What Improvements Have Been Made In Predicting Future Changes In Climate?


Climate Models Predict Cooling Effect Of Aerosols

For the past two decades, climate models have predicted the amount of increase in global average temperatures as a result of the rising concentrations of greenhouse gases. Global observations indicate that the measured increase of 0.5°C rise since the last century is only about half the increase of model projections. Recent research suggests that increased concentrations of atmospheric aerosols may be counteracting the warming influence of greenhouse gases. This increase in aerosol concentrations is mostly the result of combustion-related emissions of sulfur dioxide, hydrocarbons, and soot from fossil fuel use and biomass burning. The newer climate models, which include the effects of these aerosols, predict that they are exerting a cooling influence on global temperatures. When the effect of aerosol cooling is combined with the effect of greenhouse warming, the magnitude and geographical pattern of the combined changes have considerable similarity to the observed patterns of change.
Reference: Response of the Climate System to Atmospheric Aerosols and Greenhouse Gases, Taylor, K. E., and J. E. Penner, Nature, Vol. 369, pp. 734-737, 1994.

The Global Warming Potential For Several Greenhouse Gases Has Been Found To Be Larger Than Originally Thought

The Global Warming Potential (GWP) of greenhouse gases was developed as an index for comparing the different magnitude of influence of each greenhouse gas in contributing to climate change. It is useful as a first order tool for policy makers in evaluating mitigation strategies. The GWP of a gas is defined as "the cumulative radiative forcing between the present and some chosen later time 'horizon' caused by a unit mass of gas emitted now, expressed relative to some reference gas (usually carbon dioxide)." For several gases, the GWPs reported by the IPCC in the 1994 report were 10 to 30% larger than those reported in 1992. Uncertainty in the GWPs is about 35%.
Reference: Climate Change 1994: Radiative Forcing of Climate Change and an Evaluation of the IPCC IS92 Emission Scenarios, Intergovernmental Panel on Climate Change, Cambridge University Press, 339 p, 1995.

Climate Model Reproduces Recent Climate Trends And Corroborates The Influence Of An Enhanced Hydrologic Cycle In The Tropics

A remarkable similarity in the predicted and observed climate trends over the most recent portion of the global temperature record (1970-1992) was demonstrated using atmospheric general circulation models forced only with ocean surface temperatures. These results corroborate the suggestions that the observed increase in the globally averaged surface air temperature is a result of enhancement in the tropical hydrologic cycle driven by tropical ocean temperature increases. A trend toward an enhanced tropical hydrologic cycle has been suggested as an early signal of the greenhouse warming effects of increased atmospheric concentrations of CO2 and other greenhouse gases.
Reference: Simulation of Recent Global Temperature Trends, Graham, N. E., Science, Vol. 267, pp. 666-671, 1995.

New Parameterization Of Ocean Eddies Improves The Computational Efficiency Of Coupled Ocean-Atmosphere Global Circulation Models

Accurate characterization of mesoscale eddies (those with lengths of 10 to 100 km) in climate models is important because of their role in transporting heat, salinity, and passive tracers used to study ocean circulation. This implies that in order to resolve these important processes, ocean models should have a resolution on the order of 10 km. Most coupled GCMs use ocean grid scales on the order of hundreds of kilometers due to the excessive computational time required for the higher resolution. This new parameterization of mesoscale processes by scientists at the National Center for Atmospheric Research has been demonstrated to yield improved predictions of global temperature distributions, poleward heat fluxes, and deep-water formations without requiring excessive computational resources.
Reference: The Role of Mesoscale Tracer Transports in the Global Ocean Circulation, Danabasoglu, G., J. C. McWilliams, and P. R. Gent, Science, Vol. 264, pp. 1123-1126, 1994.

Advanced Computer Systems Allow Improvement Of Resolution In Global Ocean Models

The development of "eddy-resolving" global ocean general circulation models (OGCMs) is required to accurately simulate the global thermohaline (or "conveyor belt") ocean currents that strongly influence global and regional climate. These high resolution simulations, which divide the globe into grid cells approximately 40 km on a side, were not feasible because of the constraints imposed by limited computing power and modeling capabilities. Global eddy-resolving models have now been developed by USGCRP scientists that take advantage of the latest massively-parallel super computer technology to make multi-decade, global simulations of the world ocean circulation. These models are currently being coupled to highly optimized, state-of-the-science atmospheric GCMs to more accurately project climate changes out to centuries in advance.
References: (1) Parallel Ocean General Circulation Modeling, Smith, R. D., J. K. Dukowicz, and R. C. Malone, Physica D, Vol. 60, pp. 38-61, 1992; (2) Implicit Free-Surface Method for the Bryan-Cox-Semtner Ocean Model, Dukowicz, J. K. and R. D. Smith, Journal of Geophysical Research, Vol. 99, pp. 7991-8014, 1994.

Three Dimensional Ocean Model Developed By The World Ocean Circulation Experiment (WOCE) Wins National Award

A very realistic numerical model of the global three-dimensional ocean circulation, including important strong currents and eddies and proper representation of coastlines and bathymetry, was developed in order to improve our physical understanding of the ocean and to enable more accurate prediction of climate change. The model output was compared with both in-situ and satellite observations and found to be in very good agreement with what actually happened. The agreement of predicted surface height variability with that observed by the TOPEX POSEIDON satellite altimeter was especially impressive. Model output was distributed to more than thirty research groups worldwide for further analysis and additional scientific uses. Video animation's of ocean currents, height, temperature, and salinity have also been sent to hundreds of individual scientists, educators and others who requested them. The importance of this model development has been widely recognized, including a national award for "Breakthrough Computational Science" from the Smithsonian Institution. An even higher resolution model running on a massively parallel computer won a similar Smithsonian Award in the "Science" category a year later. (see also previous item).
Reference: Ocean General Circulation Model from a Global Eddy-Resolving Model, Semtner, A. and R. Chervin, Journal of Geophysical Research, Vol. 97, 5493-5550, 1992.

New Laboratory To Be Devoted To Large Climate Simulations For Comprehensive Earth System Modeling

A large climate model simulation requires hundreds to thousands of processor hours for its completion and often produces many gigabytes of model output that must be archived for analysis and intercomparison with other simulations and with observations. The Climate Simulation Laboratory is a special use, dedicated climate system modeling computing facility. CSL's purpose is to provide high performance computing and data storage systems to support large-scale, long-running simulations of the Earth's climate system (defined as the atmosphere, oceans, land and cryosphere, and associated biogeochemistry and ecology, on time scales of seasons to a century), including simulations of both the coupled system and appropriate model components.
Reference: The NCAR Climate Simulation Laboratory - A National Computing Facility for Climate System Modeling for the U.S. Global Change Research Program, Buzbee, W., 1995.

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