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.
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About Global Warming
