Human-induced climate change and its impacts will continue for many decades, and in some cases for many centuries. Individually and collectively, these changes pose risks for a wide range of human and environmental systems, including freshwater resources, the coastal environment, ecosystems, agriculture, fisheries, human health, and national security, among others.
The ultimate magnitude of climate change and the severity of its impacts depend strongly on the actions that human societies take to respond to these risks.
Despite an international agreement to stabilize GHG concentrations “at levels that would avoid dangerous anthropogenic interference with the climate system” (UN-FCCC, 1992), global emissions of CO2 and several other GHGs continue to increase. Projections of future climate change, which are based on computer models of how the climate system would respond to different scenarios of future human activities, anticipate an additional warming of 2.0°F to 11.5°F (1.1°C to 6.4°C) over the 21st century. A separate National Research Council (NRC) report, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia (NRC, 2010i), provides an analysis of expected impacts at different magnitudes of future warming.
In general, it is reasonable to expect that the magnitude of future climate change and the severity of its impacts will be larger if actions are not taken to reduce GHG emissions and adapt to its impacts. However, as with all projections of the future, there will always be some uncertainty regarding the details of future climate change. Several factors contribute to this uncertainty:
Projections of future climate change depend strongly on how human societies decide to produce and use energy and other resources in the decades ahead.
Human-caused changes in climate overlap with natural climate variability, especially at regional scales.
Certain Earth system processes—including the carbon cycle, ice sheet dynamics, and cloud and aerosol processes—are not yet completely understood or fully represented in climate models but could potentially have a strong influence on future climate changes.
Climate change impacts typically play out at local to regional scales, but processes at these scales are not as well represented by models as continental- to global-scale changes.
The impacts of climate change depend on how climate change interacts with other global and regional environmental changes, including changes in land use, management of natural resources, and emissions of other pollutants.
The impacts of climate change also depend critically on the vulnerability and