In order to project future changes in the climate system, scientists must first estimate how GHG emissions and other climate forcings (such as aerosols and land use) will change over time. Since the future cannot be known with certainty, a large number of different scenarios are developed, each using different assumptions about future economic, social, technological, and environmental conditions. These scenarios have increased in complexity over time, and the most recent scenario development efforts include sophisticated models of energy production and use, economic activity, and the possible influence of different climate policy actions on future emissions. Future climate change, like past climate change, is also subject to natural climate variations that modulate the expected warming trend.
After future forcing scenarios are developed, climate models are used to simulate how these changes in GHG emissions and other climate forcing agents will translate into changes in the climate system. Climate models are computer-based representations of the atmosphere, oceans, cryosphere, land surface, and other components of the climate system. All climate models are fundamentally based on the laws of physics and chemistry that govern the motion and composition of the atmosphere and oceans. The most sophisticated versions of these models—referred to as Earth system models—include representations of a wide range of additional physical, chemical, and biological processes such as atmospheric chemistry and ecosystems on land and in the oceans. The resolution of climate models has also steadily increased, although global models are still not able to resolve features as small as individual clouds, so these small-scale processes must be approximated in global models.
After decades of development by research teams in the United States and around the world, and careful testing against observations of climate over the past century and further into the past, scientists are confident that climate models are able to capture many important aspects of the climate system. Scientists are also confident that climate models give a reasonable projection of future changes in climate that can be expected based on a particular scenario of future GHG emissions, at least at large (continental to global) scales. A variety of downscaling techniques have been developed to project future climate changes at regional and local scales. These techniques are not as well established and tested as global climate models, and their results reflect uncertainties in both the underlying global projections and regional climate processes. Hence, predictions of regional and local climate change are generally much more uncertain than large-scale changes. Other key sources of uncertainty in projections of future climate change include (1) uncertainty in future climate forcing, especially how human societies will produce and use energy in the decades ahead; (2) processes that are not included or well represented in models, such as changes in ice sheets, and certain land use and ecosystem processes; and (3) the possibility that abrupt changes or other climate "surprises" (see below) may occur.
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