Last month, researchers published
the first results from a worldwide global climate experiment run on tens of
thousands of personal computers across the planet. Their results contain perhaps
the most extreme scenario yet for global warming, with the worst case coming
in at an 11-degree-Celsius increase in Earth's temperature, for a world in which
carbon dioxide levels are doubled.
In response to a doubling of carbon dioxide
in a highly sensitive climate model, temperatures (in degrees Celsius) and rainfall
(percentages) in the middle of a 45-year timespan increased dramatically in
some places. Image copyright David Stainforth; courtesy of Nature.
An online climate experiment built on the blueprint of SETI@home and other such
distributed-computing research projects, climateprediction.net has presented
researchers with a range of predictions and a way to look at model uncertainty
(see Geotimes, April 2004). The "ensemble of
ensembles," reported on in the Jan. 27 Nature, took 2,578 simulations for
a general circulation model with the atmosphere linked to a mixed-layer ocean.
The model is based on the leading model from the Hadley Centre, in Exeter, United
Kingdom, which uses two supercomputers to run its climate model for global forecasting.
Climateprediction.net, however, took advantage of idle home computer power to
run its simulations over a 45-year period to test permutations of the model's
variables, starting with different starting conditions and different carbon
dioxide levels in the atmosphere.
Each simulation varied six key components at a time, including moisture levels,
cloud cover or other parameters. Varying more than one parameter allows the
experiment to show how the system might respond as multiple variables interact.
The researchers also set the simulations to assess the response to an increase
in carbon dioxide. Eventually, the ongoing experiment will test varying 21 parameters
"over a wider range of processes and values," they report.
The resulting possible warming so far ranges from just less than 2 degrees Celsius
up to 11 degrees Celsius, with the bulk of the results falling around 3.4 degrees
of warming. The few models that were more sensitive to changes tended to warm
the most, and the researchers note that this is the first time a general circulation
model has produced such extreme changes with highly sensitive models.
Most climate scientists agree with the range of warming reported by the IPCC's
Third Assessment Report, which found future warming likely to be around 1.5
to 4 degrees Celsius with a doubling of the carbon dioxide levels in the atmosphere,
says Richard Alley of Pennsylvania State University. Because of the physics
of the system, doubling carbon dioxide generally yields a baseline temperature
increase. But, Alley says, the other end of the scale is more open: "It's
fairly possible to get a really big number" for temperature changes, so
"it's not a surprise." Alley adds that "comparison to past and
ongoing changes will help narrow the answer" on future climate scenarios,
something this study did not do.
For now, the climateprediction.net team is not able to say how probable the
worst-case scenario may be, but they also are unwilling to discount it until
it is ruled out by hard data. The project plans a future experiment that unfolds
over a longer period, 1950 to 2100, and contains a dynamic ocean in the models.
Naomi Lubick
Links:
"A Climate
of Your Own," Geotimes, April 2004
climateprediction.net
Back to top
 |
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers  |