Once there was a world...
not so very different from our own.
There were occasional natural catastrophes, massive volcanic eruptions...
and every once in a while...
an asteroid would come barreling out of the blue to do some damage.
But for the first billion years or so...
it would have seemed like a paradise.
This is what we think the planet Venus might have looked like...
when our solar system was young.
Then, things started to go horribly wrong.
The planet Venus, which once may have seemed like a heaven...
turned into a kind of hell.
The difference between the two can be a delicate balance...
far more delicate than you might imagine.
Once things began to unravel, there was no way back.
This is what Venus...
our nearest planetary neighbor, looks like today.
Venus' oceans are long gone.
The surface is hotter than a broiling oven, hot enough to melt lead.
Why?
You might think it's because Venus is 30 percent closer to the sun than the Earth is...
but that's not the reason.
Venus is completely covered by clouds of sulfuric acid...
that keep almost all the sunlight from reaching the surface.
That ought to make Venus much colder than the Earth.
So why is Venus scorching hot?
It's because the small amount of sunlight that trickles in through the clouds...
to reach the surface can't get back out again.
The flow of energy is blocked by a dense atmosphere of carbon dioxide.
That carbon dioxide gas, or CO2 for short...
acts like a smothering blanket to keep the heat in.
No one is burning coal or driving big gas-guzzlers on Venus.
Nature can destroy an environment without any help from intelligent life.
Venus is in the grip of a runaway greenhouse effect.
Why does it look like we're inside a bowl?
It's due to the intense atmospheric pressure.
That's the wreck of Venera 13.
In 1982, the scientists and engineers of what was then the Soviet Union...
successfully landed this spacecraft on Venus.
They managed to keep it refrigerated for over two hours...
so it could photograph its surroundings...
and transmit the images back to Earth...
before the on board electronics were fried.
Venus and Earth started out with about the same amount of carbon.
But the two worlds were propelled along radically different paths.
And carbon was the decisive element in both stories.
On Venus it's almost all in the form of gas:
carbon dioxide in the atmosphere.
Most of the carbon on Earth has been stored for eons...
in solid vaults of carbonate rock like this one.
Part of a chain that forms the celebrated White Cliffs of Dover...
right on the English Channel.
What titan built this wonder of the world?
A creature a thousand times smaller than a pinhead...
trillions of them.
One-celled algae.
Volcanoes supply carbon dioxide to the atmosphere...
and the oceans slowly absorb it.
Working over the course of millions of years...
the microscopic algae harvested the carbon dioxide...
and turned it into these tiny shells.
They accumulated in thick deposits of chalk or limestone...
on the ocean floor.
Later the restless Earth pushed up the seafloor...
and carved out these massive cliffs.
Other marine creatures took in carbon dioxide to build enormous coral reefs.
And the oceans converted dissolved CO2 into limestone even without any help from life.
As a result only a trace amount was left as a gas in Earth's atmosphere...
not even three-hundredths of 1 percent.
Think of it, fewer than three molecules out of every 10,000.
And yet it makes the critical difference...
between a barren wasteland and a garden of life on Earth.
With no CO2 at all the Earth would be frozen.
And with twice as many, we're still talking about only six molecules out of 10,000...
things would get uncomfortably hot and cause us some serious problems.
But never as hot as Venus, not even close.
That planet lost its ocean to space billions of years ago.
Without an ocean it had no way to capture CO2...
from the atmosphere and store it as a mineral.
The CO2 from erupting volcanoes just continued to build up.
Today, that atmosphere is 90 times heavier than ours.
Almost all of it is heat-trapping carbon dioxide.
That's why Venus is such a ferocious inferno, so hostile to life.
The Earth, in stunning contrast, is alive.
It breathes...
but very slowly.
A single breath takes a whole year.
The forests contain most of Earth's life...
and most forests are in the Northern Hemisphere.
When spring comes to the north...
the forests inhale carbon dioxide from the air and grow...
turning the land green.
The amount of CO2 in the atmosphere goes down.
When fall comes and the plants drop their leaves, they decay...
exhaling the carbon dioxide back into the atmosphere.
The same thing happens in the Southern Hemisphere...
at the opposite time of the year.
But the Southern Hemisphere is mostly ocean.
So the forests of the north that control the annual changes in global CO2.
The Earth has been breathing like this for tens of millions of years.
But nobody noticed until 1958...
when an oceanographer named Charles David Keeling...
devised a way to accurately measure...
the amount of carbon dioxide in the atmosphere.
Keeling discovered the Earth's exquisite respiration.
But he also discovered something shocking...
a rapid rise, unprecedented in human history...
in the overall level of CO2, one that has continued ever since.
It's a striking departure from the CO2 levels that prevailed...
during the rise of agriculture and civilization.
In fact, the Earth has seen nothing like it for 3 million years.
How can we be so sure?
The evidence is written in water.
The Earth keeps a detailed diary written in the snows of yesteryear.
Climate scientists have drilled ice cores from the depths of glaciers...
in Greenland and Antarctica.
The ice layers have ancient air trapped inside them.
We can read the unbroken record of Earth's atmosphere...
that extends back over the last 800,000 years.
In all that time, the amount of carbon dioxide in the air...
never rose above three-hundredths of 1 percent.
That is until the turn of the 20th century...
and it's been going up steadily and rapidly ever since.
It's now more than 40 percent higher than before the Industrial Revolution.
By burning coal, oil and gas...
our civilization is exhaling carbon dioxide much faster than the Earth can absorb it.
So CO2 is building up in the atmosphere.
The planet is heating up.
Every warm object radiates a kind of light we can't see with the naked eye...
thermal infrared light.
We all glow with invisible heat radiation, even in the dark.
This is what the Earth looks like in the infrared.
You're seeing the planet's own body heat.
Incoming light from the sun hits the surface.
The Earth absorbs much of that energy which heats the planet up.
And makes the surface glow in infrared light.
But the carbon dioxide in the atmosphere absorbs most of that outgoing heat radiation...
sending much of it right back to the surface.
This makes the planet even warmer.
That's all there is to the greenhouse effect.
It's basic physics, just bookkeeping of the energy flow.
There's nothing controversial about it.
If we didn't have any carbon dioxide in our atmosphere...
the Earth would just be a great big snowball.
And we wouldn't be here.
So a little greenhouse effect is a good thing.
But a big one could destabilize the climate and wreck our way of life.
All right, but how do we know we're the problem?
Maybe the Earth itself is causing the rise in CO2.
Maybe it has nothing to do with the coal and oil we burn.
Maybe it's those damned volcanoes.
Every few years, Mt. Etna in Sicily blows its stack.
Each big eruption sends millions of tons of CO2 into the atmosphere.
Now, combine that...
with the output of all the other volcanic activity on the planet.
Let's take the largest scientific estimate...
about 500 million tons of volcanic CO2 entering the atmosphere every year.
Sounds like a lot, right?
But that's not even 2 percent of the 30 billion tons of CO2...
that our civilization is cranking out every year.
And, funny thing, the measured increase of CO2 in the atmosphere...
tallies with the known amount we're dumping there...
by burning coal, oil and gas.
Volcanic CO2 has a distinct signature.
It's slightly heavier than the kind produced by burning fossil fuels.
We can tell the difference between the two when we examine them at the atomic level.
It's clear that the increased CO2 in the air is not from volcanoes.
What's more, the observed warming is as much as predicted...
from the measured increase in carbon dioxide.
It's a pretty tight case. Our fingerprints are all over this one.
How much is 30 billion tons of CO2 per year?
If you compressed it into solid form...
it would occupy about the same the volume as the White Cliffs of Dover.
And we're adding that much CO2 to the air every year, relentlessly...
year after year.
Unlucky for us, the main waste product of our civilization is not just any substance...
it happens to be the chief climate-regulating gas of our global thermostat...
year in, year out.
Too bad CO2 is an invisible gas.
Maybe if we could see it.
If our eyes were sensitive to CO2...
and perhaps there are such beings in the cosmos.
If we could see all that carbon dioxide...
then we would overcome the denial...
and grasp the magnitude of our impact on the atmosphere.
But the evidence that the world is getting warmer is all around us.
For starters, let's just check the thermometers.
Weather stations around the world...
have been keeping reliable temperature records since the 1880s.
And NASA has used the data to compile a map...
tracking average temperatures around the world through time.
Yellows mean warmer temperatures than the average for any region in the 1880s.
Orange means hot.
And reds means hotter.
The world is warmer than it was in the 19th century.
Back then, at the greatest fair the world had ever seen...
a forgotten genius demonstrated the solution to this problem.
Come with me.
Once there was a world that was not too hot and not too cold.
It was just right.
Then there came a time when the life it sustained...
began to notice our lovely planet was changing.
And it's not as if we didn't see it coming.
As far back as 1896 the Swedish scientist Svante Arrhenius calculated...
that doubling the amount of CO2 in the atmosphere would melt the Arctic ice.
In the 1930's, the American physicist E. O. Hulbert...
at the Naval Research Laboratory confirmed that result.
So far, it was still just theoretical.
But then the English engineer Guy Callendar...
assembled the evidence to show that both the CO2...
and the average global temperature were actually increasing.
Even now, man may be unwittingly changing the world's climate...
through the waste products of his civilization.
Due to our release through factories and automobiles every year...
of more than 6 billion tons of carbon dioxide...
which helps air absorb heat from the sun, our atmosphere seems to be getting warmer.
This is bad?
Well, it's been calculated a few degrees rise...
in the Earth's temperature would melt the polar ice caps.
In 1960, Carl Sagan's PhD thesis...
included the first calculation of the runaway greenhouse effect on Venus.
This was part of a career-long interest in the atmospheres of the planets...
including our own.
In the original Cosmos series in 1980, Carl Sagan warned:
We are releasing vast quantities of carbon dioxide...
increasing the greenhouse effect.
It may not take much to destabilize the Earth's climate...
to convert this heaven, our only home in the cosmos, into a kind of hell.
Since Carl spoke those words we've burdened the atmosphere of our world...
with an additional 400 billion tons of carbon dioxide.
And if we don't change our ways, what will the planet be like in our children's future?
Based on scientific projections...
if we just keep on doing business as usual, our kids are in for a rough ride.
Killer heat waves, record droughts...
rising sea levels, mass extinction of species.
We inherited a bountiful world made possible by a relatively stable climate.
Agriculture and civilization flourished for thousands of years.
And now our carelessness and greed put all of that at risk.
Okay, so if we scientists are so good at making these dire long-term predictions...
about the climate, how come we're so lousy about predicting the weather?
Besides, this year we had a colder winter in my town.
For all those scientists know, we could be in for global cooling.
Here's the difference between weather and climate.
Weather is what the atmosphere does in the short term, hour to hour, day to day.
Weather is chaotic...
which means that even a microscopic disturbance can lead to large-scale changes.
That's why those 10-day weather forecasts are useless.
A butterfly flaps its wings in Bali...
and six weeks later your outdoor wedding in Maine is ruined.
Climate is the long-term average of the weather over a number of years.
It's shaped by global forces that alter the energy balance in the atmosphere...
such as changes in the sun, tilt of the Earth's axis...
the amount of sunlight the Earth reflects back to space...
and the concentration of greenhouse gases in the air.
A change in any of them affects the climate in ways that are broadly predictable.
My friend's meandering represents the short-term fluctuations, that's weather.
It's almost impossible to predict what will attract his interest next...
but not hard to know what the range of his meandering will be...
because I'm holding him on a leash.
We can't observe climate directly.
All we see is the weather.
But the average weather over the course of years reveals a pattern.
I represent that long-term trend which is climate.
Keep your eye on the man, not the dog.
Weather is hard to predict, like my friend here.
But climate is predictable.
Climate has changed many times in the long history of the Earth...
but always in response to a global force.
The strongest force driving climate change right now...
is the increasing CO2 from the burning of fossil fuels...
which is trapping more heat from the sun.
All that additional energy has to go somewhere.
Some of it warms the air.
Most of it ends up in the oceans.
All over the world, the oceans are getting warmer.
It's most obvious in the Arctic Ocean and the lands that surround it.
Okay, so we're losing the summer sea ice in a place where hardly anyone ever goes.
What do I care if there's no ice around the North Pole?
Ice is the brightest natural surface on the Earth, and open ocean water is the darkest.
Ice reflects incoming sunlight back to space.
Water absorbs sunlight and gets warmer...
which melts even more ice...
which exposes still more ocean surface to absorb even more sunlight.
This is what we call a positive feedback loop.
It's one of many natural mechanisms that magnify any warming...
caused by CO2 alone.
We're at Drew Point, Alaska...
on the edge of the Arctic Ocean.
When I was born the shoreline was a mile farther out...
and it was breaking off at the rate of about 20 feet per year.
Now it's being eaten away at about 50 feet per year.
The Arctic Ocean is warming and at an increasing rate.
So it's ice-free during more of the year.
That leaves the shore here more exposed to erosion from storms...
which are also getting more powerful, another effect of climate change.
The northern reaches of Alaska, Siberia and Canada are mostly permafrost...
ground that has been frozen year-round for millennia.
It contains lots of organic matter.
Old leaves and roots from plants that grew thousands of years ago.
Because the Arctic regions are warming faster than anywhere else on Earth...
the permafrost is thawing.
And its contents are rotting.
Just like when you unplug the freezer.
The thawing permafrost is releasing carbon dioxide and methane...
an even more potent greenhouse gas into the atmosphere.
This is making things even warmer...
another example of a positive feedback mechanism.
The world's permafrost stores enough carbon to more than double the CO2 in the atmosphere.
At the rate we're going, global warming could release most of it by the end of the century.
We might be tipping the climate past a point of no return...
into an unpredictable slide.
Okay, the air, the water and the land are all getting warmer.
So global warming is really happening.
But maybe it's not our fault.
Maybe it's just nature.
Maybe it's the sun.
No, it's not the sun.
We've been monitoring the sun very closely for decades...
and the solar energy output hasn't changed.
What's more, the Earth is warming more at night than in daytime...
and more in winter than in summer.
That's exactly what we expect from greenhouse warming...
but the opposite of what increased solar output would cause.
It's now clear beyond any reasonable doubt that we are changing the climate.
The sun isn't the problem...
but it is the solution, and we've known this for a long time...
much longer than you might think.
Paris, September 1878.
The Eiffel Tower won't be built for years to come.
Witness one of the most glorious spectacles the world has ever seen.
The magnificent head of the Statue of Liberty has just been completed.
Thousands of exhibitors from around the planet covered 66 acres of Paris...
with their inventions and goods.
Edison's first public demonstration of the light bulb...
will not take place for another year.
There's no such thing as electrical appliances.
People don't flick switches and press buttons.
It's a hand-cranked, horse-drawn world.
That's the guy we came to see, the one with the crazy mustache.
He's a math teacher named Augustin Mouchot.
Remember, it's 1878, this is a world lit mostly by gaslight.
The automobile is still years away.
But Mouchot here is dazzling the crowd with his solar power concentrator.
The sun belongs to all of us.
Even though it is 150 million kilometers away from us, feel its awesome power.
My invention concentrates the free energy of the sun...
and converts it into mechanical motion.
It can power any kind of machine.
It can produce electricity or run a printing press...
or make ice on a hot day.
Ha-ha-ha! Think of it, sunlight converted into ice.
You see, my friends, what wonders we can work...
if we harness the bounteous energy of the sun.
The world will someday run out of coal.
But the magnificent sun will always be there for us.
Mouchot took home the gold medal from the fair.
But the price of coal tumbled, becoming so cheap...
that there was no interest in solar energy.
Besides, no one understood back then what the true cost was of burning fossil fuel.
Mouchot's research funding was cut off.
Thirty-five years later, in the early years of the 20th century...
another door opened to an alternative future.
It happened in Egypt, on the banks of the Nile.
Memo to future time travelers.
This would be an excellent entry point for averting climate change.
Egypt, 1913.
That's Frank Shuman of Philadelphia.
He's only had three years of schooling...
but his genius for innovation more than makes up for that.
Before he was 30, Shuman had invented safety glass.
Its use in automobiles and skylights saved countless lives...
and made him a very rich man.
Rich enough to pursue his real passion:
Solar energy.
Shuman led the team that designed and built...
an array of solar energy concentrators.
It could power a steam engine.
Shuman is hoping to use the sun's power to irrigate the desert and turn it green.
The official inauguration of Shuman's solar power plant in 1913...
was a dazzling success.
He had invented a practical way to tap the sun's energy on an industrial scale...
making solar energy even cheaper than coal.
The British and German governments both offered Shuman...
generous funding to develop his invention.
It was the ideal source of abundant power in tropical regions...
where imported coal was prohibitively expensive.
But Shuman was dreaming even bigger.
In a letter to Scientific American...
he calculated that his solar power plants...
if deployed in an area of the Sahara Desert only 150 miles on a side...
could supply as much power as consumed by all the industries of the world.
But it was not to be.
The market for a liquid fossil fuel, petroleum...
was exploding for shipping, home heating, cars and trucks.
Oil was abundant, cheaper even than coal...
and much easier to get out of the ground and process.
It took 100 men a week to fuel a ship with coal...
but with oil one man could do the job in a single day.
A year after Shuman's triumph in the desert...
World War I broke out.
His solar collectors were recycled into weapons.
Frank Shuman's dream of a solar-powered civilization...
would have to wait another century before it was reborn.
There's another inexhaustible source of clean energy for the world.
The winds themselves are solar powered...
because our star drives the winds and the waves.
Unlike solar collectors...
wind farms take up very little land...
and none at all if offshore, where the winds are strongest.
If we could tap even 1 percent of their power...
we'd have enough energy to run our civilization.
And more solar energy falls on the Earth in one hour...
than all the energy our civilization consumes in an entire year.
If we could harness a tiny fraction of the available solar and wind power...
we could supply all our energy needs forever...
and without adding any carbon to the atmosphere.
It's not too late.
There's a future worth fighting for.
How do I know?
Every one of us comes from a long line of survivors.
Our species is nothing, if not adaptive.
It was only because our ancestors learned to think long-term...
and act accordingly that we're here at all.
We've had our backs to the wall before...
and we came through to scale new heights.
In fact, the most mythic human accomplishments of all...
came out of our darkest hour.
Once there was a world...
rigged with 60,000 hair-triggered nuclear weapons.
The combatants were the two most powerful countries on earth.
And they were locked in a deadly embrace...
each vowing that they would rather see everything we love destroyed...
than submit to the will of the other.
When I was 3 years old the largest man-made explosion of all time...
was detonated by the Soviet Union.
That terror has subsided to be replaced by new fears.
The danger that the 2000 largest cities on earth would be reduced to rubble...
in the space of an afternoon is no longer one of them.
The nuclear rivalry between the United States and the Soviet Union...
had another byproduct.
The Apollo missions to the moon were an extension of the arms race...
that raged between them.
Sending people to orbit the Earth or go to the moon...
requires big, reliable, powerful rockets...
precisely the same technology you need...
to carry a nuclear warhead halfway around the planet...
to destroy your enemy's largest cities.
I believe that this nation should commit itself to achieving the goal...
before this decade is out...
of landing a man on the moon and returning him safely to the Earth.
President Kennedy's 1961 speech electrified the nation...
and it contained much that was remarkably prophetic...
but not a word about a scientific objective for going to the moon.
No questions about the moon's origin...
or the hope of bringing back samples to analyze.
No, the Apollo missions were conceived...
as a demonstration of the superior power and precision of our strategic missiles.
But a funny thing happened to us on our way to the moon.
We looked homeward...
and discovered another world...
our own.
For the first time we inhabitants of Earth...
could step back and see it as it really is.
One world, indivisible, and kind of small in the cosmic context.
Whatever the reason we first mustered the enormous resources required...
for the Apollo program...
however mired it was in Cold War nationalism and the instruments of death...
the inescapable recognition of the unity and fragility of the Earth...
is its clear and luminous dividend...
the unexpected gift of Apollo.
A project conceived in deadly competition...
made us recognize our community.
What titan built this wonder of the world?
It was the Ifugao people of the Philippines...
working with not much more than their hands.
About 10,000 years ago our ancestors all over the world...
took advantage of another form of climate change...
the gentler climate of the intermission in the Ice Age.
They invented agriculture.
They gave up the ceaseless wandering, hunting and gathering...
that had been their way of life for a million years or so...
to settle down and produce food.
They found a way to harvest 10 to 100 times more solar energy...
than the environment naturally provided for their ancestors.
People all over the world made the difficult transition...
from nomadic cultures to agricultural ones...
that used solar energy more efficiently.
It gave rise to civilization.
We stand on the shoulders of those who did the hard work...
that such a fundamental transformation required.
Now, it's our turn.
Once there was a world.
If life ever existed on Venus...
it would have had no chance to avert the hellish destiny of this world.
This runaway greenhouse effect was unstoppable.
Once there was a world...
ours.
And that world is now.
There are no scientific or technological obstacles...
to protecting our world and the precious life that it supports.
It all depends on what we truly value...
and if we can summon the will to act.
"But why, " some say, "the moon?
Why choose this as our goal?"
And they may well ask:
"Why climb the highest mountain?"
We choose to go to the moon.
We choose to go to the moon...
We choose to go to the moon in this decade and do the other things...
not because they are easy, but because they are hard.