Context of this Forum The Urgency of Current Demands by the Policy Community on the Scientific Community and the Need for High Scientific Standards

Given the various forecasts of impending climatic catastrophes, there can be little doubt that the issue of "global warming" has captured world attention. Such forecasts range from modest increases of global mean temperature to severe climatic shifts, flooding coastlines, crop failures, and beyond. Yet the term "global warming" is also a source of some ambiguity insofar as the verb "to warm" has both transitive and intransitive meanings.

There is no scientific doubt that the increase in manmade greenhouse gases (e.g., CO2) has created a warming of the lower layers of the atmosphere in the sense that these gases have caused heat to be added to the air (the transitive meaning). However, substantial uncertainty exists as to how much warming (in the intransitive sense) can be expected as a result of this additional heat energy, not least because the sensitivity of the climate to such perturbations is itself uncertain. Indeed, the sensitivity of global mean surface temperature to a given change in the content of greenhouse gases is uncertain to at least a factor of two and perhaps a factor of three (Schwartz 2008). A large portion of this uncertainty in climate sensitivity, and hence uncertainty in the climate forecast, stems from the uncertainty in the numerous effects of clouds and associated aerosols.

This high degree of uncertainty regarding clouds, combined with the urgency for societies to make firm decisions on the emissions of greenhouse gases (most especially on the continued combustion of fossil carbon fuels), places a great burden on our scientific community. Because we are the only group trained to study the details of clouds and climate, we must do our utmost to reduce the uncertainties and clarify the details of the climate forecast. In doing so, we assume the awesome obligation to communicate our research to the policy community in ways that are impeccably honest and forthright, so that the uncertainties that will always remain and which will, by nature, constrain the confi dence that can be taken regarding policy decisions are understood. Just a few decades ago, our fi elds of science contributed far less to policy making, and we enjoyed the freedom to speculate openly about the physics of clouds and aerosols. Today, however, what we say does count, and a very attentive audience is listening. We must therefore hold ourselves and our findings to an ever-higher standard of scientific proof and be candid about what we have and have not found.

Reducing the uncertainty of climate sensitivity requires vast improvements in the ways that clouds and aerosols are understood and described in the models used by decision makers. What is literally at stake is the ability of the global society to plan rationally ways to conduct its business. As stated by Schwartz (2008):

This uncertainty in climate sensitivity, which gives rise to a comparable uncertainty in the shared global resource of the amount of fossil fuel that can be burned consonant with a given increase in global mean surface temperature, greatly limits the ability to effectively formulate strategies to limit climate change while meeting the world's energy requirements.

It is of crucial importance for us to find answers to the many puzzles posed by clouds in the perturbed climate system.

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