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4. Models of the climate system

4. Models of the climate system. Earth’s Climate System. Sun. Atmosphere. Climate model components. Land. Ocean. Ice. Sub-surface Earth. Atmosphere. Composition (in particular the greenhouse gases and aerosols) determine radiative and thermodynamic properties

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4. Models of the climate system

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  1. 4. Models of the climate system

  2. Earth’s ClimateSystem Sun Atmosphere Climatemodelcomponents Land Ocean Ice Sub-surface Earth

  3. Atmosphere • Composition (in particular the greenhouse gases and aerosols) determine radiative and thermodynamic properties • Passage and absorption of radiation (energy from Sun and back into space) crucial • Conservation of mass, momentum, energy, and equation of state: general circulation • Hydrological cycle (evaporation, clouds, rainfall, runoff…)

  4. Ocean • Surface circulation driven by wind: large scale ocean basin gyres • Deep overturning circulation driven by ‘thermohaline circulation’ – competing influences of heat and salt on sea-water density • Oceans transport ~50% of heat from equator to pole (atmosphere other 50%) • Most of the climate system’s stored heat is in the ocean – it acts as a big flywheel – making the climate system respond over much longer time periods (decades) than if the Earth’s surface were all land • Ocean and atmosphere closely coupled

  5. Cryosphere - Ice • Mass balance of ice sheets: snow adds material, evaporation/sublimation, ablation, and melting removes • Rising temperatures don’t necessarily imply shrinking ice sheets, as it may mean increased precipitation (snowfall) • Ice dynamics (e.g. glacier flow rates) important • Ice albedo – clean snow vs. dirty snow • Close interaction of ice with atmosphere and oceans

  6. Biosphere • Models have simplified representations of land and ocean carbon cycles • Only ½ the emitted CO2 ends up in the atmosphere – the other ½ is taken up by the oceans and vegetation • CO2 uptake by ocean causes acidification: CO2(g) + H2O(aq)↔ H2CO3(aq) ↔ H+ + HCO3- ↔ 2H+ + CO32- With potential impacts on marine biota – harder to form a calcium carbonate skeleton – high uncertainties

  7. Coupled atmosphere / ocean climate model Radiation Atmosphere: Density Motion Water Heat Exchange of: Momentum Water Ocean: Density (inc. Salinity) Motion Sea Ice Land

  8. Typical resolution of weather forecast model – ‘nested’ high resolution region

  9. 30km 19 levels in atmosphere 2.5 lat 3.75 long The HadleyCentrethirdcoupledmodel -HadCM3 1.25 1.25 20 levelsin ocean Hadley Centre -5km

  10. The Development of Climate models, Past, Present and Future Mid 1980s Early 1990s Late 1990s Present day Early 2000s Mid 1970s Atmosphere Atmosphere Atmosphere Atmosphere Atmosphere Atmosphere Land surface Land surface Land surface Land surface Land surface Ocean & sea-ice Ocean & sea-ice Ocean & sea-ice Ocean & sea-ice Sulphate aerosol Sulphate aerosol Sulphate aerosol Non-sulphate aerosol Non-sulphate aerosol Carbon cycle Carbon cycle Atmospheric chemistry Sulphur cycle model Non-sulphate aerosols Ocean & sea-ice model Off-line model development Strengthening colours denote improvements in models Land carbon cycle model Carbon cycle model Ocean carbon cycle model Atmospheric chemistry Atmospheric chemistry Climate models → Earth System Models

  11. The Enhanced Greenhouse Effect S L 236 236 S L 236 232 S L 236 236 S L 236 236 Solar (S) and longwave (L) radiation in Wm-2 at the top of the atmosphere T = -18°C CO2 x 2 + Feedbacks H2O (+60%) Ice/Albedo (+20%) Cloud? Ocean? CO2 x 2 CO2 x 2 TS = 15°C TS = 15°C DTS ~ 1.2K DTS ~ 2.5K

  12. Feedbacks • There are many important feedbacks in the climate system that can either amplify (positive feedback) or dampen (negative feedback) forcings • Examples: • + Water vapour – a warmer atmosphere holds more water, which is a greenhouse gas • + Ice albedo – a warmer atmosphere has less ice cover, and reduces albedo • +/- Clouds – a more cloudy atmosphere tends to be warmer at night, but cooler during the day. Cloud height is also important • Uncertainties in feedbacks lead to uncertainties in future predictions – currently the main source of uncertainty

  13. Greenhouse gases Aerosols Solar forcing Volcanic forcing

  14. Climate simulations 1860-2000 Model ingrey observations inred Including natural and anthropogenic forcings makes the model producethe best simulation of20th century climate

  15. Attribution Observations • are observed changes consistent with • expected responses to forcings • inconsistent with alternative explanations All forcing Solar+volcanic

  16. Understanding and Attributing Climate Change Continental warming likely shows a significant anthropogenic contribution over the past 50 years

  17. Summary (4. Climate models) • Climate models represent the main components of the climate system, and their interactions and feedbacks • Climate models have some skill at simulating present-day climate, and also the trend over the 20th century • This gives us some confidence that they can predict the future…

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