Air Pollution as a Climate Forcing: A Workshop
The final day of the workshop was devoted primarily to discussing what we know and what we need to know in each of the areas: (1) non-CO2 gases, (2) aerosols, (3) technology, and (4) health and other impacts, as they relate to the issues that the workshop was asked to address. An introductory overview was provided by Daniel Albritton. The technical summaries were provided by Michael Prather (gases), Tami Bond (aerosols), Terry Surles (technology), Daniel Greenbaum (health), and Denise Mauzerall (agriculture).
The summaries are primarily the responsibility of the individual authors, although they are based on the discussions at the workshop. It was not anticipated that the workshop could provide answers to the overarching questions that were raised. Rather the aim was to initiate discussions and communicate our current understanding. The workshop summaries, extended abstracts, and presentations are all meant to contribute to these objectives.
Summary A. Overview
Workshop Scope. The workshop brought together the topics of air quality and climate change. The chosen scope emphasized the impact of air quality on climate change, and the emphasis of the workshop was on air pollutants that cause substantial climate forcing.
Although not an emphasis, the effect of climate change on air quality was also touched upon. Indeed, preliminary modeling studies presented suggest that climate change could have a significant effect on future growth rate of tropospheric ozone and methane.
The decision making regimes for air quality and climate change are distinctly different. The focus of the workshop was not on whether or how such decision making could be coordinated. Rather, the aim was the description of what we know and what we need to know about the commonalities between the two phenomena and the communication of this status of understanding among the participants.
Participant "Ingredients". The workshop participants were from the atmospheric science, technology, societal impacts and policy communities communities from academic, governmental, and industrial organizations in 15 countries. It was thus a multi-cultural and multi-language assembly. The individual sessions, all held in plenary, included "in-the-box" (i.e., disciplinary) discussions, but presenters aimed to speak to a broad audience. The overall workshop approach involved discussing the phenomena, issues, potential solution mechanisms, and policy perspectives. In that way, the sum of these components, discussed together, provided a comprehensive overview.
Chosen Foci: Non-CO2 Radiative Constituents. The air pollutants discussed have been addressed primarily for reasons other than climate (e.g., air quality). These atmospheric constituents are relatively short-lived, with atmospheric lifetimes from days to about a decade.
The short-lived nature of these species broaden policy options in remarkable ways. For example, they have the potential of providing relatively rapid atmospheric response to policy intervention and multiple visible benefits. As such, they could be potentially useful adjuncts to the difficult long-term issue of the growth of atmospheric CO2.
From a research perspective, it seems feasible to obtain the information that is needed to consider effectively choices associated with the non-CO2 constituents. Further, it seems that, with augmented focus, such information could be obtained within a reasonably useful period.
Workshop Framework. The first workshop sessions on gases and aerosols considered the emissions from many different sources. It is clear that there is still much work needed to usefully inventory the sources and to better quantify how the atmospheric amounts have changed in the past and are continuing to change. The short-lived nature of these chemically-active constituents and the resulting spatial heterogeneity and temporal variations underlie these challenges.
The atmospheric responses to the emissions of these species and their precursors are complex, as there are variable interrelations of constituents such as NOx, CH4 and O3. There are also interactions of climate change with their atmospheric composition.
The sessions on the impacts of air pollutants were focused on the health effects. But, effects on agricultural productivity and ecosystems were also considered.
Technical mechanisms for addressing these pollutants, and thus policy choices, were discussed. For example, the issue of diesel vs. gasoline engines received attention. Diesel engines may be more energy efficient, thus producing less CO2, but at a cost of greater emissions of black carbon aerosols. This in turn focused questions on the effectiveness of potential particulate filters.
Overall, many parts of the phenomena and issues were discussed. The workshop represents a first step toward a synthesis of aspects of the air pollution and climate issues.
Hierarchy of Useful Information. The workshop recognized the importance of the fact that there is a hierarchy of useful information on the interaction of air pollution and climate. Namely, each level of improving understanding is useful information for decision makers.
The most primitive, yet nevertheless useful, information is the sign of the coupling between climate and air quality. Namely, does potential action on one aspect of one issue positively or negatively influence the other issue? More usefully, we would want to understand the nature of the linkage and, indeed, quantify that linkage, so that "what if" scenarios could be constructed. Of course, the ultimate aim of expanded understanding would be to broaden it to the point of having a reliable cost/benefit analysis of effects and potential mitigations.
As a "sidebar", the utility of each level of information depends upon whether the constituent is mainline to either the climate or other environment issues and upon whether it represents a co-benefit or dis-benefit to the other application.
Air Quality and Climate Couplings. A clear "win/win"-coupling example is provided by black carbon. Reductions in black carbon ("soot") aerosols could yield substantial benefits to health and agricultural productivity, as well as generally a reduction in positive radiative forcing of the climate system. Similarly, any emissions that increase tropospheric ozone contribute to both health problems and greenhouse warming. For example, reductions in methane would reduce global warming directly and also decrease tropospheric ozone, thus further reducing global warming, while also improving public health and agricultural productivity.
An apparent "win/lose" example is provided by sulfate aerosols. Reduction of sulfur emissions would improve air quality, but would add to greenhouse warming because heat-reflecting sulfate aerosols would be less abundant. However, the overall effect of sulfate aerosols, especially on regional climate, needs to be understood better.
A complex case is provided by diesel engines, as an alternative to gasoline engines. Diesel is more efficient, thus producing less CO2 than gasoline, but with extant engines, diesel fuel produces more black carbon aerosols. A full accounting depends upon projecting technology (e.g., particulate-matter filters, as well as better understanding of both climate and health impacts. The point here is that a combined climate — air quality analysis and decision making could be more effective than independent analyses and decisions.
The workshop discussions underscored that many scientific investigations illuminate the evaluation of both the climate and health issues. For example, the physical structure and chemical composition of aerosols, including the coating of core material by another constituent, can influence both the health and climate effects of the aerosols. As with policy considerations, these discussions underscored the potential effectiveness of coordinated air quality and climate research.