reported in the literature are aggregated across sectors and countries and presents estimates of the marginal damage of a ton of carbon dioxide equivalent1 (CO2-eq) from various IAMs. The committee did not conduct its own modeling analyses of damages related to climate change. We determined that attempting to estimate single values would be inconsistent with the rapidly changing nature of knowledge about climate change and the extremely large uncertainties associated with estimation of climate-change effects and damages.

Climate-Change Observations, Drivers, and Future Projections

According to the Intergovernmental Panel on Climate Change (IPCC), scientists have documented that Earth’s climate system is warming, the last decade was the warmest on record, global average temperatures have increased about 1.3°F since 1990, and sea levels at the end of the 20th century were rising almost twice as fast as over the century as a whole (IPCC 2007a,b).2 Arctic sea ice and glaciers are rapidly shrinking. Economic losses from extreme weather events, such as tropical cyclones, heavy rain storms, flooding, severe heat waves, and droughts, are increasing rapidly (CCSP 2008).

The IPCC states that “most of the observed increase in global average temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic GHG concentrations” (IPCC 2007a, p.5). With high and increasing confidence, a range of “fingerprinting” techniques attribute a substantial fraction of recent warming to anthropogenic causes (IPCC 2007a).

Although the greenhouse effect is a natural process necessary for life on Earth, humans have inadvertently intervened in this process so that the greenhouse effect is now trapping additional heat in Earth’s atmosphere, which is driving climate change. Specifically, human activities have led to a significant increase in the amount of CO2 and methane (CH4) in the atmosphere. These additional GHGs absorb more energy and let less heat escape to space. Therefore, Earth’s climate is warming.3

GHG emissions have steadily grown since the Industrial Revolution, with a 70% increase between 1970 and 2004. Burning fossil fuels, agri-


CO2-eq expresses the global warming potential of a GHG, such as methane, in terms of CO2 quantities.


The IPCC is an intergovernmental scientific body given to the assessment of climate change. It does not conduct research. IPCC estimates are derived from literature reviews and assessments, not from its independent predictions or projections.


Airborne particles may have either a warming or cooling effect. Sulfate particles reflect incoming sunlight and cause a cooling effect at the surface. Other types of particles, referred to as carbon black, absorb incoming sunlight and trap heat in the atmosphere.

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