Climate System Energy Balance Kiehl and Trenberth

Review - CO 2 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, CO 2 hangs around for ages –CO 2 added to the atmosphere takes a century or two to dissipate

CO 2 from fossil fuel burning is the main contributor to warming

World use of fossil fuel

The warning

The problem

Possible Geoengineering Approaches 1.Extract CO 2 from atmosphere 2.Produce compensating global cooling CO 2 x 2 = -3.7 W/m 2 (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 CO 2 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  m 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 deflected to miss Earth starlight passing through flyers is also deflected, into donut rings sunlight entering flyers Exiting sunlight deflected

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 –r eff < 0.15 um (e.g. Bauman et al 2003) –primarily scattering in Solar part of spectrum May look like Volcanic aerosol! –r eff ~ 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 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 geo- engineering 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).

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/m 2 ) seawater CCN in the form of droplets of diameter 1μm would need to be disseminated at a rate of about 10 m 3 /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 = 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.

Stephen Salter John MacNeill

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 CO 2 concentrations. Such schemes could buy time until CO 2 reduced. 2.Elevated CO 2 causes ocean surface acidification. So we would still need to reduce CO 2 emissions. 3.By using more than one technique, there may be some scope for restraining CO 2 levels and climate change independently.