4. Models of the climate system

Earth’s ClimateSystem

Sun

IceOceanLand

Sub-surface Earth

AtmosphereClimatemodel

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 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

Coupled atmosphere / ocean climate model

Radiation

Atmosphere: DensityMotionWater

HeatExchange of: Momentum

Water

Ocean: Density (inc. Salinity)Motion

SeaIce

Land

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

19 levels in atmosphere

20 levelsin ocean

2.5lat 3.75

long

1.251.25

The HadleyCentrethirdcoupledmodel -HadCM3

30km

-5km

Hadley Centre

The Development of Climate models, Past, Present and Future

Atmosphere Atmosphere Atmosphere Atmosphere Atmosphere Atmosphere

Land surfaceLand surfaceLand surfaceLand surfaceLand surface

Ocean & sea-ice Ocean & sea-ice Ocean & sea-ice Ocean & sea-ice

Sulphateaerosol

Sulphateaerosol

Sulphateaerosol

Non-sulphateaerosol

Non-sulphateaerosol

Carbon cycle Carbon cycle

Atmosphericchemistry

Ocean & sea-icemodel

Sulphurcycle model

Non-sulphateaerosols

Carboncycle model

Land carboncycle model

Ocean carboncycle model

Atmosphericchemistry

Atmosphericchemistry

Off-linemodeldevelopment

Strengthening coloursdenote improvementsin models

Mid 1970s Mid 1980s Early 1990s Late 1990s Present day Early 2000s

Climate models → Earth System Models

The Enhanced Greenhouse EffectSolar (S) and longwave (L) radiation in Wm2 at the top of the atmosphere

S L

236 236

T = 18°C

S L

236 232

CO2 x 2

S L

236 236

CO2 x 2

S L

236 236

CO2 x 2+ Feedbacks

H2O (+60%)

Ice/Albedo (+20%)

Cloud?

Ocean?

TS = 15°C TS = 15°C TS ~ 1.2K TS ~ 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 1860-2000

Including natural andanthropogenic forcingsmakes the model producethe best simulation of20th century climate

Model in greyobservationsin 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 20th century

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