This assessment has drawn information from a variety of approaches for documenting past and present climatic conditions and projecting future climatic conditions, including: observed data (such as data from instruments like thermometers, and past climate records from tree rings, ice cores, and sediments); field experiments; computer-based climate models; and indigenous knowledge. When information from several methods converges, it offers greater confidence in the results. Still, there will always be uncertainties and surprises in projecting future changes in climate.
Projecting future climate change and its potential impacts is undertaken in a systematic manner. Two major factors determine how human activities will cause the climate to change in the future:
- the level of future global emissions of greenhouse gases, and
- the response of the climate system to these emissions.
Research over recent decades has contributed a great deal of information regarding each of these factors.
Projecting the level of future emissions is carried out by developing plausible scenarios for future changes in population, economic growth, technological and political change, and other aspects of future human society that are difficult to fully anticipate. The IPCC produced a Special Report on Emissions Scenarios (SRES) to grapple with these issues. Their scenarios encompass a range of possible futures based on how societies, economies, and energy technologies are likely to evolve, and can be used to estimate the likely range of future emissions that affect the climate.
Regarding the response of the climate system, computer models developed by research centers from around the world represent aspects of the climate system (such as how clouds and ice cover might be expected to change, and ultimately how climate and sea level might be influenced) somewhat differently, resulting in differences in the degree of warming projected.
Regardless of the emissions scenario or computer model selected, every model simulation projects significant global warming over the next 100 years. Even using the lowest emissions scenario, and the model that generates the least warming in response to changes in atmospheric composition, leads to a projection that the earth will warm more than twice as much in this century as it warmed over the past century. Model simulations further indicate that the warming in the Arctic will be substantially greater than the global average (in some regions, more than double). While the models differ in their projections of some of the features of climate change, they are all in agreement that the world will warm significantly as a result of human activities and that the Arctic is likely to experience noticeable warming particularly early and intensely.
The climate models and emissions scenarios reviewed by the IPCC generate a range of possible conditions for the future. To provide an indication of the types of impacts that are likely to occur, ACIA drew upon the results of five climate models from leading climate research centers and one moderate emissions scenario (known as B2, see Appendix 1 for more information) to be the primary basis for its assessment of the impacts of future climate conditions. The maps of projected climate conditions in this report are based on this B2 emissions scenario. A second emissions scenario (called A2) was added to some analyses to explore another possible future. The focus on these two scenarios here reflects a number of practical limits to conducting this assessment, and is not a judgment that these are the most likely outcomes.
When viewing the model results in this report, it is important to remember that these are not worst-case or best-case scenarios, but rather fall slightly below the middle of the range of temperature rise projected by global climate models. It is also important to note that for many of the impacts summarized in this report, information was also drawn from additional sources, including observed changes in climate, observed impacts, extrapolation of current trends, and laboratory and field experiments published in the peer-reviewed scientific literature.