Aerosol Workshop — June 2-3, 1997
A workshop on atmospheric aerosols was held at the NASA Goddard Institute for Space Studies on June 2-3, 1997. The workshop was organized by James Hansen and Robert Curran at the request of Robert Harriss, Director of Sciences of NASA's Mission to Planet Earth Office.
Atmospheric aerosols, or fine particles, could play an important role in global climate change. Natural variations of aerosols, especially due to episodic eruptions of large volcanos, are recognized as a significant climate forcing, that is, a factor that alters the planetary radiation balance and thus tends to cause a global temperature change. In addition, there are several ways in which humans are altering atmospheric aerosols, and thus possibly affecting climate. We are concerned here with the climate forcing due to changing aerosols, both the direct radiative forcing by the aerosols and the indirect radiative forcing caused by effects of changing aerosols on cloud properties. These climate forcings due to changes of aerosols are not determined well, especially in the case of anthropogenic aerosols. Indeed, aerosols are one of the greatest sources of uncertainty in interpretation of climate change of the past century and in projection of future climate change.
Harriss' reasoning was that NASA (and other agencies) already have made a large investment in space-based measurements capable of detecting aerosols, as well as investments in field campaigns and global numerical models relevant to aerosol studies. Thus he asked us to address whether a modest focused aerosol initiative could use existing capabilities to make significant progress in understanding the role of aerosols as a mechanism of climate change. He specifically asked us to address whether better use could be made of volcanic aerosols, especially the eruption of Pinatubo in 1991, to test understanding of climate change, and he emphasized a desire to encourage interagency cooperation in aerosol studies.
Events strategy. A strawman science strategy, dubbed an "events" or more completely an "events in global decadal context" strategy, was proposed as an organizing principal. "Events" include natural occurrences such as volcanic eruptions and desert dust episodes, as well as long-term changes of anthropogenic emissions. The objective of this strategy is to estimate the global distribution of aerosol (direct and indirect) radiative forcing for the period of satellite data (from about 1980 to the near future) and make this available to the scientific community.
If successful, this "events in global decadal context" strategy would provide the forcing information that climate modelers need for simulations of aerosol climate effects. Quantitative aerosol forcing data, by itself, would be valuable for comparison to other climate forcings. Furthermore, aerosol changes, for example from volcanos, soil dust, biomass burning, or fossil fuel use, have distinctive spatial and temporal patterns, so there is a good chance that the aerosols produce an identifiable effect on the global four-dimensional temperature field. Thus it may be possible to detect and confirm the climate impact of aerosols.
This strategy has its best chance of success if a long continuous record of aerosol climate forcing can be constructed. A long record is best able to minimize the difficulties associated with unforced climate variability, and may be able to detect decadal change as well as interannual variability. A long record, including many specific aerosol events, also minimizes the chance of misinterpretation based on a single unusual case.
Satellite data in context of other available data. Satellites are essential for obtaining global coverage of aerosol properties. But existing satellite measurements, by themselves, are not able to define aerosol radiative forcings. Thus it will be necessary to find innovative ways to combine satellite measurements with models and with surface and in situ measurements and other data to improve information on aerosol climate forcing. For example, satellites are able to obtain more quantitative aerosol data over oceans than over land. If models of specific aerosol types can be calibrated against satellite data over the ocean, it may be possible to obtain global distributions of useful accuracy.
Climate studies depend on knowledge of the changes of individual aerosol types and sources, such as volcanos, soil dust, combustion aerosols, and so forth. As most satellite measurements cannot distinguish among aerosol types, such distinctions will require multidisciplinary analyses including the use of aerosol models. The science objectives require inclusion of all major aerosols that are changing substantially, and it is desirable that satellite analyses estimate the total change of aerosols. Because of the great difficulty in obtaining accurate aerosol change data, it seems appropriate that initial studies focus on certain aerosols that seem amenable to rapid progress or results. In this regard, sentiment was expressed by some workshop participants for early emphasis on volcanic aerosols, soil dust and perhaps smoke from biomass burning.
Fixed algorithms for aerosol parameters need to be applied uniformly to each satellite data set for their full period of record. A data processing capability needs to be established that can apply candidate aerosol algorithms and make the results available to the research community. As candidate algorithms are improved, and perhaps multiple satellite instruments employed, it will be necessary to repeatedly reanalyze the full data set.
Project structure. The task of extracting twenty-year aerosol records from satellite measurements involves substantial data processing. It will be necessary to run and rerun algorithms for the full period of measurements, and eventually to run algorithms using data from more than one instrument. NASA Headquarters will need to decide whether this is handled best using a project structure with a NASA institutional base, as one of the activities called for in the NASA Research Announcement, or in some other fashion.
Field studies. It was generally agreed that the success of this aerosol climate forcing program will depend upon complementary relevant field studies. Sentiment was expressed for design of new field experiments, but also for the fact that existing field data sets need to be more fully exploited. An issue to be decided by NASA Headquarters is when such field studies should be called for, for example whether it should be after or before initial investigator teams have been chosen for studies based on existing and planned satellite and field data. It is important that field programs be coordinated among the different agencies.
Related aerosol studies. It is important to coordinate with closely related aerosol investigations that are already supported by NASA EOS, Atmospheric Chemistry or other programs. Coordination could be aided by entraining appropriate interested individuals onto the aerosol research team described in the recommendations below, which might be accomplished by the NASA Research Announcement encouraging letter proposals for participation in the project activities with modest or no supplementary funding. An effective approach may be to have a Science Steering Group for the aerosol study project, with members selected from successful proposals and letter proposals and also including relevant program managers from other agencies.
The background to the Workshop's recommendations is contained in the hundreds of aerosol and climate research papers published in the past few years. The large uncertainty that aerosols introduce in our understanding of climate change is illustrated by the Intergovernmental Panel on Climate Change (Climate Change 1994: Radiative Forcing of Climate Change, eds. J. T. Houghton, et al., Cambridge University Press, 339 pp., 1995). The National Research Council research program plan Aerosol Radiative Forcing and Climate Change (J.H. Seinfeld et al., National Academy Press, 161 pp., 1995) also contains background material that is useful in establishing the rationale for our recommendations. The workshop recommendations derive in part from such background information, as well as from the individual and panel presentations, and the workshop discussions. The workshop recommendations are:
"Events" strategy. We recommend a strategic approach that leads to definition of aerosol radiative forcing on a global scale for the approximately 20-year period of satellite observations. Aerosol changes, or "events", in such a period would include natural occurrences such as volcanic eruptions and desert dust episodes, as well as long-term changes of anthropogenic emissions. It is exceedingly difficult to determine the aerosol properties with adequate precision, and there is no guarantee that knowledge of such properties will lead to definable observable climate changes, but this strategy may be our best hope for estimating and verifying the role of aerosols in climate change. It is unlikely that satellites can directly supply the required aerosol information; rather success will depend upon appropriate combinations of satellite data, models, field measurements and surface monitoring. These considerations apply to investigation of the effect of aerosols on clouds, i.e., the "indirect" aerosol climate forcing, as well as the direct aerosol forcing. We encourage collaborative efforts to define direct and indirect aerosol radiative forcings among researchers in these various specialties.
Team approach. We recommend that researchers supported by a new initiative be organized in a team to provide scientific guidance for a strategic approach toward definition of the aerosol radiative forcing, to encourage appropriate collaborations among research groups, and to provide guidance to an aerosol satellite climatology project. This "science steering group" could include researchers who are supported already for aerosol research. We recommend that such persons be allowed the option of submitting a letter proposal for little or no funding to allow them to participate in the new aerosol initiative. Also it is anticipated that the team would include experts in in situ data, e.g., aircraft measurements.
Aerosol "satellite" climatology. We recommend systematic application of aerosol retrieval algorithms to satellite radiances. A data processing capability needs to be established that can apply candidate algorithms to the full period of satellite measurements, rerun the full period with improved algorithms, employ multiple satellite data streams, merge satellite and aerosol model results if appropriate, and make the results available to the research community.
Aerosol types. We recommend that, to the extend practical, the aerosol climatologies be obtained for each of the major aerosol compositions. Understanding of aerosol radiative forcing depends upon data for all of the aerosol types undergoing significant change, but we recommend that first priority be given to aerosols with the most promise for contributing soon to our understanding of aerosol climate effects. Volcanic aerosols already provide two major events in the period of satellite data, and we recommend that the volcanic aerosol direct, and perhaps indirect, radiative forcings be defined more precisely. Soil dust aerosols, both natural and anthropogenic, are perhaps a major radiative forcing, but that forcing remains poorly defined; we recommend an early focus on this aerosol type. Sulfates have been a focus of study for the past few years; we recommend entraining some of these existing investigations into the new aerosol initiative. Carbonaceous aerosols are among the potentially important tropospheric climate forcings; emphasis needs to be placed on better understanding of the properties and temporal changes of organic aerosols. The science steering group should help provide scientific guidance in prioritization of research focus among different aerosol types.
Tropospheric aerosols are the principal source of uncertainty in decadal global climate change and will require the greater portion of the research effort. A relatively modest effort to complete the definition of stratospheric aerosol parameters for the past two decades can provide the climate research community with a valuable data set for aerosol climate studies.
Strategic field studies. We recommend that support be provided for strategic field studies as an integral component of the "events" strategy. Data from existing field studies can provide a starting point for modeling studies that help define uncertainties and needed observations. New field studies should be designed such that, in coordination with global satellite data and models, understanding of the direct and indirect aerosol radiative forcing is advanced; that is, field studies supported by this program should be strategic, designed to yield data leading to quantitative knowledge of aerosol direct and indirect radiative forcings.
Quantification of uncertainties. We recommend special emphasis on quantifying the uncertainty in estimated aerosol radiative forcings. Present knowledge of temporal changes in tropospheric aerosol properties is so poor that practically useful limits on the aerosol forcing are absent. The strategy for combining satellite and in situ data with global modeling must be designed to define useful limits on this forcing. Candidate aerosol climatologies must be subjected to tests, including comparisons with surface and in situ data, that confirm quantitative usefulness of the data for climate forcing applications.
Quantification of the uncertainties in the tropospheric aerosol radiative forcings is expected to emphasize the inadequacy of existing and planned satellite data per se for the climate applications. It is the required high accuracy of aerosol data for climate applications, and the need to assign portions of measured aerosol optical depths to appropriate aerosol types (sulfates, desert dust, black carbon, etc.), that demands a team approach combining satellite and in situ data with global modeling.
Interagency and international collaborations. We recommend that the initiative be carried out in collaboration with the relevant agencies and with participation of international aerosol researchers. We suggest that consideration be given to providing travel support to allow foreign researchers to participate in aerosol project and science steering group activities.
Acknowledgments. Logistical support for the workshop was provided by Katrina Parris, Sabrina Hosein, Carl Codan and Mary Larson. Funding was provided by the NASA Office of the Mission to Planet Earth. The speakers provided brief summaries of their talks for the proceedings. The proceedings were edited by Jim Hansen.