February 29, 2000

NYT

Global Warming: The Contrarian View

Diagram: Two Sides, Two Data Sets

By WILLIAM K. STEVENS

Over the years, skeptics have tried to cast doubt on the idea of global warming by noting that measurements taken by earth satellites since 1979 have found little or no temperature rise in large parts of the upper atmosphere. The satellites' all-encompassing coverage yields more reliable results than temperature samplings showing a century-long warming trend at the earth's surface, they argued. In January, a special study by the National Research Council, the research arm of the National Academy of Sciences, declared that the "apparent disparity" between the two sets of measurements over the 20-year history of the satellite measurements "in no way invalidates the conclusion that surface temperature has been rising." The surface warming "is undoubtedly real," the study panel said. But the dissenters are a long way from conceding the debate, and they have

seized on other aspects of the panel's report in an effort to bolster their
case.

To be sure, according to interviews with some prominent skeptics, there
is now wide agreement among them that the average surface temperature
of the earth has indeed risen.

"I don't think we're arguing over whether there's any global warming,"
said Dr. William M. Gray, an atmospheric scientist at Colorado State
University, known for his annual predictions of Atlantic hurricane
activities as well as his staunch, longtime dissent on global climate change.

"The question is, 'What is the cause of it?' "

On that issue, and on the remaining big question of how the climate might
change in the future, skeptics continue to differ sharply with the dominant
view among climate experts.

The dominant view is that the surface warming is at least partly
attributable to emissions of heat-trapping waste industrial gases like
carbon dioxide, a product of the burning of fossil fuels like coal, oil and
natural gas.

A United Nations scientific panel has predicted that unless these
greenhouse gas emissions are reduced, the earth's average surface
temperature will rise by some 2 to 6 degrees Fahrenheit over the next
century, with a best estimate of about 3.5 degrees, compared with a rise
of 5 to 9 degrees since the depths of the last ice age 18,000 to 20,000
years ago. This warming, the panel said, would touch off widespread
disruptions in climate and weather and cause the global sea level to rise
and flood many places.

Dr. Gray and others challenge all of this. To them, the observed surface
warming of about 1 degree over the last century -- with an especially
sharp rise in the last quarter century -- is mostly or wholly natural, and
there is no significant human influence on global climate.

They also adhere firmly to their long-held opinion that any future warming
will be inconsequential or modest at most, and that its effects will largely
be beneficial.

In some ways, though, adversaries in the debate are not so far apart.

For instance, some dissenters say that future warming caused by
greenhouse gases will be near the low end of the range predicted by the
United Nations scientific panel.

And most adherents of the dominant view readily acknowledge that the
size of the human contribution to global warming is not yet known.

The thrust-and-parry of the climate debate goes on nevertheless.

With the National Research Council panel's conclusion that the surface
warming is real, "one of the key arguments of the contrarians has
evaporated," said Dr. Michael Oppenheimer, an atmospheric scientist
with Environmental Defense, formerly the Environmental Defense Fund.

But those on the other side of the argument see things differently.

To them, the most important finding of the panel is its validation of
satellite readings showing less warming, and maybe none, in parts of the
upper atmosphere, said Dr. S. Fred Singer, an independent atmospheric
scientist who is an outspoken dissenter.

For him and other climate dissenters, this disparity is key, in that it does
not show up in computer models scientists use to predict future trends.

The fact that these models apparently missed the difference in warming
between the surface and the upper air, the skeptics say, casts doubt on
their reliability over all.

Experts on all sides of the debate acknowledge that the climate models
are imperfect, and even proponents of their use say their results should
be interpreted cautiously.

A further problem is raised by the divergent temperatures at the surface
and the upper air, said Dr. Richard S. Lindzen, an atmospheric scientist
at the Massachusetts Institute of Technology, who is a foremost skeptic.

"Both are right," But, he said, increasing levels of greenhouse gases
should warm the entire troposphere (the lower 6 to 10 miles of the
atmosphere).

That they have not, he said, suggests that "what's happening at the
surface is not related to the greenhouse effect."

Skeptics also argue that the lower temperatures measured by the
satellites are confirmed by instruments borne aloft by weather balloons.

But over a longer period, going back 40 years, there is no discrepancy
between surface readings and those obtained by the balloons, said Dr.
John Michael Wallace, an atmospheric scientist at the University of
Washington in Seattle, who was chairman of the research council study
panel.

Although the climate debate has usually been portrayed as a polarized
argument between believers and contrarians, there is actually a broad
spectrum of views among scientists.

And while the views of skeptics display some common themes, there are
many degrees of dissent, many permutations and combinations of
individual opinion.

The views of some have changed materially over the years, while others
have expressed essentially the same basic opinions all along.

One whose views have evolved is
Dr. Wallace, who describes himself
as "more skeptical than most people"
but "fairly open to arguments on both
sides" of the debate.

He says the especially sharp surface
warming trend of the 1990's has
"pulled me in a mainstream
direction." While he once believed
the warming observed in recent
decades was just natural variation in
climate, he said, he is now perhaps
80 percent sure that it has been
induced by human activity -- "but
that's still a long way from being
willing to stake my reputation on it."

A decade ago, Dr. Wallace said,
many skeptics questioned whether
there even was a surface warming
trend, in part because what now
appears to be a century-long trend
had been interrupted in the 1950's,
1960's and early 1970's, and it had
not yet resumed all that markedly.

But the surge in the 1980's and
1990's changed the picture
substantially.

Today, Dr. Wallace said, few appear
to doubt that the earth's surface has
warmed.

One prominent dissenter on the
greenhouse question, Dr. Robert
Balling, a climatologist at Arizona
State University, says, "the surface
temperatures appear to be rising, no
doubt," and other skeptics agree.

There also appears to be general
agreement that atmospheric
concentrations of greenhouse gases
are rising.

At 360 parts per million, up from
315 parts in the late 1950's, the
concentration of carbon dioxide is
nearly 30 percent higher than before
the Industrial Revolution, and the
highest in the last 420,000 years.

Mainstream scientists, citing recent
studies, suggest that the relatively
rapid warming of the last 25 years
cannot be explained without the
greenhouse effect.

Over that period, according federal
scientists, the average surface
temperature rose at a rate equivalent
to about 3.5 degrees per century --
substantially more than the rise for
the last century as a whole, and
about what is predicted by computer
models for the 21st century.

But many skeptics, including Dr.
Gray and Dr. Singer, maintain that
the warming of the past 25 years can
be explained by natural causes, most
likely changes in the circulation of
heat-bearing ocean waters.

In fact, Dr. Gray says he expects that
over the next few decades, the
warming will end and there will be a
resumption of the cooling of the
1950's and 1960's.

At bottom, people on all sides of the
debate agree, the question of the
warming's cause has not yet been
definitively answered.

In December, a group of 11 experts
on the question looked at the status
of the continuing quest to detect the
greenhouse signal amid the "noise" of
the climate's natural variability.

The lead author of the study was Dr.
Tim P. Barnett, a climatologist at the
Scripps Institution of Oceanography
in La Jolla, Calif.

Dr. Barnett, who has long worked
on detecting the greenhouse signal,
describes himself as a "hard-nosed"
skeptic on that particular issue, even
though he believes that global
warming in the long run will be a
serious problem.

The study by Dr. Barnett and others,
published in The Bulletin of the
American Meteorological Society,
concluded that the "most probable
cause" of the observed warming had
been a combination of natural and
human-made factors.

But they said scientists had not yet
been able to separate the greenhouse
signal from the natural climate
fluctuations.

This state of affairs, they wrote, "is
not satisfactory."

Two big questions complicate efforts
to predict the course of the earth's
climate over the next century: how
sensitive is the climate system,
inherently, to the warming effect of
greenhouse gases? And how much
will atmospheric levels of the gases
rise over coming decades?

The mainstream view, based on
computerized simulations of the
climate system, is that a doubling of
greenhouse gas concentrations would
produce a warming of about 3 to 8
degrees.

But Dr. Lindzen and Dr. Gray,
pointing to what they consider the
models' problems with the physics of
the atmosphere, say they
overestimate possible warming.

It "will be extremely little," Dr. Gray
said. How little? Dr. Lindzen pegs it
at about half a degree to a bit less
than 2 degrees, if atmospheric
carbon dioxide doubles.

Other factors in the climate system
modify the response to heat-trapping
gases, and the United Nations
panel's analysis included these to
arrive at its projection of a 2- to
6-degree rise in the average global
surface temperature by 2100.

One factor is various possible levels
of future carbon dioxide emissions.

Dr. Singer, saying that improving
energy efficiency will have a big
impact on emissions, predicts a warming of less than 1 degree by 2100.

Dr. Balling projects a warming just shy of 1 degree for the next 50 years,
not out of line with the United Nations panel's lower boundary.

Another skeptic, Dr. Patrick J. Michaels, a climatologist at the University
of Virginia, similarly forecasts a greenhouse warming rise of 2.3 degrees
over the next century.

As is now the case, he says, the warming would be most pronounced in
the winter, at night, and in sub-Arctic regions like Siberia and Alaska.

A warming of that magnitude, he and others insist, could not be very
harmful, and would in fact confer benefits like longer growing seasons
and faster plant growth.

"It should be pretty clear," he said, that the warming so far "didn't
demonstrably dent health and welfare very much," and he said he saw no
reason "to expect a sudden turnaround in the same over the next 50
years." After that, he said, it is impossible to predict the shape of the
world's energy system and, therefore, greenhouse gas emissions.

A warming in the low end of the range predicted by the United Nations
panel may well materialize, said Dr. Oppenheimer, the environmentalist.

But, he said, the high end may also materialize, in which case, mainstream
scientists say, there would be serious, even catastrophic, consequences
for human society. "There is no compelling evidence to allow us to
choose between the low end, or the high end, or the middle," Dr.
Oppenheimer said.

If business continues as usual, the world is likely at some point to find out
who is right.

Surface or Air? The Great Debate
Continues

When the debate over global
warming burst into prominence
more than a decade ago, scientists
based their assessment that the
earth's temperature was rising on a
single set of observations, those
at the planet's surface.

Dr. John R. Christy and Dr. Roy
W. Spencer were troubled by that.

Both had been weather observers,
and both knew that the surface
temperature readings were subject
to multiple kinds of distortion --
including poor placement of
thermometers, spotty coverage of
some regions and the heat emitted
by the concrete of cities.

So Dr. Christy and Dr. Spencer,
atmospheric scientists at the
University of Alabama in
Huntsville and the NASA
Marshall Space Flight Center,
respectively, began wondering:
could global temperatures be
measured by earth satellite? If so,
the problem of distorted data
might be eliminated.

They decided to find out, and
discovered that in some ways the
satellite and surface trends
roughly matched.

Both sets of measurements found
substantial warming over Northern
Hemisphere continents, for
instance, and the timing of
year-to-year temperature
fluctuations was about the same in
both.

But unlike the surface record, the
satellites revealed no underlying
warming trend for much of the free
atmosphere, up to about five
miles, mainly in the tropics.

In fact, the raw numbers showed a
cooling there.

It was this disclosure on which
skeptics seized in their efforts to
use the satellite record to cast
doubt on the idea of global
warming.

Later, the satellite record was
adjusted to filter out the effects of
natural, short-term climate
fluctuations like the El Niño
phenomenon and temporary
cooling produced by sun-blocking
haze from volcanic eruptions.

Later still, the record was
corrected to eliminate distortions
caused by the movements of the
satellites.

Now the satellite record showed a
slight warming, but one much
smaller than the apparent surface
warming.

Other analysts had for some time
been examining the surface record
and trying to eliminate or
compensate for some of the
imperfections that had originally
concerned Dr. Spencer and Dr.
Christy.

Enter the National Research
Council, the research arm of the
National Academy of Sciences.

Dr. Christy and Dr. Spencer were
members of the panel. In January,
it published its analysis of the
disparity.

It found that the remaining margin
of error in the surface
measurements was now probably
about one-tenth as large as the
observed trend, and that the
surface warming over the 20-year
period of the satellite
measurements, from 1979 to 1998,
was "undoubtedly real." That
warming, it reported, was equal to
2.25 degrees to 3.6 degrees
Fahrenheit per century --
"substantially greater" than the
surface warming over the
preceding century as a whole.

By contrast, the study panel
found that over the same 20-year
period, the satellite record showed
an average global warming rate in
the lower to middle free
atmosphere of zero degrees to 1.8
degrees per century.

How real was the discrepancy?
The research council's panel of
scientists found that the level of
uncertainty inherent in obtaining
and analyzing temperature
observations was almost as large
as the disparity, so it was difficult
to know.

To the extent the disparity exists,
what has caused it? The panel
said this was also unclear.

Errors in measurement and
analysis of data could account for
some of the difference, the group
said, but concluded that some of
the apparent disparity was
probably real.

It noted that surface temperatures
and temperatures aloft were
subject to different influences and
were often "decoupled" from one
another; that is, the surface and
the upper air do not necessarily
behave alike.

For example, the group said,
sun-blocking haze from volcanic
eruptions is believed to cool the
upper air more than the surface.

In 1991 there was just such an
eruption, the biggest of the
century from a climatic standpoint.

Short-term effects like these
should probably wash out over
the long run, the panel said.

Therefore, it cautioned, a 20-year
temperature record is "not
necessarily indicative of the
long-term behavior of the climate
system." (That is true not only of
the satellite record but also of the
unusually sharp spike in surface
temperatures over the two
decades.)

In fact, said the study panel's
chairman, Dr. John Michael
Wallace of the University of
Washington in Seattle, weather
balloons confirm the disparity
between the surface and the upper
air over the last 20 years, but show
an upper-air warming comparable
to that of the surface over a longer
period of the last 40 years.

"The very likely possibility," Dr.
Wallace said, "is that in the next
decade we might see a different
behavior" than what has been
revealed over the last two
decades.

But for now, the great satellite
debate remains at least partly
unsettled.

WILLIAM K. STEVENS