Weather and Climate
Engineering
William R. Cotton
Professor of Atmospheric Science
Colorado State University
• This talk is based on a chapter of the
book: “Clouds in the Perturbed Climate
System: Their Relationship to Energy
Balance, Atmospheric Dynamics, and
Precipitation”, 2009. ed. J. Heintzenberg
and R. J. Charlson.Strugmann Forum
Report, vol. 2. Cambridge, MA: The MIT
Press.
Weather and Climate
Engineering
• The above is the title of the chapter that I
was tasked to write.
• This chapter has been said to be “Wicked”
by the lateTony Slingo and “Wacko” by
Graeme Stephens
• No one has called it “Wonderful”
Weather Engineering
• Deliberate cloud seeding, with the goal of
increasing precipitation by the injection of
specific types of particles into clouds, has been
pursued for over 60 years.
• It all began following following experiments by
Irving Langmuir and Vincent Schaefer
• For many years weather modification was highly
visible in the news media and most funding for
cloud research was linked in some way to
weather modification
.
I remember comic books where even
Donald Duck got into cloud seeding
Cloud seeding methods
• Deliberate cloud seeding can be divided
into two broad categories: glaciogenic
seeding and hygroscopic seeding.
Glaciogenic cloud seeding
• Glaciogenic seeding involves seeding with ice
nuclei or dry ice and has been applied to
supercooled cumulus clouds and orographic
clouds.
• The documentation of increases in precipitation
on the ground due to glaciogenic seeding of
cumuli has been very elusive
• The evidence that seeding orographic clouds
can cause significant increases in snowpack is
quite compelling
Hygroscopic Cloud Seeding
• This is the inverse of pollution aerosols
• Essentially the approach is to seed with giant
soluble aerosol particles, while pollution
aerosols are primarily small aerosols
• The results of hygroscopic seeding experiments
are quite promising but they still do not
constitute a “proof” that hygroscopic seeding can
enhance rainfall on the ground over an extended
area.
Lessons learned
• The scientific community has established a set of criteria
for determining that there is “proof” that seeding has
enhanced precipitation.
• For firm “proof” [see NRC, 2003; Garstang et al., 2005]
that seeding affects precipitation, both strong physical
evidence of appropriate modifications to cloud structures
and highly significant statistical evidence is required.
• Likewise, for firm “proof” that climate engineering is
affecting climate, or even that that CO2 is modifying
climate, both strong physical evidence of appropriate
modifications to climate and significant statistical
evidence is required.
• Another lesson from evaluating cloud seeding
experiments is that “natural variability” of clouds and
precipitation can be quite large and thus can inhibit
conclusive evaluation of even the best designed
statistical experiments.
• The same can be said for evaluating the effects of
climate engineering or that human-produced CO2 is
altering climate. If the signal is not strong, then to
evaluate if human activity has produced some observed
effect (cause and effect), one requires much longer time
records than is available for most if not all data sets.
• We do not have an adequate measure of the “natural
variability” of climate.
• Venturing into climate engineering
recognizing that potentially large “natural
variability” may exist is hazardous indeed.
Climate Forcing Factors
• Greenhouse gas variability—water vapor, CO2,
Methane.
• Changes in solar luminosity and orbital
parameters,
• Changes in surface properties
• Natural and human-induced changes in aerosols
and dust--volcanoes, desert dust, pollutants
• Differential temporal responses to external
forcing by the atmosphereand oceans.
Natural Variations do not Explain
Observed Climatic Change
• Climate models with
natural forcing
(including volcanic
and solar) do not
reproduce warming
• When increase in
greenhouse gases is
included, models do
reproduce warming
• Addition of increase in
aerosols (cooling)
improves agreement
Some Climate Engineering
Hypotheses
• Crutzen among others propose to burn S2 or
H2S carried into the strosphere by balloons,
artillery guns, or rockets to produce SO. Like
volcanoes sulfuric acid drops would then
enhance reflectance of solar radiation
• Crutzen among others proposed black carbon
seeding in the stratosphere which would absorb
solar radiation thus depleting radiation reaching
the surface but warm the stratosphere
More hypotheses
• Seed marine stratocu with small sea-spray
drops to increase their albedo—a cooling
effect(Latham [1990; 2002])
Artists concept
Black Carbon Seeding of Cirrus
• Another idea is to seed cirrus with black carbon aerosol.
The absorbed solar radiation would have a semi-direct
effect of dissipating cirrus.
• This would have to be done selectively for optically thin
cirrus which absorb upwelling LW radiation(heating) but
reflect small amounts of solar radiation(cooling). In
addition, the absorbed solar radiation would cool the
lower atmosphere.
• It would not work for optically thick cirrus and anvils as
they have high albedo, thus dissipating them would have
a warming affect as well as requiring huge amounts of
aerosol to have any effect
Other proposals
• Deploy something like 55,000 mirrors with
a surface area of 100km2 into Earth orbit.
• Introduce a solar shield at the Sun-Earth
Lagrange point (1.5X 106 km from Earth).
• Costs of either are very high and if
undesirable responses occur it would be
hard to remove and reverse the cooling
Robock, (2008): Unexpected
undesirable consequences of
climate engineering
• The list is too long(20) to enumerate here
but it includes things like:
• Impacts on the hydrological budget
• Unexpected consequences such as
stratospheric ozone depletion
• Increased demands for fossil fuel use in
response to cooling
• If it goes awry can it be turned off?
NASA is already doing Climate Engineering!!
So why Research Climate
Engineering?
• If for no other reason, we know from cloud seeding that if
there is a drought or major weather disaster, politicians
call for cloud seeding to do “something” without due
regard for the consequences—”a political placebo”
• I expect if we find ourselves in a real climate disaster,
human caused or not, politicians will likewise call for
implementation of climate engineering strategies
• It is important that it be done with the most advanced
scientific knowledge and with full understanding of the
consequences of our actions