Changes in climate and related factors have been observed in the United States. These were recently assessed in Global Climate Change Impacts in the United States23 and discussed in two of the ACC panel reports (NRC, Advancing the Science and Adapting to the Impacts), and include the following:

  • U.S. average air temperature increased by more than 2°F over the past 50 years, and total precipitation increased on average by about 5 percent;24
  • Sea level has risen along most of the U.S. coast, and sea level rise is already eroding shorelines, drowning wetlands, and threatening the built environment;25
  • Permafrost temperatures have increased throughout Alaska since the late 1970s, damaging roads, runways, water and sewer systems, and other infrastructure;26
  • There have been widespread temperature-related reductions in snowpack in the northeastern and western United States over the last 50 years,leading to changes in the seasonal timing of river runoff;27
  • Precipitation patterns have changed: heavy downpours have become more frequent and more intense;28 the frequency of drought has increased over the past 50 years in the southeastern and western United States, while the Midwest and Great Plains have seen a reduction in drought frequency;29 and
  • The frequency of large wildfires and the length of the fire season have increased substantially in both the western United States and Alaska.30


Projections of future climate change impacts are developed in three steps:

(i) Emission Scenarios: Scientists first develop different scenarios of how GHG emissions and other human drivers of climate change (such as land use change) could plausibly evolve over the 21st century. Each scenario is based on specific assumptions about future social, economic, technological, and environmental change.31

(ii) Climate Simulation: Computer-based models of the climate system32 are then used to estimate how temperature, precipitation, storm patterns, and other aspects of climate would respond to each emission scenario. Typically, a number of different scenarios and models are used to explore a wide range of possible future climate changes.

(iii) Impact Assessment: Finally, researchers evaluate the potential impacts of climate change, including their likelihood and temporal evolution by combining climate

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