we've already set into motion? While
 
we struggle to figure it all out, the face 

of the Earth as we know it—coasts, 

forests, farms and snow-capped

mountains—hangs in the balance.

 Greenhouse effect  

The "greenhouse effect" is the 

warming that happens when certain
 
gases in Earth's atmosphere trap heat.
 
These gases let in light but keep heat

from escaping, like the glass walls of a
 
greenhouse. First, sunlight shines
 
onto the Earth's surface, where it is 

absorbed and then radiates back into
 
the atmosphere as heat. In the
 
atmosphere, “greenhouse” gases trap 

some of this heat, and the rest 

escapes into space. The more
 
greenhouse gases are in the 

atmosphere, the more heat

 gets trapped. Scientists have known
 
about the greenhouse effect since
 
1824, when Joseph Fourier calculated
 
that the Earth would be much colder if 

it had no atmosphere. This 

greenhouse effect is what keeps the
 
Earth's climate livable. Without it, the 

Earth's surface would be an average 

of about 60 degrees Fahrenheit 

cooler. In 1895, the Swedish chemist 

Svante Arrhenius discovered that 

humans could enhance the 

greenhouse effect by making carbon 

dioxide, a greenhouse gas. He kicked
 
off 100 years of climate research that
 
has given us a sophisticated 

understanding of global warming. 

Levels of greenhouse gases (GHGs)
 
have gone up and down over the 

Earth's history, but they have been
 
fairly constant for the past few
 
thousand years. Global average
 
temperatures have stayed fairly
 
constant over that time as well, until
 
recently. Through the burning of fossil
 
fuels and other GHG emissions, 

humans are enhancing the 

greenhouse effect and

warming Earth. 

Scientists often use the term "climate
 
change" instead of global warming.

This is because as the Earth's 

average temperature climbs, winds
 
and ocean currents move heat around
 
the globe in ways that can cool some

areas, warm others, and change the
 
amount of rain and snow falling. As a

 result, the climate changes differently
 
in different areas

 Aren’t temperature changes natural?  

The average global temperature and
 
concentrations of carbon dioxide (one

 of the major greenhouse gases) have 

fluctuated on a cycle of hundreds of

thousands of years as the Earth's
 
position relative to the sun has varied.

 As a result, ice ages have come and
 
gone. However, for thousands of 

years now, emissions of GHGs to the

atmosphere have been balanced out
 
by GHGs that are naturally absorbed.

As a result, GHG concentrations and 

temperature have been fairly stable.

This stability has allowed human
 
civilization to develop within a 

consistent climate. Occasionally, other
 
factors briefly influence global
 
temperatures. Volcanic eruptions, for
 
example, emit particles that 

temporarily cool the Earth's surface.
 
But these have no lasting effect 

beyond a few years. Other 

cycles, such as El Niño, also work on
 
fairly short and predictable cycles.

Now, humans have increased the
 
amount of carbon dioxide in the

atmosphere by more than a third since
 
the industrial revolution. Changes

this large have historically taken 

thousands of years, but are now

happening over the course of
 
decades.