4. Models of the climate system

Earth’s Climate System Sun IceOceanLand Sub-surface Earth Atmosphere Climate model components

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…)

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

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

Biosphere Models have simplified representations of land and ocean carbon cycles Only ½ the emitted CO 2 ends up in the atmosphere – the other ½ is taken up by the oceans and vegetation CO 2 uptake by ocean causes acidification: CO 2(g) + H 2 O (aq) ↔ H 2 CO 3(aq) ↔ H + + HCO 3 - ↔ 2H + + CO 3 2- With potential impacts on marine biota – harder to form a calcium carbonate skeleton – high uncertainties

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

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

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

The Enhanced Greenhouse Effect Solar (S) and longwave (L) radiation in Wm  2 at the top of the atmosphere SL236 T =  18°C SL CO 2 x 2 SL236 CO 2 x 2 SL236 CO 2 x 2 + Feedbacks H 2 O (+60%) Ice/Albedo (+20%) Cloud? Ocean? T S = 15°C  T S ~ 1.2K  T S ~ 2.5K

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

Greenhouse gases Aerosols Solar forcing Volcanic forcing

Climate simulations Including natural and anthropogenic forcings makes the model produce the best simulation of 20 th century climate Model in grey observations in red

Attribution are observed changes consistent with  expected responses to forcings  inconsistent with alternative explanations Observations All forcing Solar+volcanic

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

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 20 th century This gives us some confidence that they can predict the future…