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CESDSAGES Scottish Alliance for Geoscience, Environment &

Society

Observing and Modelling Climate Change

Simon Tett, Chair of Earth System Dynamics & Modelling, University of Edinburgh

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Greenhouse Gas changes

Global atmospheric concentrations of CO2, methane and N2O have increased as a result of human activity since 1750

•Far exceed pre-industrial values. CO2 increase primarily due to fossil fuel use and land-use change

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Global Mean Temperature

From Brohan et al, 2006

Red bars show annual average. Grey bars the possible range. Blue line is smoothed to show slow change

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The longer perspective

Recent warming unprecedented

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What are we trying to understand?

Image created by Reto Stockli with the help of Alan Nelson, under the leadership

of Fritz Hasler

How might the earth system evolve in the future?

How and why did it evolve in the past?

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The Earth is Dynamic

From Space Science and Engineering Center, University of Wisconsin-Madison

Shows weather. Weather what you get, Climate is what you expect

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Greenhouse Effect

Surface

Cooling at about 6.5 o C/Km

Heat Energy escapes to

space. At balance energy in from

sun is the same as heat energy to

space

Energy from Sun

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Adding more Greenhouse gases to the atmosphere

Surface

Cooling at about 6.5 o C/Km Same Heat

Energy escapes to space but

from higher in the

atmosphere

Energy from Sun

Surface Temperature warms

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Feedbacks

• Act to amplify (or damp) warming from changes in carbon dioxide, other greenhouse gases and other climate drivers.

– Water vapour – warmer atmosphere can store more water vapour. Water vapour absorbs “heat” radiation so is a Greenhouse gas.

• Warmer world will have more moisture in the atmosphere and so will trap more heat. Increases warming

– Clouds have two main effects:1. Trap heat energy so add to the greenhouse effect2. Reflect energy from sun back to space so cool climate

– Relative importance of these two effects uncertain. High clouds tend to warm while low clouds cool. How relative proportions change as climate changes is a large uncertainty in estimates of future climate change

– Ice/Snow feedback. • Ice and snow are white and reflect solar energy back to space.• Melting ice and snow will cause more solar radiation to be absorbed which

in turn will warms the climate.

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Snow/Ice Feedback

Image courtesy NASA/GSFC/JPL, MISR Team.

See http://visibleearth.nasa.gov/

Summer Winter

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Climate Modelling

• Modelling of the atmospheric flow has long history – first attempts, using computers, made in 1950’s.

• General Circulation Models developed from computer models used to forecast weather

• These models represent atmosphere, ocean, ice, land-surface, carbon cycle, chemistry…

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General Circulation Models

• Three dimensional model of the circulation of the atmosphere and ocean• Build on fundamental equations but approximately solved by breaking space and

time into cells.• Cells are roughly 100 by 100 miles (150 by 150 Km) in size.• Many processes occurring that are not resolved with cells this big so their

average effect needs to be approximated. How to do this is uncertain.

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The End!

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Jan 2008