A world begins. In its acidic oceans float the corpses of suffocated sea creatures.

The coastlines are swamped by rising waters.

In between wide swaths of wasteland are a few burning forests.

The Mediterranean is almost as dry as the Sahara,

and the west coast of North America

swings wildly between drought and flood. The sky is laden with carbon dioxide and methane, and the

temperature steadily rises. Terrible things come to pass. I end that world. I start another. It's similar

in many ways to the previous nightmare planet. There

are heat waves and droughts, a thousand-year flood every decade or so. Sea levels are still high.

But after decades of work, humans have finally ceased to put carbon dioxide and methane into

the atmosphere. The temperature, still dangerously high, no longer climbs.

This is a planet coming to a new equilibrium, slowly adjusting to its new reality, making peace

with the way it is now.

Another world exists, a better one, maybe the best of all possible worlds.

By the middle of the century, human beings not only emit no carbon dioxide,

but have learned how to remove a little from the atmosphere. Nearly every structure is covered

in solar panels, as are roads, canals, parking lots, even some farms. Most people live in pleasant

green cities. On the urban outskirts, restored forests thrive. The world is far from perfect.

It's hotter than now, more dangerous with higher seas.

Even in utopia, the climate conditions under which human civilization developed are gone forever.

I don't know which of these worlds is more likely.

Science says that as long as human beings emit greenhouse gases by cutting down trees and burning fossil fuels, the planet will keep getting warmer.

...