.

There's no power or fresh water. No jobs. Her family survives on fish and octopus from the reef.

We're as good as dead thanks to the bleached coral.

Lydia, Nosy Andragnombala, Madagascar

Second cause: Agriculture

People have converted 37% of Earth’s land surface into farms and pasture.

Source: World Bank, 2000

Here’s all the land we use for crops …

… the habitat we’ve given to livestock …

… and the land we humans occupy.

The human population is exploding like never before, according to the United Nations.

There are grave consequences in the rainforest. Orangutans are on the brink of extinction because their habitat is being cleared for palm oil plantations.

Bees are also suffering.

More than 25% of bumblebees in the US are at risk for extinction. Habitat loss, pesticide use and disease are thought to be causing the decline.

That’s troubling. Bees help pollinate 35% of the world’s food — a service worth billions per year.

And bees are more diverse than you think, with 20,000 species. They’re big and small, green and blue — ugly and beautiful.

Some bees disappear with little notice.

Robbin Thorp was one of the only people to notice Franklin’s bumblebee vanished from Oregon.

Now he spends his free time searching for the bee. He was the last person to see it alive — in 2006.

I'm hoping it's still out there under the radar.

Robbin Thorp, professor emeritus, UC Davis

Third cause: Wildlife crime

Environmental crime, including wildlife trade, is valued at $91 billion to $258 billion per year — making it one of the most lucrative black markets.

Rhinos are poached for their horns, which are falsely believed to be an aphrodisiac.

Pangolins are killed for their scales, which are used in traditional medicine in China and Vietnam.

And elephants are targeted for their ivory tusks.

Mike Chase has been surveying elephant populations across Africa from the skies.

He found 30% of elephants disappeared between 2007 and 2014. That’s 144,000 missing animals.

We are failing the elephants.

Mike Chase, ecologist and founder of Elephants without Borders

If poaching rates continue, some researchers fear Africa’s elephants will disappear in 20 years.

Fourth cause: Pollution

Nearly 9 million tons of plastic end up in the ocean each year. That’s like dumping one garbage truck full of plastic into the water every minute.

By 2050, researchers expect the ocean to contain more plastic than fish by weight.

When scientists dissect seabirds, nearly all of them are found to have plastic in their stomachs.

This is especially true on Midway Island in the Pacific.

You can see all that plastic that's inside this bird.

Matt Brown, US Marine National Monuments Superintendent

The birds’ longterm prospects trouble some scientists. One study showed seabird populations worldwide declined 70% between 1950 and 2010.

Final cause: Disease

About 40% of amphibians are at risk for extinction, more than any other group of vertebrates.

Maybe you’ve never heard of the chytrid fungus, but it’s one cause of the amphibian apocalypse.

Humans likely helped carry the deadly fungus around the world by moving frogs across continents.

In Costa Rica, ecologists are installing microphones in the rainforest to listen for the vanishing.

Listen to this recording from 2008.

And then another from 2015.

I'm worried that these would potentially become acoustic fossils.

Bryan Pijanowski, professor of landscape ecology, Purdue University

Maybe all of this sounds hopeless.

But experts say we have the solutions we need.

To avoid the worst of climate change, scientists and policy experts say we can end the era of fossil fuels and switch rapidly to cleaner energy sources.

To stop farm encroachment, biologist E.O. Wilson proposes protecting half the planet’s surface. That would save 84% of species, he says.

Currently, only 15% of the land surface and about 4% of the oceans are protected, according to the UN's Protected Planet report.

And to stop the wildlife trade, we can reject ivory and other wildlife products.

We have the tools we need to fix this crisis.

What we don’t have is time.

Anthony Barnosky, Stanford University

You can help stop the vanishing.

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Story by John D. Sutter

Reporting from David McKenzie, Ingrid Formanek, John D. Sutter and Nick Paton Walsh, CNN

Visuals and video production by Deborah Brunswick, Frank Fenimore, Matt Gannon, Janelle Gonzalez, Jackson Loo, Mark Phillips, Peter Rudden, John D. Sutter, Bryce Urbany, CNN

Additional visuals from Getty Images, Great Big Story, USGS, Bryan Pijanowski

Photo editing by Brett Roegiers, CNN

Development by Curt Merrill and Sean O'Key, CNN

Design by Padraic Driscoll and Alberto Mier, CNN