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Winter 1998

Update: Climate of the 20th Century Project

The climate-simulation (labeled NCAR-CSM) temperatures compared with observations (labeled IPCC). (Illustration courtesy of Jeffrey Kiehl.)

This article follows up on "Past and future history: NCAR Climate of the 20th and 21st Century Projects," Summer 1998.

The first simulation in the Climate of the 20th Century Project was completed in mid-September. The simulation used NCAR's climate system model (CSM) and unique data sets for four greenhouse gases plus sulfate aerosols from 1870 through mid-1998--not 1990, as originally planned. "We decided to throw in the last few years, especially since 1998 is shaping up to be the hottest on record," said Jeffrey Kiehl (NCAR Climate and Global Dynamics Division, or CGD), who helped direct the project.

The figure shows how the simulation compares with observations. According to Kiehl, the agreement in the early part of the century is a matter of chance, because human forcing then was so small. "The variations at the beginning of the simulation are due to natural variations in the model," he says. The match between model and observations starting in the 1970s, however, indicates that the model is correctly responding to increases in greenhouse gases.

The mid-century disagreement, Kiehl thinks, may be due to a change in the solar output, which was not included in the model run. There was considerable solar activity--a factor that's known to cause warming--from the 1920s through the 1970s. Solar variability was not simulated in this run because the modelers are trying to distinguish the individual contributions of various changes, such as greenhouse gas increases, to climate change. "The only way to do this is by sequential simulations," Kiehl explains. "Really, we'd like to do one simulation for each feature." However, this would require more resources than are available.

To verify that solar variability is the cause of the disparity, Kiehl and his colleagues will next do a simulation including it, rather than one with only greenhouse gases as originally planned. They expect that in this simulation, temperatures from the 1970s to the present will be slightly higher than the observed increase. "The implication is that we're missing some negative forcing," Kiehl says. "It could be the indirect effects of sulfates."

Of course some of the discrepancies around the 1940s could be due to natural variability. To get a handle on the possible magnitude of this effect, Byron Boville (CGD), who also helped direct the project, has turned to a 300-year control run performed on the CSM in 1997. This simulation was not forced with human-induced effects, such as increased carbon dioxide. Instead, the modeled climate chugged along by itself for 300 years, using the fixed forcings for 1990. The result sketches the range of natural variability--although an ensemble of about ten such runs would be more reliable. But no institution today has the computer power to allow that.

Boville fitted the real temperature dip at the turn of the 20th century to a similar dip in the 300-year model run. Compared to the modeled recovery, the observed temperatures are considerably higher. This leads him to think that about half of the disparity is due to natural variability, and the rest is probably the solar signal. The same comparison also shows that the modeled and observed temperature increase of the last 20 years is far outside the natural variability of the 300-year simulation, another sign that the increase is truly anthropogenic.

The data used to force the model in this project will shortly be released to the community; the Web address for these forcing sets will be linked to the CSM's home page. About ten scientists are working on papers to be published in the upcoming months. The team is especially rushing to document the forcing sets, says Kiehl, so that other institutions will have some guidelines for using them.

The 21st century climate runs are currently under way at NCAR, with results expected before Christmas.


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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 14:56:02 MDT 2000