CHAPTER TWO THE SCIENTIFIC AND HISTORICAL CONTEXT 11
600 billion to 800 billion metric tons of carbon (mtc)—
the highest amount in at least 400,000 years.
5
About 30
percent of the increase has come from cutting timber and
clearing land for agriculture; the rest stems from extracting
coal, oil, and natural gas from the fossil reservoir and
burning them.
6
Atmospheric concentrations of methane
and nitrous oxide have also risen over the past two cen
turies—by about 150 percent and 16 percent, respectively
—as a result of various agricultural and industrial activi
ties. More recently, halocarbons have begun to accumulate
as well. The combined effect of these additions to the
atmosphere has been to enhance the greenhouse effect
slightly by raising the amount of radiation at the Earth’s
surface by about 0.5 percent—with perhaps half of that
impact offset by the effects of other human activities, such
as the cooling influence of sulfate emissions.
Current evidence indicates that since the mid19th cen
tury, the average surface temperature of the Earth has risen
by between 0.7ºF and 1.4ºF (0.4ºC and 0.8ºC). The
warming trend has been most pronounced during the past
decade and in higher latitudes. Ocean temperatures are
also rising, expanding the volume of water, and that ex
pansion, combined with water from melting glaciers, has
raised global sea level by about four to 10 inches (10 to
20 centimeters) over the past century.
Scientists generally agree that the observed warming is
roughly consistent with the expected effects of changing
concentrations of greenhouse gases and other emissions.
However, other phenomena also appear to be influencing
the Earth’s climate—for example, variations in the Sun’s
brightness and magnetic field, and poorly understood
fluctuations in the circulation of the oceans. As a result,
although scientists have dramatically improved their
understanding of the atmosphere, oceans, and climate in
recent years, they are uncertain about how much of the
observed warming is due to greenhouse gas emissions.
They are even more uncertain about whether the warming
that has occurred has caused moreextreme weather, such
as more and bigger hurricanes, floods, and droughts.
However, some evidence suggests that unusually warm
conditions may have contributed to persistent droughts
in North America, Europe, and Asia between 1998 and
2002 (Hoerling and Kumar, 2003).
Some researchers believe that if people immediately halted
emissions of greenhouse gases, gradual warming of the
oceans would ultimately contribute to an additional warm
ing of the atmosphere of between 0.9ºF and 2.7ºF, or
0.5ºC and 1.5ºC (Mahlman, 2001, p. 8). Over the follow
ing centuries, the climate would return nearly to its pre
industrial state, as the oceans gradually absorbed most of
the extra carbon dioxide from the atmosphere and other
greenhouse gases broke down.
However, as the world’s population grows and the global
economy continues to industrialize, the pace of emissions
—particularly of carbon dioxide—is accelerating. The
period since World War II has seen 80 percent of all car
bon dioxide ever emitted from the burning of fossil fuels
—and twothirds of the entire increase in atmospheric
concentrations (Marland, Boden, and Andres, 2002). Dur
ing the 1990s, annual global emissions of greenhouse gases
ran at about 10 billion metric tons of carbon equivalent
(mtce; see footnote 2), and carbon dioxide concentrations
grew by more than 4 percent. Fossil fuels accounted for
about 6 billion mtc per year; of that total, oil claimed a
share of 45 percent, natural gas, 20 percent; and coal, 35
percent.
7
Net deforestation contributed roughly 1 billion
to 2 billion mtc annually (Watson and others, 2000,
p. 32). About 2½ billion to 3 billion mtce per year of
other greenhouse gases, mostly methane, came from a wide
variety of sources, mainly agricultural activities but also
5. Atmospheric concentrations of carbon dioxide are usually measured
in parts per million (ppm). In those terms, atmospheric carbon
dioxide has increased from about 280 ppm to about 370 ppm.
6. Estimates of emissions and reabsorption of carbon from land use
are based on data for 1850 to 1990 from R.A. Houghton of the
Woods Hole Research Center and an extrapolation based on data
from Houghton and Skole (1990). Estimates of emissions from
fossil fuels are from Marland, Boden, and Andres (2002). Much
of the available data on greenhouse gas emissions, changes in
atmospheric concentrations, and changes in temperature is available
from the Carbon Dioxide Information Analysis Center at http://
cdiac.esd.ornl.gov/pns/pns_main.html. For a discussion of recent
research, see Schimel and others (2001).
7. Coal contains about 80 percent more carbon per unit of energy
than gas does, and oil contains about 40 percent more. For the
typical U.S. household, a metric ton of carbon equals about 10,000
miles of driving at 25 miles per gallon of gasoline or about one year
of home heating using a natural gasfired furnace or about four
months of electricity from coalfired generation.