Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportation's Role in Reducing U.S. Greenhouse Gas Emissions: Volume 1
2.0 Climate Change, Greenhouse
Gas Emissions, and
Transportation
2.1 CLIMATE CHANGE AND GREENHOUSE GASES
Greenhouse gases (GHG) trap heat in the earth’s atmosphere. Common
greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide
(N2O), ozone, water vapor, and chlorofluorocarbons. Many of these are naturally
occuring and necessary to maintain an atmospheric temperature that supports
human life.
GHGs are produced by both natural and human activities, and can be removed
from the atmosphere through natural processes. However, human-produced
GHGs have significantly exceeded natural absorption rates since the industrial
revolution. CO2 is the predominant GHG from human sources, with a majority
resulting from the combustion of fossil fuel. Once released, CO2 and other
greenhouse gases take many years to leave the atmosphere. Atmospheric
lifetimes are estimated to be 50-200 years for CO2, 9-15 years for CH4, and 120
years for N2O. 16 The combination of long atmospheric lifetimes, increasing GHG
output and deforestation have resulted in the increased atmospheric
concentration of these gases. Since the beginning of the industrial revolution,
atmospheric concentrations of CO2 have increased by 36 percent, CH4
concentrations have more than doubled, and N2O concentrations have risen by
approximately 18 percent. 17 Human activities over the past 70 years have also
produced synthetic chemicals that are powerful greenhouse gases with
atmospheric lifetimes ranging from years to millennia. 18
These substances
include hydroflurocarbons (HFCs), chlorofluorocarbons (CFCs) and sulfur
hexafluoride (SF6).
GHG emissions are projected to continue to rise. The Intergovernmental Panel
on Climate Change (IPCC) estimates that in the absence of additional climate
policies to reduce GHG emissions, baseline global GHG emissions will increase
16 Intergovernmental Panel on Climate Change (IPCC) (1996) Second Assessment Report.
17 Intergovernmental Panel on Climate Change (IPCC) (2007) Fourth Assessment Report,
Climate Change 2007: Synthesis Report. Valencia, Spain.
18 Intergovernmental Panel on Climate Change (IPCC) (1996) Second Assessment Report.
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