A great deal is known about past, present, and projected future climate change, especially at large (continental to global) scales. For example, there is high confidence that global temperatures will continue to rise, that the rate and magnitude of future temperature change depends strongly on current and future rates of GHG emissions, and that climate change—in interaction with other global and regional environmental changes—poses significant risks for a number of human and natural systems. Global climate models and, increasingly, regional techniques are also starting to provide useful information about future climate and climate-related changes on local to regional scales. Some of these projections—such as increases in extreme heat events and Arctic sea ice—are quite robust, while others are somewhat more speculative.
There are, however, several aspects of future climate change that remain more uncertain, and these represent some of the most important and active areas of current scientific research (see Research Needs at the end of this chapter). The uncertainties in climate projections can be categorized into two main sources: (1) uncertainties in future climate forcing and (2) uncertainties in how the climate system will respond to forcing, which includes both the known limitations of global climate models (such as an inability to resolve individual clouds) and the fact that the climate system is complex and might exhibit novel or unanticipated behavior in response to ongoing climate change.
The first of these categories, uncertainties in future climate forcing, was discussed in the Future Climate Scenarios section earlier in the chapter. The spread among the three colored curves in Figure 6.20 provides a rough indication of the importance of this uncertainty in terms of the magnitude of future climate change. As discussed above, and described in further detail in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c), future climate forcing depends strongly on the choices that current and future human societies make, especially regarding energy production and use. However, actions that might be taken to limit the magnitude of future climate change, or adapt to its impacts, have not yet been fully and systematically integrated into climate forcing scenarios and evaluated across a range of different climate models to determine how they might ultimately affect both climate and other aspects of the Earth system.
As an illustration of some of the uncertainties present in climate model projections, Figure 6.22 shows projections of temperature change over North America from 21 different models, each using the same scenario of future climate forcing. Several robust features emerge from these projections—for example, all of the models project a