Facts about sulfate aerosols

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Pacala and Socolow proposal stabilize emissions
at current levels for 50 y (atmospheric
concentrations will still reach 500 ppm)
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Reality emissions have increased 2/y since
their publication
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What if emissions dont decline?

• geoengineering
• Fe fertilization of oceans
• Injecting metallic (reflecting) particles into
  atmosphere
• Launching large mirrors into orbit
• Floating continental-sized reflector in ocean
• Injecting soot or sulfate aerosols into the
  atmosphere

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Facts about sulfate aerosols

• SO2 is produced by fossil fuel combustion
• Annual SO2 emissions are 55 Tg S/y
  
• 50 of SO2 forms SAs in atmosphere
• SAs have
• a short MRT in the troposphere (1 week)
• a longer MRT in the stratosphere (1-2 y)
• SAs cool the atmosphere through
• direct effects (scattering of solar radiation)
• indirect effects (cloud condensation nucleii)

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Present-day radiative forcing due to greenhouse
gases and sulfate aerosols (W/m2)
Greenhouse gases warm
Sulfate aerosols cool
IPCC 2001
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Annual mean forcings for different atmospheric
agents from 1750 - 2000
IPCC 2001
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Environmental Consequences of Sulfate Aerosols

• Environmental Benefits of SAs
• SAs counteract some of the warming caused by
  greenhouse gases
• Environmental Detriments of SAs
• SAs are a human health hazard contributing to
  500,000 premature deaths/y
• SO2 contributes to acid rain

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TASK Design a system to cool the planet using
sulfate aerosols
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Crutzen Proposal(P Crutzen. 2006. Climatic
Change 77211-219

• To counteract warming due to pollution control
  measures
• inject 1-2 Tg S/y into the stratosphere
• as H2S or S2 by balloon or artillery gun
• 0.75 - 1.4 W/m2 average global cooling
• US 25-50 billion
• To counteract a CO2 doubling
• Inject 5.3 Tg S/y into the stratosphere

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TASK Discuss the following

• Should we do this?
• Why or why not?

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Reaction to Crutzen(arguments against proposal)

• Bengtsson
• Climate is unpredictable
• Climate feedbacks are not well-understood
• Effects of aerosols are not well-understood
  (e.g., clouds)
• Time scale of elevated CO2
• Fossil fuel CO2 will stay in the atmosphere for a
  long time so S would have to be injected over
  hundreds of years
• Oceans will take up 70-80 of emitted CO2 but
  only after several hundred years
• Other problems with elevated CO2
• Ocean acidification dissolves calcium carbonate
  harmful to marine biota
• Kiehl
• Complexity of climate system
• Moral obligation to reduce energy consumption

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