The Earth maintains a temperature of about 60 degrees Fahrenheit
or 16 degrees Celsius, temperatures that enable people, plants
and animals to live safely within its atmosphere. Naturally occurring
gases known as greenhouse gases help capture the sun's energy,
keeping the Earth warm enough to sustain life. Without greenhouse
gases, scientists estimate temperatures would plunge to zero degrees
Fahrenheit, making it impossible for life to exist.
In the last century, the Earth has warmed by 1 degree Fahrenheit
-- a change significant enough to cause atmospheric disturbances,
some scientists believe -- with the highest increases occurring
within the past 50 years, the height of the industrial revolution,
according to the U.S. Environmental Protection Agency.
scientists believe human activity has contributed to much of the
rise in temperature over the past few decades. The increased emission
of greenhouse gases, such as carbon dioxide caused by car exhaust,
and methane and nitrous oxide from industrial plants and agricultural
activities, has trapped more heat close to the planet's surface.
The EPA estimates that in the last 50 years, the presence of
carbon dioxide in the atmosphere has increased 30 percent, methane
50 percent and nitrous oxide 15 percent. The higher concentrations
of these gases counter the effects of gases in the atmosphere
that help keep the Earth cool.
Many scientists and policy-makers see this trend, commonly referred
to as "global warming," as a threat to the environment
and its inhabitants, while others -- including advocates for large
industry -- say the danger is overstated. They say policies aimed
at stemming high levels of greenhouse gas emissions harm consumers,
who would bear the brunt of the associated costs.
The argument over who or what is to blame for global warming
and to what extent the environment could be damaged by its effect
has become one of the most urgent -- and hotly debated -- environmental
issues of the 21st century.
Evidence that the Earth is warming can be found in the melting
of Arctic glaciers, changes in weather patterns such as the increase
of hurricanes and droughts, and in rising sea levels -- globally
the sea level has risen 4 inches to 8 inches in the past century,
according to the EPA.
Some advocacy groups, criticized by their opponents as apocalyptic,
attribute these changes to poor environmental policy and predict
dire consequences for future generations.
"We will experience extreme temperatures, rises in sea levels,
and storms of unimaginable destructive fury," the nonprofit
organization stopglobalwarming.org predicts on its Web site. "Recently,
alarming events that are consistent with scientific predictions
about the effects of climate change have become more and more
Natural disasters like the 2004 tsunami that hit South Asia and
Hurricane Katrina, which destroyed major parts of the U.S. Gulf
Coast in 2005, have been attributed in part to global warming.
Because other factors affect the Earth's temperature and because
the science of global warming is not exact, some groups, who disagree
with efforts to reduce emissions, have discounted not only the
phenomenon itself, but also its effects on the environment.
"While the planet is indeed warming - probably due in no
small part to industrial greenhouse gas emissions - the warming
has been modest, benign, and largely confined to northern latitudes
during winter nights," Jerry Taylor, director of natural
resource studies at the Cato Institute, wrote in a 2004 commentary.
"There are good reasons to expect that warming pattern to
continue. And that warming pattern does not threaten to usher
in the convulsive climatic events we are warned about in the press
or in the movie theaters. In fact, some scientists and economists
can make a pretty good case that global warming will prove a net
plus to both the economy and the global environment."
Most scientists do agree, however, that global warming is occurring
and that along with other factors including natural changes in
the climate, human activity is accelerating it.
"Greenhouse gases are accumulating in Earth's atmosphere
as a result of human activities, causing surface air temperatures
and subsurface ocean temperatures to rise," a 2001 report
commissioned by the National Research Council said. "Temperatures
are, in fact, rising. The changes observed over the last several
decades are likely mostly due to human activities, but we cannot
rule out that some significant part of these changes is also a
reflection of natural variability. Human-induced warming and associated
sea level rises are expected to continue through the 21st century."
To counter the effects of global warming, 140 countries in 2005
ratified the Kyoto Protocol, an international agreement made in
1997 that forces developed nations to reduce their emission of
greenhouse gases to pre-1990 levels by 2012.
As of early 2006, the United States -- the world's largest contributor
to greenhouse gas emissions -- had not signed the agreement. The
Bush administration said that placing mandatory restrictions on
emissions could hurt large industry and damage the U.S. economy
by making it less competitive with countries such as China, which
is exempt because of its status as a developing nation.