Consensus
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instead of relatively balmy 60° F.
 Collectively, these gases make up a small portion of the atmosphere.
But their share is growing. Since the beginning of the industrial age, human-caused
emissions of carbon dioxide have steadily increased in a pattern that began
with the cutting and burning of forests and accelerated with this century's
explosion of automobiles, power plants, and factories. Since 1750, the atmosphere's
carbon component is up 30 percent. Greenhouse gases continue to rise by
about 1 percentage point each year, with about a quarter of the gases coming
from the United States alone.
Source: The San Francisco Chronicle and National Environmental
Trust
Dark shading shows terrain that would be flooded in
the San Francisco Bay area by a 1-meter sea-level rise and a record high
tide.
How the Earth's climate responds to the additional carbon is complex,
but the basic physics are well understood. "If you add these chemicals
they will trap heat-that's pretty hard to argue against," said David
Rind of NASA's Goddard Institute for Space Studies in New York. "That
fact by itself goes a long way toward convincing people that this is real."
Even at current levels, concentrations of greenhouse gases in the atmosphere
are far higher than at any time since the appearance of modern humans on
Earth. By the end of the next century-as the human population doubles again
to 11 billion or even more-there will be two to three times as much carbon
in the air as 250 years ago if emissions continue to rise at current rates.
In 1995, the more than 2,000 international scientists of the U.N.-sponsored
Intergovernmental Panel on Climate Change predicted that the changes in
the atmosphere will raise the global average temperature between 1.8 and
6.3°F by the year 2100-an extraordinarily rapid increase by geological
standards. By comparison, the average global temperature was only 9 degrees
cooler at the height of the last ice age, when a half-mile of ice covered
much of what is now the northern United States.
"Under business as usual, we'll reach carbon dioxide concentrations
that haven't been seen on this planet in the last 50 million years,"
Harvard University professor and Nobel Laureate John Holdren told a recent
White House conference on global warming. "We will have achieved that
in the geological blink of an eye, exposing, as we do it, natural systems
to a rate of temperature change faster than at any time in the last 10,000
years."
Scientists know that adding greenhouse gases to the atmosphere will
eventually cause the planet to warm. The big questions are how-how much,
how fast, and how the warmth will be distributed.
Possible clues are being found in implausible places. Scientists from
NOAA and other agencies are mining Antarctic ice sheets, mountain glaciers,
ocean sediments, and ancient coral reefs for chemical evidence of temperature
and atmospheric conditions from tens of thousands of years ago. The research
is yielding a steady flow of data that reflects the climate's sensitivity
to past fluctuations in carbon dioxide.
Other clues are found in places such as Moon Lake, a popular fishing
spot near Sanborn that was created by a retreating glacier more than 12,000
years ago. Scientists dug deep into the black mud of the lake bed and discovered
a well-preserved geological record dating to the last ice age. The silicate
remains of microscopic creatures, deposited in layer after layer, act as
a crude precipitation index, distinguishing relatively wet periods from
drier ones.
In telling the story of the lake, the microbes offer a glimpse into
previous warm-weather patterns in the past 2,000 years. One finding, according
to Queens University ecologist Kathleen R. Laird, was that warmer periods
coincided with droughts more profound than any seen in the region's recorded
history. Some dry spells lasted more than a century.
"The droughts were more extreme-with more intensity than the Dust
Bowl-and they were more frequent," Laird said.
Several studies have predicted that crop yields would remain stable
or possibly increase in the Northern Hemisphere. Many formerly frozen areas
would become arable, and higher levels of carbon dioxide-a necessary plant
nutrient-could stimulate the growth of crops. But the Moon Lake record supports
computer projections that show dryer patterns emerging in America's breadbasket.
One computer model predicts a loss of soil moisture of more than 20 percent
in the Great Plains and southeastern United States if carbon dioxide levels
double. Under quadrupled carbon dioxide, soil moisture losses climb to 50
percent and even higher.
Other parts of the globe could experience different, but equally disruptive
effects, scientists say. In some regions, changes may already be underway.
A few scientists maintain that increased precipitation at the poles
would lead to larger ice sheets and subsequently lower sea levels. But most
models forecast further melting of glaciers and Arctic ice, which could
raise the sea level anywhere from a few inches to several feet. Already,
sea level is four to nine inches higher than a century ago. If it climbs
by another meter, as some computers predict, thousands of square miles of
coastal Florida and Louisiana could be swallowed up, as well as large chunks
of Bangladesh and many island nations. Increased water vapor in the atmosphere
could cause an increase in severe storms, computer projections show. Records
compiled at NOAA's National Climate Data Center in Asheville, N.C., show
a 10 percent increase in precipitation in the United States since 1910,
with much of it coming in large doses: downpours and blizzards.
Warmer weather would surely mean fewer deaths from cold and ice. On
the other hand, there likely would be more killer heat waves such as the
one that killed 465 people in Chicago in 1995.
A rapid change in climate could also place unusual stresses on animal and
plant ecosystems, hastening the extinction of rare species. Insects and
microorganisms generally thrive in warmer [TOP] |
climates, and could spread to new areas. Most of the predicted changes
would occur in the latter half of the next century, a time when the human
population will be twice what it is now.
"If you double the number of people, you stress the resources even
more," said NOAA's Baker. "Add global warming on top of that,
and there will be lots more losers than winners."
Accurately predicting even one day's weather can be a daunting task,
as any TV weathercaster knows. To make the kinds of long-term projections
needed to forecast climate change requires truly extraordinary computing
power. And still it may not be enough.
At NOAA's Geophysical Fluid Dynamics Lab at Princeton, the technological
muscle is provided by a Cray C90 Supercomputer, a liquid-cooled, aluminum-encased,
battlestar of a machine that contains two dozen processors and row after
row of hard drives. The C90 has as much processing memory as 256 top-of-the-line
personal computers and can perform 15 billion operations in a single second.
Using computer models that simulate atmospheric processes, machines
such as the C90 can reliably forecast major climatic events, such as an
El Niño. They accurately predicted the global cooling that occurred
after the Mount Pinatubo volcano erupted in 1991, spewing ash into the upper
atmosphere. These are the same models that are used to predict the effects
of warming, and they're getting better all the time.
"These are not Johnny-come-lately calculations," said Daniel
L. Albritton, director of NOAA's Aeronomy Laboratory in Boulder, Colo. "These
models are two decades old, and they've been constantly improved and modified.
And they consistently give the same answers."
But even today's most advanced computers still cannot match the complexity
of the Earth's climate. Humans don't yet fully understand-nor can computers
simulate-how clouds form in the atmosphere. That's important, because scientists
believe warmer temperatures will increase cloud cover throughout the world.
More clouds would reflect more of the sun's rays and may offset some of
the warming.
A similar mystery surrounds tiny aerosols, or bits of dust and man-made
soot that create haze in cities. Some of these particles can also reflect
solar radiation so that, in effect, human pollution ends up heating and
cooling the planet at the same time.
Many climate scientists believe that aerosols have masked some of the
warming that computer models predict should have occurred by now. Princeton's
Mahlman believes the slow-but-sure increase in greenhouse gases is now beginning
to overwhelm the aerosol effect in a kind of tortoise-and-hare race.
But another natural process may also have helped keep warming in check.
About half of the carbon dioxide people emit each year doesn't end up in
the atmosphere but simply disappears, apparently absorbed by the oceans,
microbes and plants in ways that aren't fully understood.
This vanishing act has caused some scientists to suspect that nature
is more forgiving of human excesses than previously believed -at least to
a point. These scientists are part of a small but persistent minority that
remains skeptical of claims that humans have already caused the planet to
warm. They point to 20 years of satellite measurements that so far do not
show the kind of temperature increases that land-based thermometers have
recorded.
NASA's Roy Spencer believes the traditional, land-based measurements
have been skewed by city buildings and concrete that absorb heat and make
temperatures artificially high. He also questions the reliability of temperature
data that come from sea-bound ships and developing countries which, he says,
"have a rather poor reputation for providing information of sufficient
quality."
Other scientists argue that the satellite record could also be flawed-it
is based, after all, on a relatively small amount of data from two satellites-and
it may be misleading as well. "The satellites don't measure the same
thing. They sense temperatures two to five miles above the Earth's surface,"
Albritton said. "You'd expect the result to be different."
Albritton and other prominent climate experts are even more derisive
of claims that tiny changes in the sun's intensity may account for a significant
share of the temperature change in the last century. Mahlman dismisses that
theory-which House Speaker Newt Gingrich (R-Ga.) recently invoked in criticizing
President Clinton's climate change policy-as "nutty science."
"There is no evidence-no measurements whatsoever," Mahlman
said. "Implicitis the assertion that the Earth is extraordinarily sensitive
to tiny variations in solar radiation-Miracle No. 1-and also extraordinarily
insensitive to much greater changes in the infrared [greenhouse] balance-Miracle
No. 2A grand jury would throw it out in 20 minutes."
To Albritton, the continuing debate over satellite measurements and
possible solar influence makes for interesting discussions in scientific
journals but may not even be relevant in the long run. The search for conclusive
proof of a human link to global warming in the past century is "somewhat
of a distraction-looking for a one-degree needle in a hay stack."
"What's more important are the potential consequences," he
said. "We're going to double the amount of carbon dioxide in the atmosphere-it's
just a matter of when. And the consequences of doubling is more than a mere
half-degree Celsius."
He added: "Once that stuff is in the atmosphere, there's no way
to get it back."
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"The key scientific message is this:
The atmospheric accumulation of greenhouse gases, and the resulting climate
change, are effectively irreversible. Once the gases are emitted, they warm
the Earth for centuries. In addition, there is no known limit on warming.
In other words, we are barreling down a highway toward a blind turn with
our foot on the accelerator instead of the brakes. It's time to slow down
by reducing emissions of these gases."