from Environmental Review, A Monthly Newsletter of Environmental Science and Politics, June 1998.
Introduction:
In a policy article for Science magazine(1), Jerry Mahlman wrote "... the state of the science of greenhouse warming is often warped in differing ways by people ... with widely varying sociopolitical agendas and biases." and "...such distortions grossly exaggerate the public's sense of controversy about the value of the scientific knowledge base...".
He describes the scientific consensus about greenhouse warming and climate change as a series of facts, virtually certain projections, very probable projections, and probable projections. Virtually certain facts are that greenhouse gases in our atmosphere are increasing because of human activities and these gases act directly to heat the planet. Virtually certain projections, those that have a greater than 99 out of 100 chance of being true include stratospheric cooling and more water vapor in the air closer to the planet's surface. Very probable predictions have a greater than 9 out of 10 chance of being true and include a 1.5 to 4.5 degree Celsius global warming by the year 2100; substantial sea level rise; and an increase in global mean precipitation.
We spoke with Dr. Mahlman about his work as an "honest broker"
in the policy debates about global warming and climate change.
ER: Dr. Mahlman, what is your scientific background?
JM: My Ph.D. was from Colorado State University in atmospheric
science, and I started my career on the faculty of the Naval Postgraduate
School in Monterey, California. In 1970, I came to my current affiliation
with the Geophysical Fluid Dynamics Laboratory -- GFDL -- National Oceanic
and Atmospheric Administration. I was a research scientist working on a
number of climate, ozone, and atmospheric chemistry problems. My approach
was to use and develop mathematical models of the atmosphere to address
some of the key scientific questions. In 1984, I became the director of
GFDL. I have been broadly involved with the science, and its implication,
of the ozone depletion issue. For multiple reasons, I got entrained into
the climate change problem over the last ten years. I occupy the niche
of being a person in the middle of the fight between the right wing and
the left wing, trying to describe the science of the problem, independent
of policy-driven agendas.
ER: You wrote that people with different political agendas are
exaggerating the controversy about climate change.
JM: The left wing is saying that the science is in on climate change; we know it well enough and we have to mitigate now. The right wing is saying the science is a bunch of garbage or the effect is too small to be of concern; so don't do anything. These differing positions have produced an amplified sense of uncertainty about the science. My own approach is to try to be as honest as possible about what we know and what we don't know. I am concerned that these values-driven debates still being featured in the media are undermining the public's sense of what the science is telling us. By exaggerating the sense of scientific controversy, at least in the U.S., it effectively gives an artificial sense of differences about the reliability of the science. This may help to explain why Europe is willing to make large commitments to mitigation of carbon dioxide emissions, while here in the U.S., there appears to be no way that Congress is going to ratify the Kyoto agreement in its current form. These very different policy attitudes seem to be based upon acceptance of a pretty confident science in Europe, but perceptions of a shaky science in the U.S., at least in the minds of many people who are making policy decisions. However, the reality remains that science is just the science, and we scientists in the U.S. and Europe pretty much agree on what we know and what we don't know. The actual scientific consensus is international, not regional.
Many arguments that you hear about climate change start with a political
position, and with the "science" being lined up to buttress that
position. When I say my opinions are policy independent, I am saying that
when I talk about what we know and what we don't know, I know of no policy
motive behind my statements. They are simply my best estimates; I don't
know if they are high or low. The left wing can say that climate change
could be twice as bad as I am saying, while the right wing can say that
I am exaggerating the most likely climate change. I am left wondering how
people seem to know that the consensus among climate scientists is so systematically
far off on either the high side or the low side.
ER: What do you mean that the climate model uncertainty is distinct
from the social uncertainty?
JM: Assume for the moment that we can agree to work with a specific
scenario of the added greenhouse gases and sulfate aerosols over the next
century. Suppose that we scientists could provide a perfect prediction
of how these added substances would actually change the climate. How would
the planet's social systems respond to this information? With considerable
uncertainty and controversy, I would predict. This is because the amount
of greenhouse gases that we actually emit into the atmosphere is a social
choice with winners and losers, not a scientific problem. For example,
suppose that we begin to learn that our best estimates are low on the amount
of climate change we would be for a particular amount of added carbon dioxide
-- using carbon dioxide in its simplest sense as a paradigm for all the
other climate-altering substances we put in the atmosphere. Confronted
with the new information that the climate models have been on the low side,
we would surely re-ignite spirited debates on the painful social choices
that would achieve a sharp reduction in the use of fossil fuels. Suppose,
however, that our best calculations of global warming have been found to
be too high by a factor of two. Then the social pressure to make such politically
difficult decisions would be reduced substantially. Thus, social uncertainty
would not disappear even if scientific uncertainty were to become very
small, a prospect I don't see happening anytime soon. We will be arguing
on the specifics of what to do about this problem for a long time.
ER: What is the role of the Intergovernmental Panel on Climate
Change?
JM: Many people have pet arguments about climate change, and
many times those arguments are unconstrained by quantitative thinking.
IPCC, because it is grounded in refereed scientific publications, has attempted
to find the broad-based scientific consensus on what we know and what we
do not know about climate change. In my view, if people have contrarian
ideas about the science of climate change, then they should explain why
they disagree with the science-based IPCC assessments. Both right-of-center
people and left-of-center people have made arguments that are scientifically
bewildering to me. If they disagree with IPCC, they should explain why.
ER: The news media seems to sense that climate change is an important
story, but they don't do a good job of explaining it.
JM: The tendency of the press has been to ferret out controversy more than to educate. Until recently, the press has been running to virtually anybody who has something extreme to say, presumably because controversy sells newspapers and increases ratings. I don't think the press has a liberal bias or a conservative bias. I think they have a sexy news bias.
Incidentally, this controversy has produced a peculiar inversion: the
usual way science works has been turned upside down. In science the burden
of proof usually is on the person who thinks that the mainstream argument
is not correct; if someone has an alternative hypothesis, it should be
at least testable. But in the IPCC process, the right wing has been so
vociferous that the scientific establishment has felt obliged to respond
to every statement or claim that is coming from that end of the spectrum.
In the previous IPCC reports, essentially the entire list of the right
wing's arguments have been carefully analyzed and rebutted, even though
man of those arguments have never appeared in the refereed scientific literature.
I thus say congratulations to the counter attackers of the right wing because
they have successfully managed to flip the paradigm of science backwards.
It now seems to be the burden of the scientific mainstream to evaluate
all assertions, whether or not they show any evidence of scientific merit.
ER: In your Science article you list the virtually certain facts we know about climate change. The first is that atmospheric gases are increasing because of human activities.
JM: The ever-increasing amounts of carbon dioxide in the atmosphere
are essentially coming from fossil sources of carbon. We extract coal,
gas, and oil, and we burn it. Of the other greenhouse gases, methane, nitrous
oxide, and some chlorofluorocarbons are increasing. There is not any doubt
about the chlorofluorocarbon increase being from human activities because
it is a human-made molecule, and there are no natural sources. Methane
continues to increase, and we are pretty sure it has a strong anthropogenic
component. The source of the methane is burn off from natural gas, agricultural
practices, rice paddies, and enteric fermentation in large animals. The
increasing nitrous oxide is thought to be mainly due to use of nitrogen-based
fertilizers.
ER: And we are virtually certain that these trace gases in the
atmosphere trap heat?
JM: Yes. We can measure this trapping in the lab. We can measure
it from space. One of the peculiarities of our planet is that nitrogen
and oxygen, that compose 99 percent of the atmosphere, are diatomic molecules
and they are essentially transparent to the infrared spectrum of light.
It is these other trace gases that determine the trapping of heat on the
planet. The planet is about thirty-three degrees Celsius warmer than it
would have been had there not been water vapor, carbon dioxide, nitrous
oxide, and methane in the atmosphere.
ER: That is why Venus is hot.
JM: Yes. Venus is a very hot place even though most of the incoming
sunlight is immediately reflected away. Venus is a massive carbon dioxide
trap of heat. The simple truth is that, if you add heat-absorbing greenhouse
gases to Earth's atmosphere -- which is nearly transparent in the solar
radiation wavelengths -- sunlight gets a nearly free ticket in, is absorbed
at the Earth's surface, and is largely trapped when it tries to return
to space in the form of infrared radiation -- heat. It is not controversial
that added greenhouse gases are producing a heating of the planet of about
two watts per meter squared, about halfway to what we would expect if we
double the atmospheric carbon dioxide by 2050. So the added greenhouse
gases are acting to heat up the planet by putting an additional thin blanket
around it. Venus is a massive exaggeration of what is happening here on
Earth.
ER: How would you describe greenhouse forcing for a non-scientist?
JM: Suppose you are sleeping in your bed on a cold night and you find yourself slightly too warm, but comfortable. Now you add a thin blanket; if you are at that warm edge, you can quickly feel the uncomfortable difference the added blanket makes. The balance between the heat your body is producing under the blankets and the loss to the cooler room has been changed by reducing the heat loss; you got too warm by reducing your loss of heat.
Another way of thinking about it is by appealing to your everyday observations
of the weather and the temperature. Suppose you go outside and look up
at the sky at ten o'clock on a cloudy June night. Is the temperature going
to drop sharply overnight? Probably not. You know already that cloudy nights
tend to prevent strong overnight cooling. Now suppose it is clear, dry,
and the starts appear distinct. You know that the morning temperature will
be considerably colder after such a clear night. The reason for the difference
is that the clouds provide an efficient trapping of heat. The Earth tries
to radiate this heat out to space, but the clouds absorb it and radiate
it back like a blanket. People who live where there is a lot of variability
in the humidity will notice that, on clear nights that are very humid,
it doesn't cool off much; on a clear and dry night, the temperature drops
markedly. All these are greenhouse heat trapping effects, or lack thereof.
ER: Why are we so focused on carbon dioxide?
JM: Carbon dioxide is the dominant greenhouse gas that humans
are venting to the atmosphere. We are currently releasing roughly 6 billion
tons of carbon dioxide a year from the burning of fossil fuels, and then
another 1.5 billion tons or so from deforestation. The good news is that
this is producing a rather modest atmospheric carbon dioxide increase of
about one-half percent per year. The bad news is that it is projected to
go up one-half percent every year, inexorably, for a long time. Right now
there is major economic and social pressure to increase the burning of
fossil fuels because they are cheap, and because the developing countries
want a bigger piece of the economic action. The problem is that the molecules
of carbon dioxide that you added to the atmosphere by driving to work this
morning have an apparent lifetime in the atmosphere of about seventy-five
years. Now, imagine if we get to high carbon dioxide levels and we get
a climate warming that we don't like. A renewed action to reduce fossil
fuel burning sharply wouldn't produce an atmospheric carbon dioxide drop
to pre-industrial levels any time soon. In fact, it could be well longer
than a one-century wait.
ER: We are virtually certain that particles added to the atmosphere
actually cause some cooling effect. This is a relatively new understanding
about the role of sulfates isn't it?
JM: Yes. The sulfate particles, the same pollutants that bring
us acid rain, have the ability to reflect solar radiation. If you add sulfate
particles to the atmosphere, they reflect sunlight and produce a cooling
effect.
ER: So the sunlight doesn't get to the Earth's surface in the
first place?
JM: That's right. The sulfate particles increase the effective
cloudiness of the planet, acting to reject some of the sunlight that is
coming in.
ER: The right wing leapt upon the sulfate cooling effect, and
said that IPCC didn't get its facts right, and maybe even covered up this
cooling offset effect.
JM: That was a truly bogus assertion. This is not a question
of scientific infallibility; it is a question of scientific correctability.
If there had been a coverup where IPCC had found that the aerosol effect
is more important than we previously thought, but decided to not tell anybody,
then anyone would have a good reason to start yelling and screaming. That
is not what happened. Once the scientific community began to see this sulfate
aerosol effect is big enough to make a difference, we added to our calculations.
ER: We are virtually certain that carbon dioxide increases and
ozone decreases have already produced a one Celsius degree cooling in the
stratosphere.
JM: Yes. The stratosphere is a very different place than the
lower troposphere, the air nearer to the ground. The stratosphere is a
simpler place; it doesn't have clouds and ocean effects to complicate matters.
The measured cooling in the stratosphere is consistent because the global
warming theory does predict cooling there. In fact, we are predicting the
stratosphere cooling effect will be considerably larger in magnitude than
the warming of the troposphere, markedly so at higher elevations.
ER: We are virtually certain that the surface temperature has
increased about one-half degree Celsius over the past century?
JM: That's right. We are virtually certain of that one-half degree,
plus or minus two-tenths of a degree Celsius; the plus or minus two-tenths
gives us a generous margin to cover for all of the possible problems with
the measurements. This increase provides an example of how both the left
wing and the right wing can misuse variability in the same way that uncertainty
can be misused. The right wingers have said that this one-half degree increase
in average global temperature could all be explained by natural variability
and thus there may be no greenhouse warming effect at all. However, a person
at the other end of the spectrum might easily have said that human-caused
warming could have been one degree, but we may have had one-half degree
of natural cooling that offset it. Therefore, the human-caused global warming
part could actually be twice that one-half degree! Neither statement, however,
would have any demonstrated factual basis. Both ends of the political spectrum
can thus work natural variability to their political advantage. Unfortunately,
the natural variability effect is like uncertainty. It just is; it doesn't
have a preferred algebraic sign.
ER: You wrote that reduction of key uncertainties about climate
change will require a decade or more.
JM: Yes, there are some who say that with strong increases in
research funding, the critical uncertainties will be quickly reduced. I
strongly question that. These are hard problems that are resistant to quick
breakthroughs. Anybody who says we are going to achieve sharp reductions
in scientific uncertainty in the next few years is almost sure to be disappointed.
ER: We are virtually certain water vapor will increase in the
lower troposphere?
JM: Yes, the air holds more water vapor when it is warm than
it does when it is cold; arguments to the contrary are highly dubious.
If this were not found to be the case, it would be a stunning surprise.
I don't expect that surprise to occur.
ER: Why should we be concerned about the amount of water vapor
in the air?
JM: We should be concerned because the calculations of the temperature
changes due to global warming calculate a positive feedback for water vapor.
As you warm up the planet, you evaporate more water and the air can hold
more water before it rains out. Another way of saying it, if the air now
hold more than 6 percent water because it is warmed, then for the same
storm circulation you get proportionally more rain. Also, if the atmosphere
is wetter with more water molecules in the air, the added greenhouse blanketing
effect more than doubles the original warming effect directly due to the
added greenhouse gases. Water vapor is thus a potent greenhouse player
in its role as a feedback gas.
ER: Your computer models of climate agree pretty well with historical
records when you run them forward from the middle of the 19th
century. Why is that important?
JM: If we are projecting what the climate will be for a given
greenhouse gas scenario fifty or one hundred years from now, are we perfect
because nobody can judge whether we are right or wrong? Of course not.
The warming over the past century provides an important check of the credibility
of the models. Is this observed warming consistent with theory? It turns
out that the models do rather well when the observed greenhouse gas and
sulfate particle increases are added to the calculation. However, the remaining
uncertainties due to the unknown contributions of natural variability,
and of the magnitude of the sulfate cooling effect, will leave room for
the rather generous error bars we use in projections of future climate
warming.
ER: Could climate warming be due to the sun putting out more
heat?
JM: In part it could. The solar variability argument is rather interesting. The solar physicists have determined that, over the past century, the total solar heating of the Earth could have increased by as much as 0.3 watts per meter squared. But the straightforward infrared effect, that we can measure accurately as being real, is two watts per meter squared. The solar advocates are thus making a fascinating statement. They implicitly argue that the climate system is extremely insensitive to large changes in infrared forcing, i.e., greenhouse effects. At the same time, the climate is assumed to be extraordinarily sensitive to much smaller changes in solar forcing. This argument requires what I call a double miracle because we have no evidence that the system is either highly sensitive to small changes in solar radiative energy, or that it is insensitive to much larger changes in heat trapping. There is nothing in the refereed literature about either of these mechanisms being present in our climate system, let alone both of them operating simultaneously.
I am still estimating a greater than 90 percent probability that the
observed warming over the past century is largely due to human influences,
mainly because of the conspicuous lack of credible alternative hypothesis,
including this solar one.
ER: The models predict global increase in average temperature
that are linear with the logarithm of carbon dioxide concentration. That
sounds like good news.
JM: It is good news. It says the climate system can be resistant
to large carbon dioxide changes. It takes twice as much carbon dioxide
added between a doubling and quadrupling of carbon dioxide to produce the
same added warming as between pre-industrial and doubled carbon dioxide.
The climate change is thus somewhat buffered as the carbon dioxide levels
become large.
ER: By the year 2100 the global mean surface temperature would
be 1.5 to 5 degrees Celsius higher?
JM: This rather broad range includes a still-generous scientific
uncertainty, as well as an uncertain carbon dioxide future that will likely
range somewhere between doubled and tripled carbon dioxide levels by the
year 2100. The current social trajectory, without major mitigation of carbon
dioxide emissions, is for the atmosphere to reach tripled carbon dioxide
levels by the year 2100. However, with focus, it is feasible that we could
hold to doubled carbon dioxide levels by 2100.
ER: People think that melting glaciers are causing sea level
rise, but that isn't why it is happening.
JM: Melting glaciers is a potential mechanism that is lurking,
but the sea level rise we have seen over the last century is explained
mainly by the thermal expansion of sea water. As water gets warmer, it
gets less dense and therefore occupies more volume. Our calculations show
that for doubled carbon dioxide over 500 years, we get about a meter of
sea level rise due just to this thermal expansion of sea water effect.
Measurements do show that many lower latitude mountain glaciers have melted
noticeably over the past century. If Greenland's ice were to begin to melt,
the sea level rise could proceed much faster than that due to the thermal
expansion effect. Our confidence in projecting Greenland ice melting rates
remains quite low.
ER: We are virtually certain that we will see an increase in
average global precipitation of roughly 2 percent for each degree of warming?
JM: Yes. This is a direct consequence of the strong expectation
that the atmospheric water vapor content will increase in accordance with
the known laws of thermodynamics. The same weather system now would have
more moisture available to remove in a storm system. Notice that this is
for the global average precipitation. How this would play out in a particular
region is much more uncertain.
ER: What is the connection between big tropical storms and climate
change?
JM: The tropical storms question is tricky. Such storms don't happen very often and there are many places where they don't occur at all. Tropical storm formation requires rather rare circulation events. We have no good evidence on how those circumstances will change in a warming climate. We do know empirically and theoretically that hurricanes are energized by warm water. It is thus reasonable to think that if the oceans warm up, then hurricanes will become more intense. But most hurricanes go through a period of struggle that determines how intense they are going to be. Most of them die out before they reach their full potential: they run into cold water, they run into mountains, or they encounter unfavorable weather situations. Those tropical systems that do hang around long enough tend to crank up to near their full potential intensity. Our calculations suggest that in a greenhouse warmed Earth, the hurricanes that last long enough will likely become more intense. We still are unsure whether more or less of them will form in this warm climate.
1. 1 J.D. Mahlman. Uncertainties in Projections of Human-Caused Climate Warming, 1997 Science 278: 1416-1417.