We call the
result global warming, but it is causing a set of changes to the Earth's
climate, or long-term weather patterns, that varies from place to place. As the
Earth spins each day, the new heat swirls with it, picking up moisture over the
oceans, rising here, settling there. It's changing the rhythms of climate that
all living things have come to rely upon.
What will we
do to slow this warming? How will we cope with the changes 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.
Why is this a concern?
The rapid
rise in greenhouse gases is a problem because it is changing the climate faster
than some living things may be able to adapt. Also, a new and more
unpredictable climate poses unique challenges to all life.
Historically,
Earth's climate has regularly shifted back and forth between temperatures like
those we see today and temperatures cold enough that large sheets of ice
covered much of North America and Europe. The difference between average global
temperatures today and during those ice ages is only about 5 degrees Celsius (9
degrees Fahrenheit), and these swings happen slowly, over hundreds of thousands
of years.
Now, with
concentrations of greenhouse gases rising, Earth's remaining ice sheets (such
as Greenland and Antarctica) are starting to melt too. The extra water could
potentially raise sea levels significantly.
As the
mercury rises, the climate can change in unexpected ways. In addition to sea
levels rising, weather can become more extreme. This means more intense major
storms, more rain followed by longer and drier droughts (a challenge for
growing crops), changes in the ranges in which plants and animals can live, and
loss of water supplies that have historically come from glaciers.
Scientists
are already seeing some of these changes occurring more quickly than they had
expected. According to the Intergovernmental Panel on Climate Change, eleven of
the twelve hottest years since thermometer readings became available occurred
between 1995 and 2006.
