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:
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