SAGES Scottish Alliance for Geoscience, Environment & Society

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CESDSAGES Scottish Alliance for Geoscience, Environment & Society

Climate Change: Observing and Simulating the Past;

Predicting the FutureSimon Tett, Chair of Earth System Dynamics &

ModellingWith thanks to Gabi Hegerl, Ben Santer, Phil Jones, Keith Briffa, Peter Thorne, Philip Brohan, Nick Rayner, John Kennedy, Peter Stott, Myles Allen, Gareth Jones, John Mitchell, Geoff Jenkins, Chris Folland, David Parker, Jonathan Gregory, Bob Harwood, Richard Kenway and Claire

Jones

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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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CESDWhat are we modelling?

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

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CESDOverview

• Basic physics• Modelling the climate system• Observations of climate change• Using climate models

– Understanding 20th century climate change– Role of natural drivers in natural variability– Predictions of future change– Importance of external drivers

• Concluding thoughts

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Radiation – the driver of the climate system

• Key ideas– Lots of incoming shortwave radiation (“Visible”) from

sun– Same total energy going out from Earth but peaks in

Infra-red. (“Heat”)– Surface is warmer than you’d expect from simple

radiation budget.• The bit of the climate system that radiates energy to space is

high up (where it is cooler).• Atmosphere cools with height• So surface is warmer the “greenhouse” effect• Changing the height of the atmosphere where energy gets to

space will then affect the surface temperature

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CESDLapse Rate

Temperature falls with height

From http://tamino.wordpress.com/

Tropical Pacific lapse rate

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CESDFeedbacks

• Act to amplify (or decrease) warming from changes in CO2, other greenhouse gases and other climate drivers.– Blackbody – warmer planet emits more radiation and so cools.

(Negative feedback)– Water vapour – warmer atmosphere can store more water vapour.

Water vapour absorbs “heat” radiation so is a Greenhouse gas.• Most important in the upper troposphere• Warmer world will have more moisture in the atmosphere and so will trap

more heat. Positive feedback.– Clouds

• Positive feedback – “trap” “heat” radiation. Particularly true for high clouds• Negative feedback – reflect back solar radiation. Particularly true for low

clouds– Ice/Albedo feedback.

• Ice is white and reflects lots of solar energy back to space.• Melt ice and more solar radiation absorbed which in turn warms the climate..

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CESDSnow/Ice Feedback

Image courtesy NASA/GSFC/JPL, MISR Team.

See http://visibleearth.nasa.gov/

Summer Winter

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CESDClimate Modelling

• Atmospheric modelling has long history – first attempts, using computers, made in 1950’s.

• General Circulation Models (GCM’s) developed from numerical weather prediction models– Take physical laws and apply them to atmospheric

and oceanic motions.– Key is that GCM’s are built bottom up.– Interested in “Emergent Phenomenon”, such as

statistics of data, rather than detailed evolution.• Other approaches but not covered in this lecture.

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Karl and Trenberth 2003

Modelling the Climate System

Main Message: Lots of things going on!

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General Circulation Models

3-D model of the circulation of the atmosphere and ocean

Fundamental equations:• Conservation of momentum• Conservation of mass• Conservation of energy• Equation of state

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Parameterized Processes

Slingo From Kevin E. Trenberth, NCAR

•Unresolved motions and processes affect the large scale flow so their effect needs to be parameterized.

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What are we trying to parameterize?

What is there…

How we parameterise

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CESDNumerical Modelling

Cray Y-MP ~ 1990

HECToR –2008

L. F. Richardson circa 1920

Since the 1960’s super-computer computational power increased by factor of 16 every decade. Over my career increased 200-300 fold

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CESDObserving Climate Change

• Observing system not stable• Climate changes slowly compared to observing

system.

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CESDGlobal Mean Temperature

From Brohan et al, 2006

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CESDThe longer perspective

Recent warming unprecedented

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Changes in Upper Ocean temperatures

From Palmer et al, 2007

The upper ocean is warming at, when looked one way, at roughly the same rate everywhere

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Changes in the free atmosphere: Large Observational Uncertainty

From Thorne et al, 2005 & Titchner et al, 2008

Left plot shows cooling in the tropical atmosphere. Contradicts climate models which predict largest warming in the tropics.

Right hand plot shows range of possible temperature changes in tropical free atmosphere due to uncertainties in observations. Sometimes models are more reliable than observations!

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CESDModel Applications

• Understanding 20th century climate change• The role of natural and human drivers in

climate variability• Future scenarios• Summary: external drivers important in

explaining observed climate variability and future climate change

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What might cause observed change?

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Internal variability – variability generated within the climate

systemRecent tropical Pacific ocean temperatures from IRI

source http://www.ldeo.columbia.edu/NAO by Martin Visbeck

The North Atlantic Oscillation

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Natural Factors that might effect climate: Volcanoes

20001850

Volcanic Aerosol depth

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0.2

Large tropical volcanoes inject sulphur dioxide into the Stratosphere where it stays for 2-3 years. Effect is to make an aerosol that scatters light and so cools climate.

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Natural Factors that might effect climate: Solar Irradiance

Solar activity (sunspots etc) & irradiance changes with 11-year solar cycle. There are long term changes in solar activity – the Maunder Minimum being one example. Converting this to changes in solar irradiance can be done though very uncertain. “Sun-like” starts which show activity variations have been used to estimate irradiance changes. Recent work (astronomical) and modelling (Lean et al) suggests there may be no significant long term variation in solar irradiance.

1700 2000

200

0

Sunspot Number

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Human Factors that might affect climate: Aerosols

Thanks to Met Office

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Human Factors that might affect climate: Greenhouse gases

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Ice cores

Flasks

1700 1800 1900 2000Year

600800

10001200

140016001800Mauna Loa

Observatory

Ice cores1700 1800 1900

Year260280300320

340360380

CO2 MMR*106 CH4 MMR*109Greenhouse gas concentrations have changed over the last century. Their effect is to decrease the transmission of heat radiation by the atmosphere. So should warm climate.

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Understanding and Attributing Climate Change in the 4th Assessment

Globe, Land, Ocean and individual continents all likely show human induced warming. Warming effect of greenhouses gases likely offset by other human and natural drivers

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CESDModelling the last 500 years

• How important are external drivers compared to internal climate variability?

• Simulation with fixed drivers – “internal” variability alone.

• Simulation with only natural drivers– Sun & Volcanic eruptions

• Simulation with human and natural drivers– Natural + changes in greenhouse gases,

aerosols, and land-surface properties

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CESDNatural Drivers

Annual: Slow changes with large negative forcings (from volcanoes)

25-year Gaussian filter. Solar and Volcanic forcing as important as one

another. “Maunder Minimum” includes volcanic contribution. Tambora is

largest eruption of last 500 years. Late 20th century is also a volcanically active

period.

Solar

Volcanic

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CESDEffect of natural drivers

Both hemispheres change together as does the land & ocean though there are some differences. Natural variability is about ±0.3K compared to “internal” variability of ±0.1K. So Natural forcings are an important driver of global-scale temperature

variability

SH has less variability (as more ocean) than does NH

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CESDNaturally driven variability

Effect of natural drivers is to increase variability in the tropics

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CESDAdding human drivers

Greenhouse gases

AerosolsVolcanoes

SunTot. Natural

Total Human

Aerosols and volcanoes offset some GHG and solar warming

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Temperature Changes with human drivers included

Effect of human drivers is to warm climate so that it warms outside envelope of natural variability by mid-late 19th century in southern hemisphere land and by mid 20th century in northern hemisphere

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Effect of human drivers of climate

Shows impact of human drivers on zonal-average temperature. Tropics warm first and warming is significant by mid 19th century. Northern hemisphere warming delayed by aerosol cooling in simulation

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Predicting the Future

Material in this section from IPCC 4th assessment report.

Results based on multi-model archive. Typically show average across all model simulations with uncertainties from range

Scenarios used to drive models. Self-consistent atmospheric concentrations of CO2 and other greenhouse gases. Based on different human development paths

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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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Is climate changing faster than we thought it would?

• Lot of argument has been about reality of climate change– Are observations good?– Is the sun responsible for warming?– Feedbacks are weak so that future warming not likely

to be a great threat?• General consensus (see 4th Assessment report)

is that climate is changing, likely due to human influences and agreement between different models as to likely warming.

• But could models be underestimating future climate change?

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CESDWhat does the future hold?

Ensemble of “perturbed physics” models showing large uncertainty range of future warming. Which are right?

Climate Sensitivity – measure of feedbacks. “Long tail” suggests there may be strong feedbacks.

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Sea-ice (its ½ what is should be)

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

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CESDSea-Ice

NASA/GODDARD SPACE FLIGHT CENTER SCIENTIFIC

VISUALIZATION STUDIO; (DATA) ROB GERSTON, GSFC

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Circulation change important for regional changes

Observations

Model mean

Human influence detected on Sea Level Pressure BUT magnitude under-simulated in Northern Hemisphere (e.g. Gillett et al., 2005)

These problems will affect regional model simulations and regional predictions

Multi-model archiveFrom Gabi Hegerl

NH SH

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CESDUK Extreme events

Tewkesbury 2007Photograph: Daniel Berehulak/GettyImages

Met Office 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?

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CESDUK changes

Precipitation (blue) and temperature (red) for 1931-80 and 1981-date (dashed)

High summer drying and warming. Rest warming and moistening

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CESDChange over last century

Observations distinct from zero, consistent with all and inconsistent with natural. Implies human influence on UK climate.

Does model underestimate high summer changes?

Natural

All

Obs

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CESDSummary & Conclusions

• Basic understanding of the climate system explains greenhouse effect and why would expect warming in response to changing atmospheric composition

• Details of response come from feedbacks• Climate models are built “bottom up” not top down.

Uncertainties arise from need to parameterize unresolved phenomenon

• Interested in the emergent behaviour which is not easily predictable from basic physics in model.

• Instrumental observations of surface temperature back to mid 19th century

• This, and other observations, show clear evidence of warming and climate change.

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CESDSummary & Conclusions

• Using models and observations to establish that:– 20th century climate change is likely to be human driven with

greenhouse gas warming being offset by natural and other human drivers

– That external drivers are an important driver of natural climate variability

– That humans might have affected 19th century tropical climate.– Climate change has already happened and will continue to

happen regardless of what we do.– But will be large if emissions are not reduced.

• Models may be underestimating changes to come particularly those related to changes in atmospheric circulation.– This has important consequences for regional (i.e. UK) climate

change.

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The End!

Thanks for listening

Any Questions?

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CESDExtra Material

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(Natural) Variability in Extreme events can be large

From Allan et al, 2008

20’s 60’s 90’s

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CESDTrends since 1800

1800-2006July-AugCET

1800-2006Oct-MayCET

1800-2006July-AugEWP

1800-2006Oct-MayEWP

Obs 0.42 K/Cent

0.50 K/Cent

-8.2 % /Cent

6.3% /Cent

Model 0.35 K/Cent+/- 0.2

0.41K/Cent+/- 0.3 Biggest difference is +/- 0.3

2.6 % /Cent +/- 9%

1.7% /Cent +/- 4%Biggest difference is 10%

Model not capturing drying trend

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• Available observed weather data are limited before 1950 and almost non-existent before 1850.

• Many more observations exist, in logbooks, reports and other paper records (mostly in the UK). If we digitised them we could improve the climate record and extend it back to 1800.

• Hadley Centre digitised observations from Royal Navy Ships logbooks for WW2. These give a much-improved picture of 1940s climate.

Digitisation as a source of new data

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CESD1998 & 2007

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Another feedback: The Carbon cycle

From Friedlingstein et al, 2006. Plots shows additional CO2 from feedbacks between climate change and carbon cycle. Values vary between 25 and 225 ppm at 2100 mostly due to land-carbon cycle feedbacks.

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CESDRelative Contributions

Greenhouse gases

Other human

Natural

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CESDAssessing Recent Change

Observed trend (K/decade) marked with X where outside maximum absolute 50-year trend from Natural. + where outside maximum trend. Recent changes are outside simulated natural variability over large parts of the world. Suggests that natural systems are already being affected by climate change

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Fig 9.18b

Observed (black) and simulated 1901-1998 precipitation trends

ObservationsMulti-model mean

Model range

Thin solid line model’s all forcing response detected in obs

Figure from IPCC WGI Ch9 (Hegerl, Zwiers et al)

Zonal 20th century precipitation change

Changes in rainfall over NH underestimated by models?

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CESDRanking

From John Kennedy & the Met Office

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CESDRadiation

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CESDTimeseries of UK records

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CESDActual heat waves

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Future changes in the Hydrological Cycle