phenomenon we described above is known as the natural greenhouse
effect, and is responsible for keeping the temperature of the Earth
a full 33° C warmer than it would be otherwise. It creates a
climate in which humans and other life forms can live under relatively
hospitable conditions. However, human activities are causing a rapid
increase in the concentrations of greenhouse gases, and we are now
facing an "enhanced" greenhouse effect. The result of
the enhanced greenhouse effect is an increase in the global average
surface temperature of the Earth -- and possible changes in climate
on a global scale.
11: Scheme of greenhouse effect.
(From John R. Herman and Richard A.
Sun, Weather, and Climate. Washington D.C.: NASA, 1978.)
noted earlier that even slight increases in the concentrations of
greenhouse gases in the atmosphere result in more heat being trapped.
In this section, we will summarize the scientific evidence that
the increase in concentration of greenhouse gases impacts global
climate conditions. We will also discuss the human activities that
are causing the increase, and the policies that have been put in
place to slow or reduce the effects of global climate change.
from the previous section that there is a natural greenhouse effect
that is necessary to maintain temperatures warm enough to sustain
current ecosystems. This "temperature bath" occurs due
to the absorption of short-wave (visible) solar radiation by surfaces
on the Earth, and the subsequent transformation of that radiation
into longer-wave infrared. Infrared is then absorbed and "trapped"
by greenhouse gases, causing the troposphere to maintain a significantly
warmer temperature than it would without this effect. Natural sources
of greenhouse gases are part of a balanced chemical cycle that has
been relatively steady during the time of human evolution to the
current ecosystems evolved over long time periods to acclimate to
the environmental temperature, a permanent increase of even 1°
on the average can be very disruptive, especially when this change
occurs too quickly for the system to co-evolve.
evidence has been used to understand the correlation between the
amount of CO2 in the atmosphere and global temperature
and climate. For example, it is believed that when life began around
4 billion years ago, the sun was about 30% fainter than it is today.
However, much higher levels of CO2 in the air (about
1000 times what they are today) made for enough warmth on the surface
of the earth so at least some regions were above the freezing point
of water, and began to provide conditions necessary for life to
emerge. Analysis of CO2 in the frozen layers of ice in
Antarctica provides evidence that over the past 160,000 years, climatic
change and levels of carbon dioxide are closely related.
greenhouse effect can become a "problem" when the amount
of heat-absorbing gases in the atmosphere rapidly rises far above
the levels at which they have been historically present. Since the
Industrial Revolution, there has been a high rate of increase in
the concentration of greenhouse gases, due in large part to the
combustion of fossil fuels and the destruction of large plant systems
such as tropical forests. Carbon dioxide concentrations, for example,
have risen by 30% since the late 1800's. Furthermore, scientists
predict that CO2 concentration will continue to rise,
likely reaching 2 to 3 times its pre-industrial level by 2100.
hypothesis that the known increase in greenhouse gas concentrations
has led and will continue to lead to changes in the Earth's climate
has been hotly debated in the past decade. However, a vast majority
of scientists are now in agreement that evidence is sufficiently
strong to prove the relationship. They are now mostly concerned
with how to predict
the impacts and scale of climate change, and how society can adapt
to and minimize the harmful effects of these changes.
temperature records only exist of the last century or so, and scientists
use paleohydrologic studies to extract longer-term records. These
data show that global average surface temperature can vary greatly
over short periods of time. For example, there was an apparent temporary
cooling during the 1940's, 50's, and 60's. However, the past century
has seen an overall increase in temperature by 1° F (or 0.6°
C), with about half of that increase occurring since the late 1970's.
Seventeen of the eighteen warmest years of the 20th century occurred
between 1980 and 2000.
studies have also shown a positive correlation between greenhouse
gas concentration and temperature, as shown by Figures 12.1 and
12.2. Notice on each graph that the blue line indicates readings
taken from historical records, tree rings, corals, and air trapped
in Antarctic ice, while the shift to a purple line indicates temperatures
and CO2 concentrations directly measured and recorded.
fossil fuel combustion is one of the primary sources of greenhouse
gases, fuel use is indicative of those countries that have contributed
most to global climate change. The US leads the world in the gross
amount of carbon emissions from fossil fuels, followed by China.
Overall, developing countries contribute a very small percentage
as compared to industrialized nations. In speaking of CO2
emissions, we normally speak in terms of carbon emitted, rather
than CO2 emitted. The US is also the leader in terms
of the amount of carbon dioxide emitted per person, while China
is the leader in terms of carbon emitted per dollar GNP.
developed countries produce the vast majority of the carbon emissions
from fossil fuel use, they often use more environmentally efficient
technology. As highly-populated developing countries are becoming
more industrialized, we risk a further leap in greenhouse gas concentrations
due to the use of outdated (cheaper) technology.
have been many international meetings regarding climate change since
1979 and they continue today. The most recent meeting that resulted
in international agreement was the Kyoto conference in 1997. The
debate there was between industrialized and the developing nations.
The points made by each are as follows:
countries claim that most of the population growth is occurring
in the developing countries, and most of the negative effects
of climate change will affect those countries. Therefore, the
developing world should be active participants in curtails on
greenhouse gas emissions.
countries claim that industrialized countries caused these problems
as they achieved their socioeconomic status. As such, the industrialized
countries should be solely responsible for minimizing emissions
etc., since emission limits would slow development in the developing
following preliminary agreement was reached as part of the Kyoto
industrialized countries agreed to lower their greenhouse gas
emissions by a combined 5.2% of 1990 levels by 2008. Of this,
the US agreed to 7%, Japan to 7%, and Europe to 8%.
were no emissions reductions for the developing countries.
analysis of these agreed reductions would mean that due to population
growth, the USA would have to reduce its carbon emissions by 33%
of what it would have been in 2008. However, because the
US lobbied for the inclusion of the effects of carbon sinks (or
the "recapturing" of carbon by new forest growth, etc.)
the reduction of actual emissions of greenhouse gases by
2008 only amounts to 2-3% less what the emissions were in 1990.
spite of the efforts of industrialized nations to reduce greenhouse
gas emissions, it is understood that the overall atmospheric concentration
of greenhouse gases would still increase due to the impacts of
the developing world.
order for this initial agreement to be considered binding, at least
55 countries must have the Treaty officially ratified and put in
place by their individual governments. Furthermore, the 55 countries
that ratify the Treaty must produce at least 55% of the world's
greenhouse gas emissions. Otherwise, the Kyoto Treaty is not binding
to the countries that signed it, and results will not be achieved.
the US signed the Kyoto Treaty within a year of its proposal, it
has yet to be put before the Senate, where it would need a 2/3 vote
to ratified. Shortly after taking office in 2001, President Bush
suggested that the US may be withdrawing from the Kyoto treaty.
His administration points to the "energy crisis" in California,
possible threats to the economy, and lack of regulation of emissions
in developing nations as the primary reasons for pulling out.
more detailed explanation of the Kyoto Protocol can be found at
This document is a Congressional Research Service Report for Congress,
and was written by Susan R. Fletcher, Senior Analyst in International
Environmental Policy. It was last updated March 2000.