Weather and Climate Engineering

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Weather and Climate Engineering

• William R. Cotton
• Professor of Atmospheric Science
• Colorado State University

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• 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.

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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

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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

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• .
• I remember comic books where even Donald Duck got
  into cloud seeding

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Cloud seeding methods

• Deliberate cloud seeding can be divided into two
  broad categories glaciogenic seeding and
  hygroscopic seeding.

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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

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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.

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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.

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• 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.

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• Venturing into climate engineering recognizing
  that potentially large natural variability may
  exist is hazardous indeed.

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Climate Forcing Factors

• Greenhouse gas variabilitywater 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.

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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

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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

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More hypotheses

• Seed marine stratocu with small sea-spray drops
  to increase their albedoa cooling effect(Latham
  1990 2002)

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Artists concept
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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

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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

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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?

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NASA is already doing Climate Engineering!!
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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 consequencesa 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