research groups have estimated the probabilities of various
outcomes, based on variations in the physical response of the
climate system itself. But the MIT model is the only one that
interactively includes detailed treatment of possible changes in
human activities as well - such as the degree of economic
growth, with its associated energy use, in different countries.
co-author Ronald Prinn, the co-director of the Joint Program and
director of MIT's Center for Global Change Science, says that,
regarding global warming, it is important "to base our
opinions and policies on the peer-reviewed science," he
says. And in the peer-reviewed literature, the MIT model, unlike
any other, looks in great detail at the effects of economic
activity coupled with the effects of atmospheric, oceanic and
biological systems. "In that sense, our work is
unique," he says.
new projections, published this month in the American
Meteorological Society's Journal of Climate, indicate a median
probability of surface warming of 5.2 degrees Celsius by 2100,
with a 90% probability range of 3.5 to 7.4 degrees. This can be
compared to a median projected increase in the 2003 study of
just 2.4 degrees. The difference is caused by several factors
rather than any single big change. Among these are improved
economic modeling and newer economic data showing less chance of
low emissions than had been projected in the earlier scenarios.
Other changes include accounting for the past masking of
underlying warming by the cooling induced by 20th century
volcanoes, and for emissions of soot, which can add to the
warming effect. In addition, measurements of deep ocean
temperature rises, which enable estimates of how fast heat and
carbon dioxide are removed from the atmosphere and transferred
to the ocean depths, imply lower transfer
rates than previously estimated.
Prinn says these and a variety of other changes based on new
measurements and new analyses changed the odds on what could be
expected in this century in the "no policy" scenarios
- that is, where there are no policies in place that
specifically induce reductions in greenhouse gas emissions.
Overall, the changes "unfortunately largely summed up all
in the same direction," he says. "Overall, they
stacked up so they caused more projected global warming."
While the outcomes in the "no policy" projections now
look much worse than before, there is less change from previous
work in the projected outcomes if strong policies are put in
place now to drastically curb greenhouse gas emissions. Without
action, "there is significantly more risk than we
previously estimated," Prinn says. "This increases the
urgency for significant policy action."
To illustrate the range of probabilities revealed by the 400
simulations, Prinn and the team produced a "roulette
wheel" that reflects the latest relative odds of various
levels of temperature rise. The wheel provides a very graphic
representation of just how serious the potential climate impacts
"There's no way the world can or should take these
risks," Prinn says. And the odds indicated by this modeling
may actually understate the problem, because the model does not
fully incorporate other positive feedbacks that can occur, for
example, if increased temperatures caused a large-scale melting
of permafrost in arctic regions and subsequent release of large
quantities of methane, a very potent greenhouse gas. Including
that feedback "is just going to make it worse," Prinn
The lead author of the paper describing the new projections is
Andrei Sokolov, research scientist in the Joint Program. Other
authors, besides Sokolov and Prinn, include Peter H. Stone,
Chris E. Forest, Sergey Paltsev, Adam Schlosser, Stephanie
Dutkiewicz, John Reilly, Marcus Sarofim, Chien Wang and Henry D.
Jacoby, all of the MIT Joint Program on the Science and Policy
of Global Change, as well as Mort Webster of MIT's Engineering
Systems Division and D. Kicklighter, B. Felzer and J. Melillo of
the Marine Biological Laboratory at Woods Hole.
Prinn stresses that the computer models are built to match the
known conditions, processes and past history of the relevant
human and natural systems, and the researchers are therefore
dependent on the accuracy of this current knowledge. Beyond
this, "we do the research, and let the results fall where
they may," he says. Since there are so many uncertainties,
especially with regard to what human beings will choose to do
and how large the climate response will be, "we don't
pretend we can do it accurately. Instead, we do these 400 runs
and look at the spread of the odds."
Because vehicles last for years, and buildings and powerplants
last for decades, it is essential to start making major changes
through adoption of significant national and international
policies as soon as possible, Prinn says. "The least-cost
option to lower the risk is to start now and steadily transform
the global energy system over the coming decades to low or zero
greenhouse gas-emitting technologies."
This work was supported in part by grants from the Office of
Science of the U.S. Dept. of Energy, and by the industrial and
foundation sponsors of the MIT Joint Program on the Science and
Policy of Global Change.