Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Global Warming Professor Lennart Bengtsson Max Planck Institut für Meteorologie, Hamburg ESSC, University of Reading, UK

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

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson 5 year global temperature change in the last two decades

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson presently 3mm/year

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Change in Arctic summer sea-ice

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Reduction in Arctic sea-ice observations and modeling results (H Drange, NERSC)

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

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

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson 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 years or longer ( e.g. SF6) Residence time for carbon dioxide is of the order of a few hundred years.

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson 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 (Harris et al 2001, Nature)

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson The greenhouse effect During cloud free conditions over the central tropical Pacific.

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson The greenhouse effect

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Radiative effects from anthropogenic greenhouse gases.

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Carbon dioxide measurements at Mauna Loa

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

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson ca.60% PgC annually Emission now 8PgC

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson 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/m 2 increases the temperature of the surface by C

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Results from climate model. What is a climate model? How credible are the climate models? How well can we validate the models?

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson 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 +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

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Modeled climate Observed climate. Köppen climate zones

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Storm tracks ERA40 (left) ECHAM5 ( right) NH(DJF) Observations Model

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Delworth and Knutson, 2000 Monte-Carlo simulations with a coupled AO GCM: one out five simulations almost perfectly reproduced the observed global temperature variability. obsexp 3

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Results from the latest IPCC evaluation. Observed and calculated temperature change. Red with observed greenhouse gases, blue no change in atmospheric composition since 1900.

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

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

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson 1998 JFM SST [ o C] JFM SST Climatology [ o C] 1998 JFM SST Anomaly [ o C] El Nino/Southern Oscillation

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Natural climate variations, how do they occur?

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson The North Atlantic Oscillation Negative phase Cold winter in northern Europe

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson The North Atlantic Oscillation Positive phase Warm winter in Northern Europe

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

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson What is A1B? Middle of the line scenario Carbon emission peaking in the 2050s (16 Gt/year) CO 2 reaching 450 ppm. in 2030 CO 2 reaching 700 ppm. in 2100 SO 2 peaking in 2020 then coming done to 20% thereof in 2100

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson MPI Hamburg climate model Present climate Future climate

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Change in storm track density

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

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson Predictability of snow in Germany

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

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson 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 More intensive tropical cyclones but probably less frequent Increased frequency of intense precipitation Melting of glaciers Rising sea level

Bern, 9 November 2007 Annual meeting 2007 Global Warming Lennart Bengtsson END