Transcript Comments on Global Cooling Schemes

Climate System Energy Balance
Kiehl and Trenberth
Review - CO2 in atmosphere
works just like water vapor
• Lets in warming sunlight in the day
• when humidity is high heat is trapped and nights stay
warm
• when humidity is low heat escapes and nights are
cold
• But unlike water vapor, CO2 hangs around for ages
– CO2 added to the atmosphere takes a century or
two to dissipate
CO2 from fossil fuel burning is the main
contributor to warming
World use of fossil fuel
China
18%
Rest
45%
USA
22%
Europe
15%
The warning
The problem
Possible Geoengineering Approaches
1. Extract CO2 from atmosphere
2. Produce compensating global cooling
CO2 x 2 = -3.7 W/m2
(a) Reduce solar flux by 1.8%
(b) Increase planetary albedo by 3.5%
(marine cloud albedo by 13%)
and
How to slow it down
Space Sunshade
• Advantages of space
– Changes only solar flux, not atmospheric
chemistry
– 50 year life, doesn’t need annual renewal
• Proposed by James Early 1989:
– “A thin glass shield built from lunar materials
and located near the first Lagrange point of the
Earth-sun system could offset the greenhouse
effects caused by the CO2 buildup in the
Earth’s atmosphere”
L1 orbit is a million miles from
Earth, 4 times further than moon
Lightest possible screen material
•
•
•
•
glass 1 mm thick (1/25,000 inch)
Transmits sunlight but deflects it away from Earth
Million square miles needed weighing 6 million tons
With structural support and controls, total 20 million tons
What does the sunshade look like?
• cloud of many small independent
spacecraft
– Each one has small solar sails to set its
orientation to face the sun and to stay within
the cloud, in line with sun
flyer cloud seen
from the side
sunlight entering
flyers
Exiting sunlight
deflected
sunlight
deflected to
miss Earth
starlight passing
through flyers is
also deflected,
into donut rings
L1 Sunshade conclusions
• Lifetime
– Estimate 50 years
• Cost estimate
– $4 trillion
– $100 billion/year, 0.2% of world GDP over lifetime
• Launch environmental impact ok
– 1 ton of fuel launches sunshade area enough to
mitigate 1000 tons of atmospheric carbon
• Possible show-stoppers need further work:
– Sunshade debris
– Aerodynamic drag after magnetic launch
Geo-Engineering Climate Change
with Sulfate Aerosols
Philip Rasch
Paul Crutzen, Danielle B. Coleman
with additional thanks for advice to
C. Amman, J. Kazil, D. Keith, M. Mills, O.B. Toon, T.M. Wigley
Fundamental Thesis
• Injection of SO2 at 25km in tropics will form
sulfate aerosol. This will act to cool the planet
• Estimates based on Crutzen (2006) suggest
1-2Tg S/year (as sulfate) would suffice
– 2-4% of current anthropogenic surface emissions
– Cost ~$25Billion/yr ($25/capita/yr in the affluent
world)
 1-3 W/m2 reduction reduction in incoming solar
radiation
NOTES ON SCHEME
•
What would be the impact of injecting precursors of sulfate aerosols
into the middle atmosphere, where they would act to increase the
planetary albedo, and thus counter some of the effects of greenhouse
gas forcing?
•
This geo-engineering approach may be a natural analogue
to a volcanic eruption
•
Follow up to a study by Crutzen (Climatic Change, 2006)
– Back of the envelope calculation
– A more detailed and comprehensive treatment of an old idea than
performed previously
•
This study uses a relatively sophisticated General Circulation Model
for a somewhat more quantitative, and comprehensive look at the
problem, but it is still far too simple to be a believable characterization
sulphur aerosol in
stratosphere?
Caused warming dip
Important processes for stratospheric aerosols
(from SPARC Assessment of Stratospheric Aerosols, 2006
What size are the aerosols?
• May look like tropospheric or background
stratospheric sulfate
– reff < 0.15 um (e.g. Bauman et al 2003)
– primarily scattering in Solar part of spectrum
• May look like Volcanic aerosol!
– reff ~ 0.45 um
– absorption in
• Near IR of Solar energy spectrum
• Terrestrial longwave spectrum
Experimental Setup
• The General Circulation model used is a version of the
Community Atmosphere Model (CAM), a component of
the more comprehensive Community Climate System
Model (CCSM).
• This version includes a relatively comprehensive
“physical” characterization of the atmosphere and land.
– No Biogeochemistry (particularly as it contributes to
Carbon and Nitrogen Cycles, Ocean Ecosystems)
– Prescribed Ocean and Sea Ice Dynamics (but does
include thermodynamics) --- So called “slab ocean
model” + “thermodynamic sea ice model”
– No Aerosol/Cloud Microphysical formulations relevant
to the “indirect aerosol forcing effect”
Simulations performed
Fixed aerosol and greenhouse forcing at present day values
Doubled CO2 (2XCO2)
– Injection of SO2 at 25km, 10N - 10S
• Pinatubo thought to inject 10-30 Tg S
(over a week or so)
• 1 Tg S/yr assuming a small (or background)
aerosol size distribution forms
• 2 Tg S/yr small particles
• 2 Tg S/yr as large (or volcanic) aerosol forms
– Doubled CO2 + the above permutations of emission amount
and aerosol size
Summary
•
•
•
•
•
Sulfate in a model world acts to cool planet, return some features to “present day”
– Surface temperature over much of the globe
– Cross tropopause transport
A number of features are “different” from either present day, or 2xCO2 world
– Precipitation
– Polar winter surface temperatures
Demonstrable interactions exist between the greenhouse forcing and geoengineering forcing, e.g., burden of geosulfate in presence of CO2 forcing.
Feedbacks are important.
Numerous obvious remaining topics for exploration in this run
– Influence on hydrologic cycle
– Seasonal and higher frequency transient aspect of simulation
– Sea Ice
Numerous obvious issues to explore with model augmentation
– Aerosol formulation improvements
• Microphysics, size and number resolution
• Independence of geo-sulfate from other aerosol components
• Interaction with clouds (cirrus?)
– Chemistry (particularly ozone depletion)
– Dynamical Ocean and Sea-ice Models
– Biogeochemistry (Land Ecosytems, and Ocean pH!)
Amelioration of Global Warming by Controlled Enhancement
of Albedo and Longevity of Low-Level Maritime Clouds
John Latham (MMM / ASP, NCAR, USA) & Stephen Salter (U of Edinburgh, UK)
Collaborators:
Tom Choularton, Keith Bower – University of Manchester, UK
Mike Smith, Alan Gadian – University of Leeds, UK
Idea: Advertently to enhance the Droplet Concentration in Low-level
Maritime Stratocumulus Clouds, thereby increasing Cloud Albedo & Longevity
- i.e. a cooling effect.
(Latham, 1990, 2000, Bower et al., 2006).
M1
Bubble of about 0.5mm diameter about to burst at water surface
Droplets ejected during catastrophic bursting of bubble film
Operational Requirements for Possible Global system
Calculations/computations indicate that to produce a global
cooling sufficient to balance global warming produced by CO2
doubling (- 3.7 W/m2) seawater CCN in the form of droplets of
diameter 1μm would need to be disseminated at a rate of about
10 m3/s.
For fixed CCN dissemination rate (or droplet concentration
increase to value N) albedo-enhancement ∆A increases as the
original droplet number concentration No decreases (i.e. the
effect is stronger in clouds formed in purer air).
∆A = 0.075 ln(N/No)
Meteorological/Physical Points for Further Study:1.
Losses en route to cloud base.
(Earth’s
electric field (qE>>mg) advertent charging of droplets)
2.
Need to dominate condensation process
(If N too low, albedo is reduced – warming!)
3.
Role of ultra-giant nuclei (UGN) (reducing cloud longevity)
4.
Explicit treatment of cloud longevity (cover) vvs N
5.
Advertent heat & water-vapour fluxes
6.
Influence of CCN on higher clouds
7.
Lateral dispersal of ascending CCN
8.
Changes in earth’s temperature distribution & concomitant effects
9.
Possible reduction of rainfall in sensitive areas.
Priority Technological Requirements
Possible Operational Global system
Further development (Stephen Salter) of wind-powered, satellite
controlled unmanned specially constructed vessels.
Further development (Stephen Salter) of a seawater droplet
production / dissemination system housed on these vessels and
powered by batteries deriving their energy from wind-power.
Limited-Area Field-Experiment to Assess Cooling Scheme
Seawater droplet production & dissemination system
Technique for determining advertent albedo-changes.
Disadvantage of Scheme. Needs continuous operation.
Advantages of Scheme.
•Low ecological impact: only ingredients seawater & air.
Energy derived from wind. Relatively inexpensive.
•Easy termination: System can be shut down immediately,
conditions returning to normal within a few days
•Precise & rapid control: Via satellite measurements of
albedo & cloudiness fed back through global model.
Concomitant adjustment of dissemination rates
Comments on Global Cooling Schemes
1. Solar flux diminution & albedo modification affect
climate, but not atmospheric CO2 concentrations. Such
schemes could buy time until CO2 reduced.
2. Elevated CO2 causes ocean surface acidification. So we
would still need to reduce CO2 emissions.
3. By using more than one technique, there may be some
scope for restraining CO2 levels and climate change
independently.