Evaluation of climate models,Attribution of climate change

IPCC Chapters 7,8 and 12.

John F B MitchellHadley Centre

• How well do models simulate present climate?• How well do they simulate past climate change?• Can natural factors explain the last 100 years?• Can human factors explain recent changes?

“Simulations of the response to natural forcings alone … do not explain the warming in

the second half of the century” SPM

Global mean temperature from an ensemble of 4 simulations using natural and anthropogenic forcing

Stott et al, Science 2000

“Reconstructions of climate data for the last 1000 years also indicate that this warming was unusual and unlikely to be entirely natural in origin” SPM

Arctic Sea Ice Cover: Observation and Simulation (Vinnikov et al., 1999, Science; Chapter 7)

Glacier mass balance

Cumulative balance of glacier mass in some regions

During the 20th century, glaciers and ice

caps have experienced widespread mass

losses and have contributed to sea level

rise

Decline of mountain glaciers projected

to reduce water availability in many

regions

“The warming over the last hundred years is very unlikely to be due to internal variability alone as

estimated from current models” SPM

Optimal detection• Based on spatial and temporal patterns, not

global means • Different components can be scaled separately

(e.g. greenhouses gases, aerosols)• “..most model estimates that take into account

both greenhouse gases and sulphate aerosols are consistent with observations [over the last 50 years]”

• The observations can be used to “correct” model predictions, with uncertainty limits

Substantial GHG warming with small sulphate cooling

Greenhouse warmingslightly larger thanobservedObservations

Time

Tem

pera

ture

Small sulphatecoolingGreenhouse

warming

Different Computer Models

Key technologies to reduce emissions

Efficient lighting; efficient appliances and aircodition; improved insulation ; solar heating and cooling; alternatives for fluorinated gases in insulation and appliances

Efficiency; fuel switching; renewable (hydropower, solar, wind, geothermal and bioenergy); combined heat and power; nuclear power; early applications of CO2 capture and storage

Energy Supply

Transport More fuel efficient vehicles; hybrid vehicles; biofuels; modal shifts from road transport to rail and public transport systems; cycling, walking; land-use planning

Buildings

Key mitigation technologies and practices currently commercially available

Key policies to reduce emissions Appropriate incentives for

development of technologies

Changes in lifestyle and behavior patterns, especially in building, transport and industrial sectors

Effective carbon price signal to create incentives to invest in low-GHG products, technologies and processes

Appropriate energy infrastructure investment decisions, which have long term effects on emissions

A technological society has two choices. First it can wait until catastrophic failures expose systemic deficiencies, distortion and self-deceptions…Secondly, a culture can provide social checks and balances to correct for systemic distortion prior to catastrophic failures.