16 January 2005 Lennart Bengtsson Celsius lecture 2005 Uppsala Unversity The modelling of the climate system Professor Lennart Bengtsson ESSC, University

16 January 2005Lennart Bengtsson

Celsius lecture 2005Uppsala Unversity

The modelling of the climate system

Professor Lennart Bengtsson

ESSC, University of Reading

Max-Planck-Institut für Meteorologie, Hamburg



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Temperature change1854 - 2004 ( land areas only)

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Temperature change 1954 - 2004


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The modelling of the climate system

What is climate? Climate variations on different time-scales

The modelling of climate Atmospheric modelling and weather prediction

Modelling of the Earth climate system How predictable is climate?

Model simulation of the present climate Why is the climate changing?

Climate change prediction Concluding remarks



The classical view on climate

Climate as a stationary concept



Köppen climate zones

Main groups• A: Tropical rainy climate, all months > +18 C• B: Dry climate, Evaporation > Precipitation• C: Mild humid climate, coldest month +18 C - -3 C• D: Snowy - forest climate, coldest month < -3C but warmest > +10• E: Polar climate , warmest month < +10 C• ET: Tundra climate, warmest month > 0 C

• Subgroups• f : Moist, no dry seasons• w: Dry season in winter• s : Dry season in summer



GPCP (Prec) CRU2 (Temp)

ECHAM5

T159




What is climate?

• Climate is nowadays generally defined as a comprehensive statistical description of weather ( including extremes) over a sufficiently long period of time (30-100 years)

• There is no sharp distinction between weather and climate• There are free atmospheric modes of circulation that have time-scales of up to about two years ( quasi-biennal

oscillation)• There are coupled ocean-atmospheric modes that have

time-scales from weeks to several decades. A dominant feature is the El-Nino phenomenon in the eastern tropical Pacific ocean with a time-scale of about four years.



The present view on climate

Climate as a dynamical system



Climate variations on different time-scales

• Climate variations are dominated by chaotic weather events

• Other variations are due to coupled ocean-atmosphere processes which could cover longer periods of time. They are probably also mainly chaotic.

• Climate would also vary due to changes in solar irradiation( regular or otherwise) and under the influence of volcanic aerosols

• Climate also varies due to changes in the composition of the atmosphere such as greenhouse gases and aerosols



Shorter term climate variations and their likely causes



Natural temperature variationsENSO influence





Natural temperature variationsPinatubo and ENSO





The modelling of climate

• Prior to the 1950s climate was essentially a descriptive science, but general ideas of the general circulation of the atmosphere and the oceans existed

• Over the past 50 years the direction of climate research has changed driven by space based observations and mathematical modelling of the climate system.

• Climate modelling has occurred along three lines:

• - increased numerical resolution and more accurate treatment of individual physical and chemical processes

• - coupling of individual model components of the climate into Earth system models including aspects of the biosphere

• - ensemble predictions to be able to separate signal from noise.



• What is a CLIMATE MODEL?

• A COMPUTER PROGRAM which numerically solves the MATHEMATICAL EQUATIONS that represent the LAWS OF PHYSICS.

• Components include the: ATMOSPHERE, OCEAN, LAND, CRYOSPHERE and BIOSPHERE and all the dynamics, physical processes and interactions between them.

• The most comprehensive climate models include:GENERAL CIRCULATION MODELS (GCMs) as atmospheric and oceanic components.

• An AGCM follows the evolution of all the weather systems, clouds, and rain, and the interactions with the land and ocean.



• Climate system in pictorial form





• Climate system as a principle system



• Climate model components



• The strategy of climate research



• Significant atmospheric processes



• Aspects of numerics & physicalparameterizations for AGCMs



• Resolution issues



Modelling error as a function of horisontal and vertical resolution

(ECHAM climate model 2005)

0

20

40

60

80

100

Normalized RMSE [ % ]

T21_L19T31_L19T42_L31T42_L19T85_L19T106_L19T63_L19T63_L31T85_L31T106_L31T159_L31



How well can model simulate present climate?

Some examples



Typical cyclone storm tracks

Tracks Intensities



Storm track density ERA40 (left, 1979 - 2002)ECHAM5 ( right, 1979 - 1999, atmos. model run)

for the relative vorticity at 850 mb



Number of extra-tropical storms at the Northern Hemisphere as a function of intensity during winter





ECHAM5simulated

ERA40determined

from analyses.




How predictable is climate?

• Do we have a unique climate?

• Predictability of weather • and predictability of climate



• CLIMATE PREDICTION AND CHAOS

• “ For want of a nail, the shoe was lost;

• For want of a shoe, the horse was lost;

• For want of a horse, the rider was lost;

• For want of a rider, the battle was lost;

• For want of a battle, the kingdom was lost “



• Predictability of weather



ECMWF EPS: Forecast Started 8th January 00UTC



ECMWF EPS: Forecast Started 6th January 00UTC



• Predictability of snow in Germany



Why is the climate changing?

• There are natural changes on a variety of time scales.

• Some of these changes are chaotic and unpredictable.

• Sometimes chaotic events are inadvertently interpreted as due to specific external events ( e.g. solar forcing, volcanic eruptions, human influences etc.).

• However, increasing greenhouse gases exercise a real influence on climate and observations and model results are supporting each other



• The greenhouse effect



• The greenhouse effect



Carbon dioxide increase1957-2003



• CO2 and CH4 Concentrations Past, Present and

Future

Compiled by K. Alverson



• Svante Arrhenius 1859-1927



• The carbon cycle







Annual increase in GHG forcing1958-2003



The problem of climate change prediction

• How credible are climate change predictions?

• How will climate forcing change?

• What aspects of climate change is predictable?• What is unpredictable?

• Some important processes are not yet generally considered in climate models. This include feedback with the biosphere



Climate feedback



• The feedback problem









• Feedback results from different models



Delworth and Knutson, 2000

Monte-Carlo simulations with a coupled AO GCM: one out five simulations almost perfectly reproduced the observed global temperature variability.

obs exp 3



Ensemble climate trends averaged fordifferent time-periods

(T/decade)

1-30 years 1-80 years





Storm track density difference between scenario A1B ( aver. cond. 2071-2100 and ( aver. cond. 1961-1990) for the ECHAM 5 model. NH left and SH right.

Note the poleward change of the storm track at the SH



CoupledModelT63L31





Concluding remarks

• Climate model can reproduce most features of the general circulation of the atmosphere( and to a lesser degree) of the oceans. Results generally improves with higher numerical accuracy (resolution)

• The treatment of small scale physical processes, turbulence, clouds, boundary layer fluxes etc. are parameterised ( only expressed in terms of the resolvable parameters) and thus to some extent subject to ad hoc assumptions. However climate change feedbacks are likely to be influenced by such assumptions

• Climate change predictions over larger areas and longer time-scales are dynamically robust (albeit model dependent).

• Regional climate change predictions of a few decades and shorter are most likely unpredictable, but an ensemble over many cases could indicate a change in the overall probability distribution of climate and weather events.

Documents

16 January 2005 Lennart Bengtsson Celsius lecture 2005 Uppsala Unversity The modelling of the climate system Professor Lennart Bengtsson ESSC, University