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Jason E. Schaff, Ph.D*
Dr. Klaus Hasselmann's recent article in Science(1) provides a
clear and accurate summary of the technical issues involved in attempting
to determine if human activity is responsible for the increases
in average global temperature seen over the past century and what,
if any, degree of anthropogenic global warming might be expected
in the future. Dr. Hasselmann also provides some editorial commentary
on the issue, which is to be expected in an article from the "Perspectives"
section of Science. This commentary, while well thought out in some
aspects, shows, in stark contrast to the excellent technical discussion,
a disturbing lack of objective detachment in other areas. This article
will provide a summary of the theory of anthropogenic global warming
and a reprise of the technical portion of Dr. Hasselmann's article.
The editorial component of that work will then be discussed, and
counterpoints to the more questionable suggestions will be provided.
The theory of anthropogenic global warming is based upon the possibility
that emissions of certain gasses resulting from human activities
may serve to enhance the natural greenhouse effect, leading to unnaturally
elevated average global temperatures. These elevated temperatures
could have deleterious effects upon the world's ecosystems and upon
human standards of living.
Consequences of severe global warming might include a spread of
desert regions, disruption of normal crop cycles, rising sea levels,
an increase in ground water salinity, and expanded geographic ranges
for tropical pests and diseases such as malaria. Perversely, a sharp,
sudden rise in average global temperatures might cause a decrease
in local temperatures at high latitudes in the northern hemisphere
by disrupting south to north ocean and air currents such as the
Atlantic Conveyor and the Gulf Stream.(2) There have even been plausible
suggestions that anthropogenic global warming could exacerbate ozone
depletion, particularly at high latitudes.(3)
The greenhouse effect, the perturbation of which is the key component
of all global warming theories, is an established and well understood
natural phenomenon. A large portion of the sunlight falling on the
Earth's surface (the exact amount depends on the Earth's average
reflectivity, or albedo) is reflected directly back into space.
A small fraction of the remainder provides the energy which drives
most of the basic processes in the biosphere, including photosynthesis
and weather. The majority of the energy which is not reflected back
into space, however, is absorbed by the ground and the ocean surface
and then slowly re-emitted as infrared light, or radiant heat. Much
of this heat escapes back into space, but there are compounds in
the atmosphere which absorb certain wavelengths of infrared light,
blocking some of the heat from escaping, and warming the atmosphere
to a higher temperature than would be expected from a purely radiative
model.
This is similar to the mechanism by which the glass roof of a greenhouse
allows sunlight to enter and warm the interior, but prevents the
heat from escaping; hence the name greenhouse effect. In the lower
atmosphere, the two most important greenhouse gasses are water and
carbon dioxide. Water vapor is very abundant (over 3% of air by
volume on a muggy day) and absorbs infrared light over a wide range
of wavelengths, but is a fairly inefficient infrared absorber. Carbon
dioxide, on the other hand, is much less abundant (about one-thirtieth
of one percent of air by volume), and absorbs infrared light over
only a very narrow range of wavelengths, but, within that band,
it is an incredibly efficient infrared absorber.
Theories of anthropogenic global warming note that the massive
fossil fuel use that accompanied the industrial revolution has resulted
in unprecedented emissions of carbon dioxide, as well as significant
emissions of a few more esoteric greenhouse gasses. These theories
postulate that the increased levels of carbon dioxide could enhance
the greenhouse effect to such a degree as to produce a large increase
in average global temperatures. Much of the warming predicted by
most of these theories does not come directly from the increased
levels of atmospheric carbon dioxide. The CO2 already present in
the atmosphere is such an effective greenhouse gas that truly massive
increases would be required to produce the calculated levels of
warming. Rather, most theories predict that increased carbon dioxide
levels would produce slight warming of the oceans, leading to an
increase in the levels of atmospheric water vapor as well as a decrease
in the solubility of carbon dioxide in the oceans. This feedback
effect could potentially produce much greater warming than that
from increasing levels of carbon dioxide alone.
The key question then becomes one of trying to prove or disprove
the theories of anthropogenic global warming, or as Dr. Hasselmann
puts it in the title of his article, "Are we seeing global
warming?" As that article points out, there are four key issues
which need to be addressed in order to answer that question. First,
are average global temperatures increasing? Second, is any increase
outside the range of normal short to medium term (decade to century)
variability in average global temperatures? Third, can we construct
models that accurately describe long term global temperature patterns?
Finally, if we can construct accurate models, do they reliably predict
future changes in temperature and do they indicate that these changes
result wholly or in part from human activity? The last two questions
are important because they would make it possible to distinguish
anomalous temperature increases due to human influences from those
which might be caused by some unsuspected or poorly understood natural
phenomenon.
The answer to the first question is an unequivocal yes. A large
body of meteorological data going back to the middle of the last
century clearly shows that, over the last hundred years, mean global
temperatures have increased by about 1 degree Farenheit. It is important
to understand that these measurements are an average of temperatures
from a wide distribution of points around the globe, and that extreme
local variations in temperature, either high or low, say nothing
of importance about possible global warming.
The answer to the second question is much less clear. Good continuous
meteorological data go back only about a century and a half, making
it difficult to determine exactly what is "normal" variability
in average global temperatures. Meteorological data can be supplemented
by indirect evidence of past temperatures gained from glacial ice
cores and deep ocean sediments, but these measurements are fragmentary
and have a fairly high level of uncertainty.(4) Still, there is
an increasing consensus within the community of geophysicists and
atmospheric scientists that the observed recent increases in global
temperatures are, if not outside the range of normal variability,
at least near the top end of the range of normal behavior. The answer
to the fourth question obviously depends on the answer to the third.
If we can construct accurate predictive models of global temperature
variation, it should be a fairly easy task to separate global warming
caused by human activity from that caused purely by natural phenomena.
The mature state of the fields of statistical data analysis and
signal deconvolution almost guarantee this.
This leaves the reliability of our models of long term atmospheric
processes as the key factor in determining if the detection of anthropogenic
global warming is feasible. The answer, as Dr. Hasselmann clearly
indicates, is that models of atmospheric dynamics are becoming much
more accurate, but they are still not accurate enough to allow us
to detect the onset of any human induced changes in climate. The
two key problems are an inability to devise computationally tractable
small-scale models of atmosphere and ocean dynamics and a lack of
adequate data about certain key climatological processes. These
two deficiencies require many atmospheric processes to be included
in standard models as semi-arbitrary parameters, which are sometimes
little more than well-educated guesses. Even when there is sufficient
data to provide reliable estimates for these parameters, their evolution
in time still cannot be handled well by the models and must be inserted
as yet more decoupled parameters.
The intractability of small scale dynamic models is probably the
more serious problem. Standard models divide the atmosphere and
oceans into a large number of slabs and then simulate the transport
of matter and energy between adjacent slabs based on known thermodynamic
processes. At present, the smallest scale at which even supercomputers
can handle these models in a reasonable amount of time is for slabs
about 100 km on a side and 1 km thick. At this scale, the relevant
portions of the atmosphere contain over one trillion slabs, with
another few hundred billion slabs for the oceans.
Many processes, however, occur only on much smaller scales, and
must be included in standard atmospheric models as parameters which
evolve in some assumed manner with time. These include cloud formation,
snowfall and the seasonal advance and retreat of sea ice, all of
which are critical in predicting the Earth's albedo and total water
vapor content of the atmosphere. Unfortunately, models which could
include these processes directly would require the atmosphere to
be divided into over one hundred quadrillion (100,000,000,000,000,000)
elements. No existing computer could run such a program in anything
less than a century.
Even some processes that could be modeled at currently accessible
scales are not understood well enough to be included as anything
other than decoupled adjustable parameters. These parameters include:
the cycling of water through soil moisture, periodic change in solar
radiance, the coupling of tropical ocean temperature anomalies (El
Nino events) to atmospheric processes, the dynamics of atmospheric
aerosols, the influence of metastable salinity gradients on ocean
circulation patterns, and the structure of deep ocean currents.
Sufficient experimental measurements would almost certainly allow
for a better understanding of each of these processes and for their
inclusion in global atmospheric models, although at the cost of
a modest increase in computational difficulty. In total, the answer
to the question, "Are we seeing global warming?" is, and
Dr. Hasselmann frankly acknowledges this: We don't know yet. Certainly,
it is possible that anthropogenic global warming is happening, but
this cannot yet be clearly demonstrated, and currently observed
warming could be mostly or entirely a natural process. The editorial
content of Dr. Hasselmann's article is confined almost entirely
to the last two paragraphs and consists of two primary points, one
of which is quite reasonable, and one of which is highly questionable.
Dr. Hasselmann argues for the support of focused research into those
areas of atmospheric sciences and climatology which are still not
well understood, in order to allow for the construction of better
models for predicting global temperatures. Given the seriousness
of the potential consequences of anthropogenic climate change, this
is an eminently sensible suggestion. It is important to determine
if human activity is contributing to recent observed increases in
global temperature and what, if any, increases can be expected from
such activity in the future. This research could also yield incidental
benefits in the form of improved accuracy in weather forecasting.
Two additional areas not specifically mentioned in the Science article
where a focused research effort could help in answering questions
about global warming are development of task specific high speed
computational systems capable of handling smaller scale atmospheric
simulations and research into mathematical methods for simplifying
intractable "complex" computational problems.
The other editorial assertion comes in the final paragraph, where
Dr. Hasselmann states:
The. . .uncertainties in the detection of anthropogenic climate
change can be expected to subside only gradually in the next few
years.... It would be unfortunate if the current debate over this
ultimately transitory issue should distract from the far more serious
problem of the long-term evolution of global warming once the signal
has been unequivocally detected above the background noise.
One can only conclude that Dr. Hasselmann's intent is to suggest
that we should assume that improved knowledge will ultimately demonstrate
significant anthropogenic global warming and that we should act
on that assumption. This is, at best, a highly questionable assertion.
For starters, if one were to act on this assumption, it would be
unreasonable to follow the earlier call for further intensive research.
After all, why bother expending considerable time, money, and effort
researching a theory if you are going to act on the assumption that
it is true anyway? It would have been reasonable to suggest that
we must be prepared to act if theories of anthropogenic global warming
are ultimately proven to be correct, but to suggest disruptive and
wide reaching actions of the sort which would be required to substantially
reduce carbon dioxide emissions in the absence of a preponderance
of evidence for these theories is irresponsible.
Ill- considered comments such as those at the end of Dr. Hasselmann's
article also tend to have an unfortunate effect upon the treatment
of other serious environmental issues. When reasonable, but technically
uniformed, people are presented with such calls for action on some
environmental issue in the absence of persuasive evidence for the
actual seriousness of the problem, they often tend to become dismissive
of environmental issues in general, assuming that most environmentalists
are just Chicken Littles. The presence of such attitudes can make
it very difficult to convince people, and the politicians they elect,
to take seriously proven environmental problems such as acid rain
and ozone depletion. This backlash effect is singularly annoying
for people, such as this author, who consider themselves pragmatic
environmentalists, as they are often forced to fight the reflexive
dismissal of environmental problems before they can even get someone
to listen to a discussion of the issues.
While the jury is still out on the global warming issue, there
are some actions which might be taken to prepare for the possibility
that the theories will be proven correct. For the most part, these
are planning and diplomatic steps which would allow the issue to
be dealt with quickly if needed. A few concrete steps that would
have other demonstrable environmental benefits could be encouraged.
Foremost among these would be some sort of program, probably involving
tax incentives, to encourage utilities to switch coal fired power
plants to natural gas. Coal burning is responsible for a large portion
of the emissions of acid rain and smog causing compounds, produces
large volumes of toxic ash, and, incidentally, coal produces far
more carbon dioxide per unit of energy generated than does natural
gas.
On the planning front, there has been considerable discussion among
social scientists and economists about how best to achieve compliance
with desirable environmental goals without the use of disruptive
and draconian regulation.(5)(6) Proposals have included tax breaks
for voluntary environmental measures by businesses, markets in tradable
"pollution credits," graduated energy taxes to encourage
efficiency, and government funding for development of environmentally
"friendly" replacements for some of the worst polluting
technologies in current use. It might be wise for governments to
institute small scale, voluntary pilot programs to test some of
these ideas and see which actually work, so that if it becomes necessary
to sharply curtail greenhouse emissions, we have some idea of how
best to proceed.
In summary, had Dr. Hasselmann stopped his article prior to the
final paragraph, it would have been an excellent and thoughtful
piece of work. As is, it is still a valuable reference for those
interested in the scientific issues involved in trying to measure
global warming, but the ill-conceived editorial commentary at the
end detracts noticeably from the effectiveness of the piece.
*Jason Schaff can be contacted at the following addresses and telephone
number: 225 East North Street, #1304, Indianapolis IN 46204; internet:
jeschaff@champion.iupui.edu;
(317) 972-0885.
- K. Hasselmann; Science, 1997, 276, 914-915.
- W. Broecker; Scientific American, 1995, November,
62-68.
- J. Austin, N. Butchart, K. Shine; Nature,
1992, 360, 221-225.
- H. Pollack, D. Chapman; Scientific American,
1994, June, 44-50.
- H. French, Scientific American, 1994, December,
94-97.
- D. Pearce, N. Adger, D. Maddison, D. Moran,
Scientific American, 1995, June, 52-56.
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