Imagine that aliens landed and gifted us a clean, limitless energy source.

And instead of killing each other over this technology, we decided to immediately transform the world into a carbon-free society.

[animation shows closing drilling rigs, oil refineries, and coal and natural gas plants.]

This wonderous source would power our homes, industries, cars and planes, and humanity's annual rate of carbon pollution would almost instantly fall to zero.

So if we kicked our carbon addiction tomorrow, what would that mean for global warming?

[Intro] Hey, I'm Joe.

So, if we stopped burning carbon tomorrow, would the climate suddenly go back to the cooler, calmer atmosphere humans lived under before the Industrial Revolution?

Short answer: Not exactly.

The climate would continue to change and temperatures would remain high for many, many centuries.

So, that's not great.

I know, we've been told for years that slashing greenhouse gas pollution is the key to tackling climate change.

But unfortunately those cuts wouldn't immediately stop the planet from warming up.

One reason is that over the last 50 years, 90% of the extra warming has gone into the oceans.

[average depth on screen: 2.3 miles, note that "oceans mix slowly"] It takes decades for oceans to heat up, but once you quit adding additional heat they'd still give off the heat that had previously accumulated.

It's kind of like how a pot of boiling water continues to give off heat long after you've turned off the stove.

Because water heats up slowly, it cools off slowly too.

And as all this extra heat fully mixes in the deep ocean [show heat equilibrating in the graphic] the oceans would continue to expand, raising sea levels for centuries.

The other reason Earth would stay hot is that CO2 molecules -- which cause more than 80% of the warming -- remain in the atmosphere for a remarkably long time.

If you burn 5 gallons of gas [show car driving] today you create about 100 pounds of CO2 [illustrated in animation].

But way off in the year 3000, as much as 40 pounds of that gas will still be floating in the sky, warming up the planet.

Earth has warmed by about 0.8ZC [1.4Z Fahrenheit] since the industrial revolution.

If we turned off greenhouse gas emissions tomorrow, shorter-lived greenhouse gases like methane and nitrous oxide would be chemically broken down and dissipate first.

That would cause the atmosphere to cool by maybe half a degree, over about a century.

But CO2 stays in the atmosphere way longer, because it's so chemically stable.

So that would keep , the earth warmer for at least two thousand years.

So, many of the impacts we're experiencing now, like the melting arctic, drier droughts, and fiercer thunderstorms would probably also continue.

In the end, CO2 might take as long as 100,000 years to finally return to pre-industrial levels.

So even if we cut all emissions tomorrow, the truth is we are guaranteed some amount of climate change and warmer temperatures.

But switching to a carbon-free energy system would still give us a lot of benefits immediately.

Health is a big one.

Over three billion people worldwide breathe air that is so polluted that it doesn't meet World Health Organization guidelines.

Air pollution from fossil fuels or stoves is thought to cause more than 5 million deaths per year, thanks to things like lung cancer, heart disease, and stroke.

The majority of those deaths occur in developing countries, where there's more pollution.

So halting fossil fuel burning would immediately improve the lives of millions of people.

Wild places would also benefit.

Some of the world's most pristine environments contain plentiful fossil fuels.

Stopping emissions would mean ending fossil fuel extraction, which means less construction in wild areas, less noise pollution, and cleaner air and water.

Halting emissions sooner rather than later means that future generations might still face climate impacts, but they'll probably be less severe.

For example, with each degree of additional warming, the amount of area burned by wildfire in the western United States doubles.

Each degree of additional warming is also thought to reduce crop yields by as much as 10 percent.

Lowering emissions means fewer fatalities in extreme weather, fewer severe storms, and more children spared the hardship of migration to cooler places.

And who knows, maybe one of those kids could grow up to develop some technologies to actually suck the extra carbon straight out of the sky and speed up the cooling.

The idea of aliens providing us a magical energy source is of course a fantasy.

But today we're already installing real clean energy technologies that could wean us off fossil fuels, things like solar cells and wind turbines.

The decisions we're making today will affect not only our grandchildren but their grandchildren's grandchildren.

's grandchildren.

's grandchildren.

's grandchildren.

Our addiction to carbon has put us and our descendants into a deep hole.

Sometimes it's hard to imagine how we'll climb out, but the best thing we can do for ourselves, and for them, is to just stop digging.