Earth was a tropical planet.

Behemoth and tiny dinosaurs alike roamed its lands, while reptiles and tentacled ammonites swept its seas.

But in an instant, everything would change.

A roughly 12-kilometer-wide asteroid was careening toward Earth at around 20 kilometers per second.

From where the sauropod stood, there would have been no early warning signs.

The asteroid barreled through Earth's atmosphere in a matter of seconds and struck the Yucatan submerged continental shelf.

It exploded upon impact, instantaneously creating a 100-kilometer-wide hole and ejecting sedimentary and crystalline rocks.

Within minutes, the impact crater, known today as Chicxulub, began collapsing inwards.

Meanwhile, the base rebounded some 20 kilometers above the Earth's surface, then fell back down and moved outwards, creating a ring of mountains.

The energy released from the asteroid's impact is estimated to have been several billion times that of a nuclear bomb.

The force sent seismic energy across the planet at a much greater magnitude than any earthquake a tectonic fault could ever produce.

Massive landslides ensued, and a tsunami sped from the newly formed crater, potentially reaching 1,500 meters high.

Countless lives were extinguished, some instantly.

All life within 1,500 kilometers of the impact site was incinerated, others right after by colossal waves, landslides, and hurricane-force winds.

But many organisms across the planet survived.

It was what came next that would bring about the end for many species, including almost all dinosaurs.

This was just the beginning of one of the most devastating periods in the history of life on Earth.

When the asteroid struck, it sent hundreds of gigatons of carbon dioxide-rich limestone and sulfur-saturated sediments into the atmosphere.

The sulfur combined with water vapor to create sulfate aerosol.

This plume of limestone dust, soot, and sulfate aerosols spread from the impact site at several kilometers per second, blanketing the globe in a matter of hours.

It's thought to have blocked the sun, plunging Earth into an extended period of darkness and dropping the temperature in many places by at least 25 degrees Celsius.

The asteroid's immediate impact was devastating, but it seems to have been the rapid climate change it triggered that ended the roughly 165 million year reign of the dinosaurs.

Plants and plankton rapidly died, causing the collapse of food webs worldwide.

An estimated 75% of life on Earth went extinct, including almost all dinosaurs.

Small birds were the only kinds that remained, perhaps because they relied on hardy seeds that weathered the catastrophe.

It's unclear why exactly the lifeforms that survived the extinction did.

Many smaller organisms, like insects, persisted.

So did early mammals, perhaps because of their ability to burrow and hibernate.

And photosynthetic lifeforms, like algae, that had ways of withstanding low-light conditions also survived.

Traces of the asteroid scattered worldwide, and the scar of the Chicxulub crater attest to this period of monumental destruction.

So what are the chances of another Chicxulub happening?

Space programs are continuously identifying and tracking near-Earth asteroids.

Fortunately, the likelihood of one as large and cataclysmic striking in the next thousand or so years seems to be small, something like a 7 in a million chance.

However, we are facing the consequences of another kind of rapid climate change.

This time, because of humanity's own emissions.

Animals are going extinct faster than ever in our history.

And people are being displaced from their homes.

But unlike the dinosaurs, we have the opportunity to avoid the large-scale devastation that will come if governments continue with the status quo.

Fortunately, we've already proven our ability to make life-saving changes on a planetary scale.

Watch this video to find out what happened to that hole in the ozone layer, and discover what role you can play in Earth's destiny at ed.ted.com slash earthschool.