The climate on the Earth depends on a number of factors, including the mix of gases in the atmosphere, the
amount of energy the Earth receives from the sun, and the conditions at the Earth's surface.
Scientists have shown that the levels of carbon dioxide (CO2)
, a major greenhouse gas (GHG) in the
atmosphere, have significantly changed over the last 600 million years and that temperature has
varied as well.
Even over just the last half-million years, the CO2
levels in the atmosphere and the average
global temperature have varied (see diagram).
The examination of ice cores from the arctic region indicate at least four periods over the past half
million years when there have been relatively high levels of CO2
in the atmosphere. These
have corresponded to warm periods.1
Since the early 1800s, atmospheric concentrations of CO2
have increased nearly 30%, and the
concentrations of other GHGs like methane (more than doubled) and nitrous oxide (up by
about 15%) have also increased.2
Scientists agree that CO2
levels in the atmosphere are currently high and are
increasing. Scientists also agree that the average annual temperature at the surface
of the Earth has increased by about 1°F over the past 150 years.3
The greenhouse effect
traps the heat energy that drives
the Earth's weather and climate. Continuing growth in the level of GHGs in the atmosphere
and a variety of other observations are together raising concerns that the Earth's climate is
changing and that change is increasing emissions of
CO2 from human activity
may be driving the change.3
Temperature and CO2 Concentration in the atmosphere over the past 400,000
years (from the Vostok ice core), UNEP/GRID-Arendal (accessed on February 12, 2009).
www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html (accessed November 2004).
Summary for Policy Makers, Contribution of Working Group II to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change (accessed on February 12, 2009).