Hotter heat waves foreseen: climate change may lead to worse weather.

Cities in the Northern Hemisphere will experience hotter, longer, and more-frequent heat waves during the twenty-first century than in the twentieth, according to a new climate-modeling study by the U.S. National Center for Atmospheric Research (NCAR).

As greenhouse gases collect in

the atmosphere, they intensify an unusual atmospheric circulation pattern that has been observed during heat waves in Europe and North America. This pattern has become more pronounced, with severe heat waves extending into the Mediterranean region and to southern and western United States.

Heat waves hitting megacities like Chicago and Paris can kill so many people in such a short time that they are considered one of the deadliest climate events. A Chicago heat wave in summer 1995 claimed 739 lives. In France, 15,000 people died from heat during 2003. In both instances, changes in atmospheric pressure produced clear skies and longer periods of hot weather on the ground; the nighttime temperature did not drop enough to offer cooling relief.

The model developed by NCAR scientists Gerald Meehl and Claudia Tebaldi used data from 1961-1990 and projections for 2080-2099 to predict the effects of greenhouse gases and sulfate aerosols on the climate in Europe and the United States, focusing specifically on Chicago and Paris. The researchers conclude that the worst heat waves will be hotter by more than 3[degrees]C (5.4[degrees]F) in minimum nighttime temperatures.

The model also shows that Chicago will experience an average of 2.08 heat waves a year, up from 1.66 currently--a 25% increase--and will last 8.5-9.24 days, up from 5.39-8.85 days. Paris will experience 2.15 heat waves a year on average (a 31% increase over the current 1.64 per year), and they'll last 11.39-17.04 days, up from an average of 8.33-12.69 days.

"It's the extreme weather and climate events that will have some of the most-severe impacts on human society as the climate changes," says Meehl.

Stabilizing Climate Change

The NCAR model assumes that no new actions are taken to slow the buildup of greenhouse gases, but a new report from Princeton University scientists offers hope that climate change could be curbed with already existing technologies--and the will to use them.

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To keep emissions from increasing over the next 50 years, humanity will have to reduce carbon output by 7 billion tons a year, according to Princeton professors Stephen Pacala and Robert Socolow. Strategies that can achieve this reduction--while still meeting the world's need for energy--include:

* Doubling fuel efficiency of 2 billion cars, from 30 to 60 mpg.

* Halving the number of car miles traveled.

* Replacing 1,400 coal electric plants with natural gas-powered facilities.

* Capturing and storing emissions from 800 coal electric plants.

* Producing hydrogen from coal at six times today's rate and storing the captured carbon dioxide.

* Increasing wind electricity capacity 50 times today's capacity, adding 2 million large windmills.

* Installing 700 times the current solar electricity capacity.

* Increasing ethanol production 50 times by creating biomass plantations equal to one-sixth of world cropland.

* Eliminating tropical deforestation and creating new plantations of non-forested lands to quintuple current plantation area.

* Adopting conservation tillage in all agricultural soils worldwide.

The researchers emphasize that actions can be taken now and need not wait some miracle breakthrough technology.

"If we decide to act, we will need to reduce carbon emissions across the whole global economy," says Socolow. "Fortunately, we have the tools to do this, especially if we think in terms of 50-year campaigns, not instant solutions."

Sources: National Center for Atmospheric Research, UCAR Communications, P.O. Box 3000, Boulder, Colorado 80307. Web site www.ucar.edu.

Princeton University, Office of Communications, 22 Chambers Street, Suite 201, Princeton, New Jersey 08542. Web site www.princeton.edu.

[Editor's note: Both studies were reported in the August 13, 2004, issue of Science magazine.]