IPCC Secretariat, WMO, 41, Av. Giuseppe-Motta, C. P. No 2300, 1211 Geneva 2 SWITZERLAND warming condition. Whether the frequency of tropical storms will increase is uncertain, in part because GCMs are currently not run at the appropriate spatial resolution to simulate hurricane formation and other factors that might lead to changes in hurricane generation. (15) Details of the climate change over the next 25 years are uncertain. Basis-Uncertainties in the factors controlling the natural variability of the climate, in the model simulations (as described above), and in the perturbations to atmospheric composition make it extremely difficult to predict or even suggest the details of changes in the global or regional climate on the time scale of only a few decades. In any given decade, the changes in temperature and related variables could be substantially less than or more than the model-predicted trend. Warming estimates in terms of degrees per decade and their use to analyze a single decade are, therefore, unwarranted. Focus on a decadal analysis of observations can be equally misleading. (16) Biosphere-climate feedbacks are expected, but how much these feedbacks will amplify or moderate climate change is uncertain. Basis-Processes governing changes in the distribution and character of Meeting participants identified a number of areas where sustained or intensified research efforts would bring important gains in understanding and predictive capabilities. (1) While progress is clear as a result of ongoing research efforts and important steps can be taken over the coming decade that will bring new insights, significant reductions of the uncertainties in projecting changes and trends in the climate will require sustained efforts that are very likely to require a decade or more. Basis-Progress will require significant effort because the problems are complex, because improvements in model parameterizations will require a sustained and long-term program of research and observations, and because the records of past changes and influences require careful reconstructions to make them more complete and more useful. Although progress may be modest, there are a number of processes and feedbacks on which research must be sustained because of the large leverage to be gained from improved understanding. These processes and feedbacks include (a) cloud-radiation-water vapor interactions, including treatment of solar and infrared radiation in clear and cloudy skies (also including resolution of uncertainties concerning anomalous solar absorption); (b) ocean circulation and overturning; (c) aerosol forcing, requiring information on aerosol character and extent; (d) decadal to centennial variability; (e) landsurface processes, including the climate-induced changes in the structure and functioning of ecological systems with resultant changes in global chemical cycles; (f) short-term variability affecting the frequency and intensity of extreme and high impact events (e.g., monsoons, hurricanes, mesoscale storm systems, etc.); (g) non-linear and threshold effects that create the potential for surprises; and (h) interactions between chemistry and climate change and improved representation of atmospheric chemical interactions within climate models, thereby leading to improved understanding of the causes of trends in CH4, N2O, O3, CFCs, and aerosols. (2) Improved spatial resolution in atmospheric and oceanic models will improve the representation of the present climate and potential global-scale changes. Basis-In the atmospheric component of climate models (as in weather forecast models), major storm systems and circulation patterns are significantly better represented in models having resolutions finer than about 2.5 degrees in latitude and longitude (as opposed to the 5- to 7-degree resolution often used in past modeling studies), in part because of improved representation of major orographic features and in part because of improved ability to represent weather systems, the Intertropical Convergence Zone, the Hadley PART 3. FOR REDUCING 121 (5.6" x 5.6°) T42 (2.8" x 2.8°) T63 (1.9" x 1.9) Figure 9. Comparison of T106 (1.1" x 1.1") circulation, and other circulation features. In the oceanic component of climate models, ocean current pattems are significantly better represented in models having resolutions finer than about 0.5 to 1 degree (as opposed to the 3- to 5-degree resolution in past modeling studies), in large part because important ocean current systems (e.g., the Gulf Stream and Kuriosho), ocean variability (including ENSO events), and the thermohaline circulation and other vertical mixing processes can be bet ter represented. Improved resolution in both atmosphere and ocean components of global climate models has also proven to reduce flux imbalance problems arising in the coupling of these components. With the increasing parallelism of supercomputers and the availability of massively parallel computers, the only impediment to the gain in model accuracy by improving model resolution is the commitment of computational resources. However, a concomitant increase in efforts for process studies and diagnosis and analysis of model results is required. (3) Climate changes at the surface and as a function of altitude can be better represented by inclusion of significantly more representative parameterizations of the atmospheric boundary layer and of vertical convection processes. Substantially improved observations and improved representations of water vapor in climate models will reduce a major source of uncertainty in model predictions. Basis-By including more complete representations of the atmospheric boundary layer, weather forecast models have achieved important gains in their representation and prediction of surface conditions, and of water vapor transport to the free troposphere. A 1% change in relative humidity leads to about a 1 W/m2 change in top-of-the-atmosphere fluxes. Observational uncertainty in relative humidity is more than 10% in some regions, which can translate to more than twice the flux change expected from a CO2 doubling. It is difficult to separate possible model error from observational error, especially in relatively dry regions. Important debate continues as to the nature of the positive water vapor feedback associated with global warming. Improved representations of precipitation and of convective transport of water vapor into the upper troposphere will significantly reduce uncertainties about the amplifying feedback due to water vapor and the extent of changes in storm tracks. Such improved representations are being developed and tested, and will be able to be implemented, provided computational time is allocated for testing, and diagnostic and analysis efforts are strengthened. An increased emphasis on observations of water vapor and evaluation and testing of models is crucial to the global warming hypothesis. (4) Improving the linkages coupling the atmosphere, oceans, and land surface will reduce uncertainties in estimates of the overall climate response by improving the accuracy of the climate simulations, by eliminating the need for ad hoc adjustments to fluxes between components that are used in some models, and by allowing fuller exploration of natural climate variability over all time scales. Basis—The climate is a result of the complex interactions of the atmosphere, the oceans, and the land surface. The dynamics, thermodynamics, and hydrodynamics (and to an increasing extent the chemical and vegetation dynamics) must all be treated in order to provide a realistic simulation of dimate. The focus has initially been on the atmosphere, then increasingly on the ocean; coupling of the atmosphere and oceans has not been completely successful due to limitations in understanding of ocean mixing and air-sea exchange mechanisms, in addition to limitations in model resolution and the full range of processes internal to each domain. Increased attention to improved representation of the coupling is starting to lead to improved representations of temperature and other climatic variables. Corresponding improvements are needed in representations of the land surface and landatmosphere interactions and fluxes. Because vegetation and chemical composition can affect radiative forcing and water vapor concentrations, these must also be treated in coupled simulations. The emerging results from the World Ocean Circulation Experiment (WOCE), the Global Energy and Water Cycle Experiment (GEWEX), and other field and analysis programs are providing the opportunity for improving the performance of coupled models. (5) More explicit representation of land-surface processes, including vegetation, soil characteristics, and CO2 and 03 effects on stomatal resistance, will reduce uncertainties in estimates of soil moisture, summertime continental drying, and changes of regional climate. Basis-Explicit treatment of vegetation provides representation of the seasonally varying albedo, the diurnally and seasonally varying pattern of Figure 10. Schematic dia- IMETER evapotranspiration, and other (6) Sub-continental and Basis-Weather forecasts are successfully tailored to specific regions using both finer scale models and empirical techniques that derive local and regional features from large-scale atmospheric conditions. These techniques, including both regional models and statistical methods, have been demonstrated in studies that predict local conditions given largescale conditions. Application is starting to be done and awaits only commitment of resources and analytic effort. Results of this type will permit significant testing and improvement of the capabilities for estimating regional vulnerabilities of ecosystems and socio-economic systems to climate change. (7) Model comparisons against observations and with other models can accelerate the rate of model improvement. Basis-Important new data sets are being developed that document the climatic changes and fluctuations of the 20th century, including improved |