Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Global Warming Professor...

Bern, 9 November 2007 Annual meeting 2007

Global WarmingLennart Bengtsson

Global Warming

Professor

Lennart Bengtsson

Max Planck Institut für Meteorologie, Hamburg

ESSC, University of Reading, UK





5 year global temperature changein the last two decades

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presently 3mm/year



Change in Arctic summer sea-ice 1979-2007



Reduction in Arctic sea-ice observations and modeling results (H Drange, NERSC)

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Why does the climate of the Earth vary?

• Variation in solar irradiation

• Orbital variations

• Changes in atmospheric composition

• Variations in natural and antropogenic aerosols

• Internal variations in the climate system



What are the most likely causes to the global warming during the last 100 years?

• Variations in solar irradiation except the small 11-year cycle is likely to be excluded at least after 1978 when we have accurate satellite measurements.

• The effects of volcanic aerosols are well known after studies of the eruptions of El Chichon(1984) and Pinatubo(1991). The surface cooling is limited to 1-3 years.

• Non-climate related changes in surface albedo can be excluded with high likelihood.

• Natural variations on longer time scales are generally more regional.

• The method of exclusion points to changes in atmospheric composition and changes in aerosols as the most likely cause.



What do we know of the greenhouse effect?

• Is known and demonstrated since the early part of the 19th century.

• A simple radiation balance calculation shows that the Earth’s temperature would be 33 C lower without greenhouse gases. Present concentration of carbon dioxide contributes ca 7 C.

• Water vapor, carbon dioxide, methane, nitric oxide and CFC’s in this order are examples of natural and artificial greenhouse gases.

• There are large differences is both absorption efficiency and in residence time. Residence times varies from a little more than a week (water vapor) to 10000 years or longer ( e.g. SF6)

• Residence time for carbon dioxide is of the order of a few hundred years.



How do we measure/determine the greenhouse effect in the atmosphere?

• Difference in the net radiation at the tropopause.Unit Wm-2

• Increasing greenhouse gases reduces the outgoing terrestrial radiation.

• Such a reduction has been determined from satellite

measurements between 1970 and 1997. (Harris et al 2001, Nature)



• The greenhouse effect

During cloud free conditions over the central tropical Pacific.



• The greenhouse effect



Radiative effects from anthropogenic greenhouse gases.



Carbon dioxide measurements at Mauna Loa1957-2006

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ca.60%

2002-06

4.9 PgC

annually

Emission now 8PgC



How does the climate system respond?• Feedback effects enhance the warming.

• Water vapor follows temperature according to Clausius-Clapeyron relation.

• Surface albedo (less snow and ice)

• Latest model calculations suggest that there is also a net warming contribution from clouds ( mainly through reduced stratiform clouds)

• A reduced outgoing radiation of 1W/m2 increases the temperature of the surface by 0.5-1.0 C



Results from climate model.

• What is a climate model?

• How credible are the climate models?

• How well can we validate the models?



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



Modeledclimate

Observed climate.

Köppen climate zones



Storm tracks ERA40 (left)ECHAM5 ( right) NH(DJF)

Observations Model



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



Results from the latest IPCC evaluation. Observed and calculated temperature change.

Red with observed greenhouse gases, blue no change in atmospheric composition since 1900.



Natural climate variations.

• Climate varies due to natural causes.

( I exclude here changes due to orbital effects which have a time scale of 10K - 100K years)

• One of the largest contributions is due to El Nino.

• There are large climate variations in Europe and Arctic. They are most likely due to internal chaotic processes.

• These variations are significant and can dominate climate for several decades.





1998 JFM SST [oC]

JFM SST Climatology [oC]

1998 JFM SST Anomaly [oC]

El Nino/Southern OscillationEl Nino/Southern Oscillation



Natural climate variations, how do they occur?



The North Atlantic Oscillation

Negative phaseCold winter in northern Europe



The North Atlantic OscillationPositive phase

Warm winter in Northern Europe



How is climate likely to change?

• Results are based on a large number of model calculations by research groups over the whole world. Such calculations have been undertaken regularly during the last 20 years.

• Calculations are based on expected emission scenarios provided by IPCC including all greenhouse gases and anthropogenic aerosols mainly sulfur based aerosols.



What is A1B?

• Middle of the line scenario

• Carbon emission peaking in the 2050s (16 Gt/year)

• CO2 reaching 450 ppm. in 2030

• CO2 reaching 700 ppm. in 2100

• SO2 peaking in 2020 then coming done to 20% thereof in 2100



MPI Hamburg climate model

Present climate

Future climate



Change in storm track density





• Predictability of snow in Germany



How reliable are the models?

• Models are based on the basic physical equations including sources and sinks conservation of energy, momentum and water. Models include empirical parameters derived from the present climate.

• Largest uncertainties are due to clouds which are crucial for the heat balance and functioning as a climate thermostat.

• Other uncertainties are aerosols ( having mainly a cooling effect)• and the degree of heat absorption in the oceans• However, models are tested continuously against reliable data and

also on data from paleo climate as well as on other planets. Systematic intercomparisons indicate an ongoing improvement.

• A global warming due to a doubling of the greenhouse gases have been in the interval of 2-5 C. This has been produced by models during the last 30 years.



What climate changes could be the most severe during the next 50 years?

• Reduced precipitation in several areas typical of a Mediterranean climate due to pole ward movements of storm tracks.

• Increased tendency of periods of very high summer temperatures of the kind we had in central Europe in 2003.

• More intensive tropical cyclones but probably less frequent

• Increased frequency of intense precipitation

• Melting of glaciers

• Rising sea level



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Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Global Warming Professor...

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