[music playing]
NEIL DEGRASSE TYSON: It used to be
hard to keep food from spoiling in the summertime.
There was a person called the ice man.
He would come to your house and sell you a big block of ice.
You'd keep it in something called
an ice box, to preserve the kinds
of food that spoil quickly.
But that was a drag because the ice kept melting.
It would drip all over the floor.
So somebody thought up another way to keep food cold.
It was a gas-powered system that used ammonia
or sulfur dioxide as a coolant.
No more lugging blocks of ice.
What could be bad about that?
The chemicals were not only poisonous,
they smelled terrible.
And there were leaks.
A substitute coolant was badly needed.
One that would circulate inside the refrigerator, but would not
poison anyone if the refrigerator leaked.
Or pose a danger if it was sent to the junkyard.
Something that wouldn't make you sick.
Wouldn't burn your eyes or attract bugs.
Or even bother the cat.
But in all of nature, no such material seemed to exist.
So chemists in the United States invented a class of molecules.
Little collections of even tinier things
called atoms that had never existed on Earth before.
They call them chlorofluorocarbons, or CFCs,
Because they were made up of one or more carbon atoms and some
chlorine and/or fluorine atoms.
These new molecules were wildly successful.
Far exceeding the expectations of their inventors.
Not only did CFCs become the chief coolant in refrigerators,
but also in air conditioners.
There were so many things you could do with CFCs.
People used them to propel great fluffy mounds of shaving cream.
And to protect your hair from wind and rain.
It was also the propellant that made
fire extinguishers and spray paint cans so much fun.
It was good for foam insulation, industrial solvents,
and cleansing agents.
The most famous brand name of these chemicals
was freon, a trademark of DuPont.
It was used for decades and no harm
ever seemed to come from it.
Safe as safe could be, everyone figured.
Until in the early 1970s, two atmospheric chemists
at the University of California Irvine
were studying Earth's atmosphere.
Mario Molina was a Mexican immigrant,
a young laser chemist.
Sherwood Rowland was a chemical kineticist--
someone who studied the motions of molecules and gases
under varying conditions.
He was from a small town in Ohio.
Molina wanted to grow as a scientist.
He was looking for a project that
would take him as far from his previous research
experience as possible.
He wondered, what happens to those freon molecules
when they leak out of the air conditioner?
This was a time when the Apollo astronauts
were still making regularly scheduled trips to the moon.
And NASA was contemplating weekly launches
of a space shuttle.
Would all that burning rocket fuel pose
a danger to the stratosphere--
that place where Earth's atmosphere
meets the blackness of space?
And this is how science works a lot of the time.
You set out to solve one problem and you
happen on a completely different,
unexpected phenomena.
Those wonderfully inert, harmless CFCs,
the magic molecules of shaving cream and hairspray,
didn't simply vanish when we were done with them.
They had an afterlife at the edge of space,
where they accumulated in the trillions.
They were silently congregating high above the Earth
and they were up to no good.
Molina and Roland were alarmed to discover that the CFCs had
thinned the protective layer that shielded us from the sun's
harmful ultraviolet radiation.
And it was getting worse all the time.
When UV light hits a CFC molecule,
it strips away the chlorine atoms.
Once that happens, the chlorine atoms start devouring
the precious ozone molecules.
It wasn't until our planet developed an ozone layer,
about two and a half billion years ago,
that it was safe for life to leave the ocean for the land.
A single chlorine atom can destroy
100,000 ozone molecules.