Projections of Future Climate Change Indicate Continued Warming

The most recent comprehensive modeling effort to date included more than 20 different state-of-the-art climate models from around the world. Each of these climate models projected future climate change based on a range of different scenarios of future GHG emissions and other changes in climate forcing. Continued warming is projected by all models, but the trajectory and total amount of warming varies from model to model and between different scenarios of future climate forcing. Based on these results, the IPCC estimates that global average surface temperatures will rise an additional 2.0°F to11.5°F (1.1°C to 6.4°C), relative to the 1980-1999 average, by the end of the 21st century. The wide spread in these numbers comes from uncertainty not only in exactly how much the climate system will warm in response to continued GHG emissions, but also uncertainty in how future GHG emissions will evolve.7 Hence, the choices that human societies make over the next several decades will have an enormous influence on the magnitude of future climate change.

As with observed climate change to date, there are wide geographic variations in the magnitude of future warming, with much stronger projected warming over high latitudes and over land areas (see Figure 2.5). In the United States, temperatures are projected to warm substantially over the 21st century under all projections of future climate change (USGCRP, 2009a). Temperature increases over the next few decades primarily reflect past emissions and are thus similar across different scenarios of future GHG emissions. However, by midcentury and especially at the end of the century, higher emissions scenarios (e.g., scenarios with continued growth in global GHG emissions) lead to much warmer temperatures than lower emissions scenarios.

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