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 bable occurred between 1995 and 2006