WMOD00435

• Title: Limits on Global Warming
• Date: 5/9/1991
• Weather Modification - Doc Type: Report

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8. POSSIBLE COUNTERMEASURES

Global warming would have serious effects on
agriculture and many other human activities, and it is
commonly assumed that nearly all of the effects would be
undesirable. Ausubel (1991) has questioned that
assumption, as well as several other pieces of prevailing
wisdom about global climate change. Furthermore, the
rates of change in temperature and precipitation predicted
so far to accompany global warming fall within the range
of natural regional variations experienced over the past few
hundred years. Nevertheless, the idea persists that global
climate change will be harmful, and considerable thought
has Ijeen given to ways to mitigate its effects.

The most obvious countermeasure is a passive one,
namely, to reduce emissions of greenhouse gases, thereby
slowing, but not preventing, global warming. Control of
emissions of CFCs is critical because, as already noted,
they have absorption bands different from those of CO2,
There are now international agreements in place to control
emissions of CFCs. An agreement to restrain emissions of
CO2 will be harder to come by, because of the close link
between fuel consumption and the level of economic
activity. Any reduction large enough to make a significant
difference over the long term will require radical
socioeconomic adjustments on a global scale.
Nevertheless, inter-governmental committees are at work
and hope to have an agreement covering all greenhouse
gases ready for possible ratification at the United Nations
Conference on Environment and Development (UNCED)
scheduled for Rio de Janeiro, Brazil in June, 1992. The
UNCED will consider climate change and a number of
related problems, including ozone depletion and air
pollution (Miller, 1991).

Many persons have suggested reforestation, or at
least a halt to deforestation, as a way to slow global
warming. Kulp (1990) has calculated recently that
reforestation could remove a significant fraction of the
expected net input of CO2 to the atmosphere over the next
50 years. Idso's (1991) paper points to a similar
conclusion; he emphasizes that deforestation not only adds
to the CO2 in the atmosphere but destroys a sink that could
absorb future emissions from other sources. However,
only growing trees take more carbon from the air than they
release. A mature forest is in balance with the atmosphere;
the only way to reduce its CO2 output significantly would
be to log the mature trees and preserve them from decay.
Therefore taking lumber from a forest and using it to build
houses, as opposed to letting it rot on the ground, is one
way to combat global warming. Over the long haul,
though, it does not appear practical to preserve enough
wood to match the fossil fuel reservoir, which contains
perhaps 7 times as much carbon as all the organisms that
are alive at this moment (table 1).

. As previously noted, marine organisms deplete CO2
In the mixed layer and carry it down to the deep ocean as
shells or organic carbon. There is evidence that the growth
of marine organisms is limited in some areas by a lack of
trace elements, notably iron (Archer 1990), so one could in
theory deplete atmospheric CO2 by fertilizing selected
regions of the ocean surface. Enthusiasm for this

suggestion should be tempered by the realization that
marine biota contain only a small fraction of the total
carbon stored in living organisms. One would also have to
consider carefully the possible implications for all types of
marine plant and animal life, to say nothing of possible
effects on cloud droplet populations. Plankton give off
dimethylsulfide, which contributes to the concentration of
cloud condensation nuclei. Therefore promoting the
growth of plankton could affect cloud albedos as well as
deplete CO2 levels in seawater. Foley et aI. (1991)
developed a simple model to test the effects of changes in
cloud albedos related to variations in the output of
dimethylsulfide by plankton. They concluded that the
effect on global temperature is small compared to those that
might be produced by changes in areal extent of cloudiness.

Cloud seeding has been suggested as a response to
climate change. One can assume a continuation of weather
modification projects designed to alleviate local effects of
droughts. As it will not be possible to distinguish drought
associated with global warming from droughts associated
with natural climate fluctuations, such weather modification
projects will not be identifiable as specific responses to
climate change.

Cloud seeding to offset global warming could take a
more active form. Increasing the areal coverage of shallow
clouds, decreasing the coverage by deep clouds, or both
would exert a cooling effect. While there is no established
method to change the height distribution of existing clouds
over any appreciable area or to increase the areal extent of
low cloud decks, it might be possible to develop such
techniques. It has also been proposed that changes in
drop-size distributions be induced to make clouds better
reflectors of sunlight. These suggestions are interesting,
but so far I have not come across any detailed feasibility
studies.

Dust in the atmosphere tends to cool the f2fth's
surface by scattering solar radiation. Particulate matter in
industrial pollution may be offsetting the expected
greenhouse effect, at least in part. Artificial clouds of
particulate matter could be generated for the express
purpose of offsetting global warming. In order to avoid
having the artificial clouds washed out by precipitation in a
few weeks, they would have to be placed in the
stratosphere. It has been suggested also that large mirrors
could be placed in orbit around the earth to reflect part of
the incoming sunlight. I have not seen any engineering
studies for these two suggested responses to global
warming, but am skeptical about them. There is no
evidence to date of an impending climate change so
catastrophic as to justify radical technological fixes.

Acknowledgements. Preparation of this paper was
supported by the Global Climate Change Response
Program of the Bureau of Reclamation, U.S. Department
of the Interior. Figure 2 was prepared by Lloyd Timblin
using data from a number of sources.

REFERENCES

Archer, D., 1990: Modeling pC02 in the Upper Ocean: a
Review of Relevant Physical, Chemical, and Biological

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