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 ClimateEngineering

• 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