Climate Experts Clash on Global Warming

TECHNOLOGY DAY

Climate Experts Clash on Global Warming

Sharp differences as well as common ground were apparent when two  
experts on atmospheric dynamics and modeling engaged in their climactic 
debate on world climate change last Friday, Technology Day 1990.  

Dr. Stephen Schneider of the National Center for Atmospheric Research in 
Boulder, Colorado, who believes that a significantly warmer world 
climate is likely because of increasing greenhouse gases, challenged 
Sloan Professor of Meteorology Richard S. Lindzen of MIT's Department of 
Earth, Atmospheric, and Planetary Sciences, a well-known skeptic about 
global warming.

The debate drew a full house of alumni, press, and interested citizens 
to Kresge auditorium. 

Moderating the good-natured though serious sparring was Professor Ronald 
G. Prinn of EAPS. He began with a crisp summary of present knowledge 
about the basic ingredients of potential global warming: the  rising 
concentrations of gases that can absorb and re-emit infrared radiation 
that comes from the solar-heated surface of the Earth.

For some of these gases, said Professor Prinn, sources are reasonably 
well understood, but the sources and sinks of others, including methane 
and even carbon dioxide, are less clear. Professor Prinn reminded the 
audience of an often overlooked fact: omnipresent water vapor itself is 
one of the most important and potent of the greenhouse gases.

Concentrations of chlorofluorocarbons (CFC's) greenhouse gases, he said, 
are rising rapidly from five-11 percent per year, and these molecules 
have typical lifetimes in the atmosphere ranging from 75-180 years. By 
contrast, the greenhouse gas nitrous oxide rises at only 0.2 to 0.3 
percent per year, but has a 150 to 180 year lifetime in the atmosphere. 
Its main source is in the tropics, not from fossil fuel combustion

Fossil fuel, however, is responsible for most of the carbon dioxide 
increase we see today, said Prinn, though some of that increase may be 
due to deforestation whose effects are still a matter of controversy.

Regarding the very long atmospheric lifetimes of many greenhouse gases, 
Professor Prinn spoke of a risk factor. "We cannot just simply stop what 
it is we're doing and have these gases decay out of the atmosphere on 
very short time scales," he said. The gases are long-lived. Just 
bringing the rate of increase of some of these gases to zero would 
require cutting their emission by factors of three to six, according to 
Professor Prinn.     

Prinn sharpened the debate: "Greenhouse gases are increasing today at 
very substantial rates. Projected into the future, these rates, when 
included in most current climate models, lead to predictions of a 
significant global warming over the next century, but are these 
predictions reliable ? "

He showed the controversial variation of carbon dioxide concentration 
and air temperature plotted for the past 160,000 years, a graph that was 
determined from air bubbles in glacial ice. Superficially, the two 
curves look like they track one another remarkably well. But Prinn 
asked, "Is this a chicken and egg problem? Is carbon rising merely 
because temperature is rising and the biosphere responds, or, is the 
temperature rise itself due to changes in carbon dioxide?  We don't have 
an answer to that yet."



Warming: Better than Even Odds

Arguing in the affirmative on warming, Dr. Stephen Schneider spoke on 
the reliability of climate models: "The real question isn't whether they 
are reliable, yes or no," he said. The main issue is what decision-
making purposes the models will be used--"reliability for what," as he 
put it.

"I'd love to tell you we've made great progress [over the last 15 to 20 
years], but we haven't. The estimate has been, if carbon dioxide were to 
be doubled and held fixed--what we call equilibrium--then we'd warm up 
the climate something between 1.5 and 4.5 degrees Celsius. . . The 
question is, is that range reliable and what are the elements of it that 
are subject to question."

After explaining the basic principles of computer modeling of global 
climate,  Schneider termed as much as a 10oC temperature rise by the end 
of the next century "a low probability but still possible case."

The most uncertain components of the models, he said, are the complex 
effects of clouds, including both cooling and warming effects. He also 
showed how difficult it is to forecast the specific geographic patterns 
of climate change, including temperature and moisture distributions.

Schneider worried about the rapidity of change, saying that nature took 
about 10,000 years to warm 5oC (from 15,000 to 5,000 years ago) as 
compared to a possible impending rate of change five to 100 times that 
natural rate.

"What is the probability of the catastrophic curve or the mild curve? 
The answer here is that there is no objective way to assign it," 
Schneider said.

"Simply looking backwards in the last century is not very instructive to 
give us reliability in the future, because too many things that we 
weren't measuring were occurring. We've only been measuring what's 
important for about the last ten years.

"The bottom line is what's the probability of these curves and the 
answer is it depends on the intuition of experts. . . It's probably a 
better than even bet, according to most people who I've asked, that the 
truth will be somewhere in this part of the range [2oC increase]. My own 
view is that it's not likely to be up at the catastrophic end or at the 
low end--when I say not likely I mean maybe a 10 percent chance."

"Are the models reliable? . . . In detail, 'no.' I'll join [Dr. Lindzen] 
in that. On the other hand, are they reliable to say that we have a 
better than even chance that there could be unprecedentedly large 
climate change [larger than 2oC]--when I say unprecedented, I mean in 
the 10,000 years of human civilization, then I think the answer is 
'yes.'"

    "Facing 50 percent or so odds of that kind of change is a reliable 
enough prediction in my value system to take quite seriously that we 
need to examine what we're doing that's causing these global changes 
that Ron Prinn showed, rather than waiting the 10 or 20 years to resolve 
the details."



Minimal or No Warming Likely

Speaking of the great difficulties inherent in computer modeling of 
climate, Professor Lindzen said, "I think the situation as we look at it 
today is that the warming, indeed, is much smaller than suggested by 
current large models, if indeed there is warming at all."

Professor Lindzen believes that there will be a warming "under half a 
degree," though he does not even rule out the possibility of a cooling.

Since he believes that  the models are making erroneous predictions, he 
argued, "I think it is a pressing obligation of meteorologists and 
oceanographers to find out why the models are wrong."

Professor Lindzen complained that atmospheric modeling ". . .used to be 
considered a tool of theory." Now, he suggested, the community of 
modelers and theoreticians has grown apart. "What I find increasingly 
worrisome is the notion that models are assessed by comparison with 
other models," he said.

Professor Lindzen took  exception to Dr. Schneider's range of warming 
prospects, saying, "whereas the small end of the diagram he last showed 
was quite likely [below 1oC rise]  the high end would violate many, many 
things."

Lindzen believes, for example, that the temperature record of the past 
is not compatible with the models that predict significant warming in 
the next century. He believes that there are "documented errors in 
models that are crucial to warming predictions."  He said, "Much less 
certain, though potentially extremely important, the models have a 
behavior in the tropics that is crucial to present predictions and seems 
inconsistent with present and past tropical behavior."

Lindzen said that models that currently predict a 4oC temperature rise 
some time next century for a doubling of carbon dioxide, suggest that a 
2oC rise should already have occurred for the carbon dioxide already put 
into the atmosphere by human activity--something that has not happened.

Perhaps Professor Lindzen's sharpest criticism dealt with what he called 
the "circumvention" of Earth's "greenhouse" by  vertical currents that 
bring water vapor upward and cause heat to be radiated back into space.

He said, "The surface of the Earth cools bodily by motion that carries 
the heat around the bulk of the greenhouse gas and the radiation is 
emitted from upper levels which have much less infrared opacity. Thereby 
they circumvent about  three-quarters of the greenhouse substance. . . 
Speaking of trapping of heat by greenhouse gases in the bulk of the 
atmosphere is no more impressive than the trapping of Germans by the 
Maginot line--there are plenty of good ways of getting around it."

I think that the current evidence suggests that the overall feedback 
ought to be negative [restraining warming]. And, indeed, if we were not 
caught up in the politics of this problem, the normal response to the 
data and the models would be to intensify our search."

	

What to Do

Two MIT professors followed with brief remarks on what should be done, 
given our present incomplete knowledge. Professor of Economics Henry D. 
Jacoby of  the MIT Sloan School of Management said, "I don't think that 
we are near a circumstance with a strong enough consensus to justify 
imposition of panic. We are not anywhere near, I think, the political 
consensus that would allow us to pull real cost out of people to solve 
this problem. . . I think we have to prepare ourselves for decades of 
work, and I think we have to pour more resources into that. The third 
thing we have to do is to buy options--primarily in the area of 
technology and institutional development. We're not going to severely 
restrict by regulation or pricing the use of carbon dioxide."

Dr. Nazli Choucri, Professor of Political Science and Associate Director 
of the Technology and Development Program at MIT, emphasized the 
importance of deciding first what criteria we should use to decide what 
to do. Her advice was to "increase options," particularly by stabilizing 
global population, promoting institutional resiliency and international 
cooperation, and establishing appropriate priorities for research.