CESD

SAGES

Scottish Alliance for Geoscience, Environment & Society

Observing and Modelling Climate Change

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

Outline

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• Observing Climate Change • Modelling Climate • Causes of Historical Climate Change • Projections of Future Climate Change

Observing Climate Change: What is the problem?

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• Observing system not stable • Climate changes slowly • Examples:

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Global mean temperatures are rising

Warmest 12 years: 1998,2005,2003,2002,2004,2006, 2001,1997,1995,1999, 1990 ,2000 Period Rate 50 0.128



0.026

100 0.074



0.018

Years



/decade

From Kevin E. Trenberth, NCAR

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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?

Climate Modelling

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• 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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Modelling the Climate System

Main Message: Lots of things going on!

Karl and Trenberth 2003

HadCM3

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19 Atmospheric Levels Atmospheric resolution: 3.75 by 2.5

Ocean resolution :1.25 by 1.25

20 Ocean Levels

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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 Volcanic Aerosol depth

might

effect climate Volcanoes inject aerosol 0.2

into the upper 0 2000 atmosphere where it stays for 2-3 years. There it scatters sunlight back to space cooling the planet 1850 Sunspot Num ber 200 0 1700 2000 The sun may be a variable star with amount of energy reaching the earth changing over decades

Important Human Factors

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CO 2 MMR*10 6

Mauna Loa Observatory 320 300 280 260 Ice cores 1700 1800 Year 1900 2000 1800 1600 1400

CH 4 MMR*10 9

Flasks 1200 1000 800 600 1700 Ice cores 1800 Year 1900 2000

Greenhouse gas concentrations have changed over the last century. As have emissions of sulphur and other aerosols

Attribution

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• are observed changes consistent with  expected responses to forcings  inconsistent with alternative explanations

All forcing Observations Solar+volcanic

Understanding and Attributing Climate Change

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Continental warming

likely

shows a significant anthropogenic contribution over the past 50 years

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Projections 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).

Projections of Future Changes in Climate

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Projected warming in 21st century expected to be greatest over land and at most high northern latitudes and least over the Southern Ocean and parts of the North Atlantic Ocean

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Projections of Future Changes in Climate Precipitation increases

very likely

in high latitudes Decreases

likely

in most subtropical land regions

Extreme events

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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?