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Summer 1998

Past and future history: NCAR Climate of the 20th and 21st Century Projects

A proverb says, "Those who don't know the past are doomed to repeat it." In NCAR's Climate and Global Dynamics (CGD) Division, scientists are taking that saying a step further. Besides reproducing the past with unprecedented faithfulness, they're trying to predict our future course with equal accuracy.

Led by Jeffrey Kiehl (CGD) and Susan Solomon (NOAA), scientists in the Climate System Model (CSM) Chemistry and Climate Change Working Group are making this attempt with the CSM, a coupled ocean-atmosphere general circulation model (i.e., a global model whose ocean, atmosphere, land, and sea-ice components can interact). The working group has produced unique data sets for past and future climate, and unique climate change scenarios for the next century. The modelers are currently in the middle of their 20th-century simulations, which cover 1870-1990, after which they'll be on their way to the year 2100.

Top: The global forcings, in watts per square meter, that are being used in the Climate System Model for increases in sulfate aerosols, greenhouse gases, and ozone from preindustrial to present times. Bottom: Total forcing that the CSM will experience from the direct and indirect effects of sulfate aerosols from preindustrial to present times. (Illustrations courtesy of Jeffrey Kiehl.)

Of course, climate modelers have been simulating the last century's climate since global modeling began, for some 30 years now (but with much simpler models until recently). So what makes these model runs better? Two things: the model and the data set.

The CSM is one of a handful of models that lead the field in their ability to reproduce global climate. Although each of these models has its own strengths and weaknesses, the CSM has one unique advantage: it does not require flux corrections--in-flight adjustments that other models need to produce a realistic simulated climate. Says Tom Wigley (CGD), who is also involved in the Climate of the 20th Century project, "Most experiments have flux corrections because there are inconsistencies between the state of the ocean and the state of the atmosphere, so modelers have to fake it up a little to get over that problem. This doesn't seriously affect the results of any simulations, but it's still really not the best way to go." With the CSM, however, Kiehl reports, "We ran it for 300 years and it produced a very stable climate." This means that the equations used to reproduce climate processes are very close to reality.

To feed the model, the CSM group has spent the last couple of years putting together the best possible data set for four important greenhouse gases--carbon dioxide, ozone (in both the troposphere and the stratosphere), methane, and nitrous oxide--a set of halocarbons, and sulfate aerosols. "Susan [Solomon] and I found experts in each of these fields and invited them to participate," says Kiehl. Many models use only carbon dioxide as a proxy for all greenhouse gases to save computer time, but a realistic simulation requires more, since all absorb and emit radiation at different rates and have different chemical reactions.

Jeffrey Kiehl (left) and Tom Wigley. (Photo by Carlye Calvin.)

For the 20th century, the modelers put in yearly changes for each gas modeled, plus a seasonal cycle for sulfur dioxide emissions, which have stronger seasonal changes than the other gases. For the simulated end point, 1990, gas and aerosol amounts were constrained to match the observational data. Kiehl points out, "Although the data used for the change in atmospheric composition are constrained by observations, the climate response is determined by feedbacks in the model." Thus, if the modelers have not reproduced these feedbacks correctly, the effects of the changes in atmospheric composition might not interact as they do in nature, and the model could show unrealistic warming or cooling. If this is the case, figuring out exactly what went wrong "is where the art of climate modeling comes in," says Kiehl.

Kiehl hopes the project will be able to reproduce the observed regional pattern of climate change. For example, temperature changes in the eastern half of the United States differ from those in the West, with relative cooling in the East. Scientists believe this difference is caused in part by the higher amount of sulfate aerosols in the East--a difference the model should be able to reproduce.

The effects of sulfate aerosols remain one of the great unknowns in the climate system (see related story on p. X). The CSM is unusual in including both the aerosols' direct effect in scattering solar radiation in a clear sky and their effect on clouds, which also changes the planet's reflectivity. This is a new area for all global climate modelers, and NCAR is rapidly building its capacity to model these effects.

The 20th-century simulations will be completed in mid- to late June. A simulation with sulfate aerosols will run at NCAR. The Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Melbourne, Australia, currently plans to run a simulation with greenhouse gases only, using the CSM on their supercomputer, as a collaborative venture.

Into the future

Once the 20th-century simulations are accomplished, "with barely a heartbeat in between," Kiehl says, the team moves on to the next challenge: the climate of the next century.

Some of the problems in foretelling the future are the same as in reproducing the past. It's just as important for the model to simulate the climate system faithfully, and the modelers still need a good understanding of today's climate. The new challenges relate to how well we can predict human behavior. This is where Wigley's group, A Consortium for the Application of Climate Impact Assessments (ACACIA), comes in.

Steve Smith (ACACIA), in collaboration with Wigley and Hugh Pitcher (Battelle Pacific Northwest Laboratories), has created new scenarios for future emissions of sulfur dioxide (SO2), the gas that reacts to make sulfate in the troposphere. The new scenarios include a number of factors that weren't considered in earlier efforts, such as the widely quoted Intergovernmental Panel on Climate Change (IPCC) 1992 scenarios. The IPCC was mandated only to take into account existing climate-related legislation, not the likelihood of the passage of future laws or other environmental factors.

In the new scenarios, the ACACIA group takes into account probable future regulation of air pollution and acid precipitation on a worldwide basis, including China, Southeast Asia, and India. "Sulfur emissions are very costly to the economy," says Wigley. "In North America, we've introduced legislation to drastically reduce SO2 emissions simply because we know how expensive they can be." In the ACACIA scenarios, which consider the history of the introduction of regulation in developed countries and the likely growth in developing nations' gross domestic products, the public-health costs of unregulated pollution lead the Asian countries to begin introducing controls over emissions in the next few decades.

Starting with the 1990 data, the CSM will use a fully integrated sulfur chemistry model developed by Philip Rasch (CGD), Mary Barth (NCAR Atmospheric Chemistry and Mesoscale and Microscale Meteorology Divisions), and Kiehl to predict the sulfate aerosols for the future scenarios.

"With luck," says Wigley, "we will have these future results by mid- to late July." Again, the scientists hope to get a detailed regional picture of sulfate cooling influences and a better understanding of feedbacks. For example, China may initially have less warming because of the cooling produced by local SO2 emissions, but the influence of such emissions details on global precipitation changes in the context of global warming is a "rather more complicated thing to figure out," Wigley says.

Sharing the science

These projects have already generated collaborations, as mentioned above, and Kiehl and Wigley hope the group efforts will continue. The CSIRO group will run the same simulations on their model; the NCAR scientists hope that other modeling centers in this country and abroad will do the same, allowing an international intercomparison of global climate models.

The scientists have also been asked to share the new data sets. "After our simulations, we will put the data on an anonymous FTP site and anyone can use them," says Kiehl. He expects to accomplish this by the end of the summer. The UCAR Quarterly will publish the URL when it's available.

Eventually, Kiehl hopes to work with several university scientists to prepare a new data set on 20th-century natural variability--e.g., known volcanic eruptions and solar variability--for another series of simulations. But this will have to wait. "By mid-July we'll have 400 years of data to digest, so I don't think we'll be running back to do another 400 years very soon."

"Climate of the 20th Century Project," Winter 1998

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 14:55:01 MDT 2000