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Greenhouse gases such as carbon dioxide trap heat in the Earth’s atmosphere.  According to the Intergovernmental Panel on Climate Change (IPCC), greenhouse gas emissions from human activities, primarily the burning of fossil fuels, are causing global warming.  The IPCC’s latest report lists several projected impacts of climate change, including sea level rise, more intense storms and droughts, biodiversity loss, reduced agricultural yields, and water supply stress.  The report represents the consensus of world’s leading climate scientists and was approved by member governments, including the United States.  The report concludes that greenhouse gas emissions must be reduced by 50 to 85% by 2050 in order to limit global warming to 4 degrees Fahrenheit, avoiding many of the worst impacts of climate change.* 

Transportation accounts for 29 percent of U.S. greenhouse gas emissions.**  Strategies for reducing transportation emissions fall into four categories: 1) increasing vehicle efficiency, 2) lowering the carbon content of fuels, 3) reducing vehicle miles traveled (VMT), and 4) improving system efficiency.  Public transportation can reduce greenhouse gas emissions by providing a low emissions alternative to driving and facilitating compact development (thus reducing VMT), as well as by minimizing the carbon footprint of its operations. 

Climate-related changes are already observed in the United States and will increase in the future, according to the U.S. Federal government’s Global Change Research Program.***  These changes are projected to grow even if aggressive action is taken to reduce greenhouse gas emissions because of the level of long-lived emissions already in the atmosphere.  Public transportation infrastructure and operations will need to be adapted to perform under changing climate conditions.   For instance, extreme heat can cause deformities in rail tracks, resulting in speed restrictions and derailments.  Subway tunnels, busways, rails, and roads are vulnerable to an increase in the frequency of flooding from sea-level rise, storm surge, and more intense rain storms.  While widespread climate impacts are occurring now and expected to increase, the extent of climate change, and its impacts, depends on choices made today to mitigate human caused emissions of greenhouse gases.

Definitions

• Mitigation: An intervention to reduce the causes of changes in climate, such as through reducing emissions of greenhouse gases to the atmosphere.
• Adaptation: Adjustment in natural or human systems to a new or changing environment that exploits beneficial opportunities or moderates negative effects.
• Carbon Dioxide Equivalent (CO2e):  Carbon dioxide (CO2) makes up 84% of all U.S. greenhouse gas emissions and 96% of all U.S. transportation greenhouse gas emissions.  The other main human induced greenhouse gas emissions are methane (CH4), nitrous oxide (N20), chlorofluorocarbons (CFCs), hydro fluorocarbons (HFCs), per fluorocarbons (PFCs), and sulfur hexafluoride (SF6).  Greenhouse gas emissions are commonly reported in terms of carbon dioxide equivalent (CO2e) to provide a common unit of measure.  Other greenhouse gases are converted into CO2 equivalent on the basis of their global warming potential.  For instance, one pound of CH4 is estimated to have the same global warming effect as 21 pounds of CO2.
• Resilience: A capability to anticipate, prepare for, respond to, and recover from significant multi-hazard threats with minimum damage to social well-being, the economy, and the environment.
• Vulnerability: The degree to which a system is susceptible to, or unable to cope with, adverse effects of climate change, including climate variability and extremes. 
  
  

*Intergovernmental Pane on Climate Change (IPCC), Fourth Assessment Report, 2007 http://www.ipcc.ch/.

**U.S. Department of Transportation, Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions, 2010.

*** U.S. Global Change Research Program, Global Climate Change Impacts in the United States, 2009.