(soft ambient music)
[Kristen] Why is the world warming up?
Let's think of Earth for just a moment as a greenhouse.
Every second, a huge amount of energy
smashes into our greenhouse.
Our atmosphere, the glass roof and walls,
reflect some of that energy,
but most of it passes right through and heats everything up.
Much of that heat used to escape back into space,
keeping us at a stable, warm temperature.
But then something changed.
Humans began to spew enough carbon dioxide
to fill billions of greenhouses each day.
What does all that greenhouse gas do?
It makes the glass of our greenhouse thicker.
Thicker glass means heat escapes into space
much more slowly.
And don't forget, energy from the sun
is still streaming in just as quickly as before.
The result is a planet with temperatures
too hot for our liking and, eventually, living.