January 1, 2000

NYT

BIOLOGY

Turning Up the Heat: Two Forecasts for Earth 2100

What will the weather be like in 2100?

Don't laugh. While forecasting the next century in any field is a
dicey exercise, climate experts have more tools than most
prognosticators. Hence the widespread concern about global
warming. Computer models enable climatologists to project the
effects of the ever-increasing emissions of industrial gases, like
carbon dioxide, that scientists contend are warming the
atmosphere.

Of course, the picture that the models paint depends on the
scientists' assumptions. A 1999 study by Dr. Tom Wigley, a
climatologist at the National Center for Atmospheric Research in
Boulder, Colo., ran four possibilities for the future through
climate models to examine how global warming might alter the
average global temperature. Then he combined the results of 15
models to explore the effects on temperature and precipitation in
North America.

Presented below are projections for the year 2100 under two of
Dr. Wigley's cases: those assuming the highest and lowest levels
of emissions. A word of caution. Computers do not yet have the
capacity to make more than relatively crude simulations of the
climate's behavior, and scientists have not yet firmly pinned down
the climate's sensitivity to warming by greenhouse gases. Dr.
Wigley's analysis, conducted for the Pew Center on Global
Climate Change in Arlington, Va., assumed a moderate
sensitivity of about 4.5 degrees Fahrenheit for a doubling of
atmospheric carbon dioxide.

WILLIAM K. STEVENS

Regions shown in dark blue in the maps below are some
of the areas that could be flooded at high tide if global
warming causes the sea to rise 2 feet in the next 100
years. The indicated areas account not only for the
effects of global warming, but also for other factors like
the sinking of the earth's crust in some coastal areas.

Maps from a forthcoming article by James G. Titus and Charles Richman
in the journal Climate Research.

The Highest-Emissions Future ASSUMPTIONS High population growth Relatively little emphasis on technological change No specific steps by governments to control greenhouse gas emissions GLOBAL CONDITIONS IN 2100 Average surface temperature increase of about 5 degrees Fahrenheit Sea level would rise about 2 feet

The Lowest-Emissions Future ASSUMPTIONS Low population growth Emphasis on global action to reduce environmental problems, but no policy steps on greenhouse gas emissions specifically Rapid technological change Use of cleaner-energy technologies GLOBAL CONDITIONS IN 2100 Temperature increase of about 3.5 degrees Sea level would rise about 1.5 feet

TEMPERATURE CHANGES

An average global warming of 4.5 degrees, more or less, might not
sound like much until it is realized that the world has warmed by
only 5 to 9 degrees since the depths of the last ice age, 18,000 to
20,000 years ago.

The heating would not be uniform. North America would warm
more than the global average. Northern latitudes would heat up
more than southern ones. Climatic zones would shift north 100 to
350 miles, making the climate of New York, for instance, more like
that of Washington today.

In either the high- or low-emissions case, winter would probably be
shorter. Northern growing seasons would lengthen, an effect that
is already discernible. Northern wheat-growing areas could thrive,
while some Southern farmers might have to switch to new crops,
like citrus. New England could lose its brilliant fall maples, and
Midwestern prairies would expand at the expense of forests.

Scientists warn that the warming would be enough to push
summer heat waves to higher, more dangerous combinations of
temperature and humidity, a trend that federal scientists say is
already under way.

Change in average temperature from 1999 to 2100, in degrees Fahrenheit

Summer	Winter

PRECIPITATION CHANGES

One of the most striking effects of a warmer atmosphere, many
scientists say, would be a worsening of floods and droughts. The
reason is that a warmer atmosphere holds more moisture and
makes more water evaporate from the earth's surface. This creates
heavier rainstorms but dries out the land faster between storms.

These extremes would have a seasonal and regional character.
Most of the country, and especially the heart of the plains, would
experience less summer rainfall. But winter precipitation would
generally increase, especially in northern regions.

Climate experts say much of the increase in precipitation would
result from more heavy rainstorms - ''gully washers'' - that cause
flash flooding. Scientists have already detected a trend toward
more of this intense rainfall in the United States.

Also, some snowstorms could become heavier, even though winter
might be shorter. Global warming is unlikely to cancel winter
altogether. Cold waves would still occur, and when they coincided
with storms, a moister atmosphere could generate worse
blizzards.

Percentage change in average precipitation from 1999 to 2100

Summer	Winter