CESDSAGES Scottish Alliance for Geoscience, Environment &

Society

Observing and Modelling Climate Change

Prof. Simon Tett, Chair of Earth System Dynamics & Modelling: The University of Edinburgh

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Outline

• Observing Climate Change

• Modelling Climate

• Causes of Historical Climate Change

• Projections of Future Climate Change

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Observing Climate Change:What is the problem?

• Observing system not stable

• Climate changes slowly

• Examples:

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100 0.0740.018

50 0.1280.026

Warmest 12 years:1998,2005,2003,2002,2004,200

6, 2001,1997,1995,1999,1990,200

0

Period Rate

Years /decade

From Kevin E. Trenberth, NCAR

Global mean temperatures are rising

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Sea-ice (its ½ what it was)

Is this unexpected? Are we missing something fundamental in our understanding of the Earth system?

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Climate Modelling

• Climate modelling has long history – first attempts made in 1950’s.– Developed from numerical weather prediction

• Which is how weather forecasting is done

– Take physical laws and apply them to atmosphere and oceans.

– But now very complex.

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Karl and Trenberth 2003

Modelling the Climate System

Main Message: Lots of things going on!

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HadCM3

20 Ocean Levels

19 Atmospheric LevelsAtmospheric resolution: 3.75 by 2.5

Ocean resolution :1.25 by 1.25

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Many important processes occur on scales below that explicitly

modelled.

What is there… How we model

Uncertainties how to do this lead to uncertainties in prediction of climate change

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Natural Factors that might effect climate

20001850

Volcanic Aerosol depth

0

0.2

Volcanoes inject aerosol into the upper atmosphere where it stays for 2-3 years. There it scatters sunlight back to space cooling the planet

1700 2000

200

0

Sunspot NumberThe sun may be a variable star with amount of energy reaching the earth changing over decades

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Important Human Factors

2000

Ice cores

Flasks

1700 1800 1900 2000Year

600

800

1000

1200

1400

1600

1800Mauna Loa

Observatory

Ice cores1700 1800 1900

Year

260

280

300

320

340

360

380CO2 MMR*106 CH4 MMR*109

Greenhouse gas concentrations have changed over the last century.

As have emissions of sulphur and other aerosols

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Attribution

• are observed changes consistent with

expected responses to forcings

inconsistent with alternative explanations

Observations

All forcing

Solar+volcanic

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Understanding and Attributing Climate Change

Continental warming

likely shows a significant anthropogenic contribution over the past 50 years

CESDProjections of Future Changes in Climate

Best estimate for low scenario (B1) is 1.8°C (likely range is 1.1°C to 2.9°C), and for high scenario (A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C).

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Projected warmingin 21st century expected to begreatest over land and at most high northern latitudesand least over the Southern Ocean and parts of the North Atlantic Ocean

Projections of Future Changes in Climate

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Projections of Future Changes in Climate

Precipitation increases very likely in high latitudes

Decreases likely in most subtropical land regions

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Extreme events

Tewkesbury 2007Photograph: Daniel Berehulak/GettyImages

Met Office provisional figures show that May to July in the England and Wales Precipitation is the wettest in a record that began in 1766.

We must learn from the events of recent days. These rains were unprecedented, but it would be wrong to suppose that such an event could never happen again…. (Hazel Blears, House of Commons, July 2007)

Is it human induced climate change or natural variability?