摘要：Scenarios of global greenhouse gas emissions have played a key role in climate change
analysis for over twenty years. Currently, several research communities are organizing to
undertake a new round of scenario development in the lead-up to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change (IPCC). To help inform this
process, we assess a number of past efforts to develop and learn from sets of global
greenhouse gas emissions scenarios. We conclude that while emissions scenario
exercises have likely had substantial benefits for participating modeling teams
and produced insights from individual models, learning from the exercises taken
as a whole has been more limited. Model comparison exercises have typically
focused on the production of large numbers of scenarios while investing little in
assessing the results or the production process, perhaps on the assumption that later
assessment efforts could play this role. However, much of this assessment potential
remains untapped. Efforts such as scenario-related chapters of IPCC reports have
been most informative when they have gone to extra lengths to carry out more
specific comparison exercises, but in general these assessments do not have the
remit or resources to carry out the kind of detailed analysis of scenario results
necessary for drawing the most useful conclusions. We recommend that scenario
comparison exercises build-in time and resources for assessing scenario results in
more detail at the time when they are produced, that these exercises focus on
more specific questions to improve the prospects for learning, and that additional
scenario assessments are carried out separately from production exercises. We also
discuss the obstacles to better assessment that might exist, and how they might be
overcome. Finally, we recommend that future work include much greater emphasis on
understanding how scenarios are actually used, as a guide to improving scenario
production.
