Climate change: facts and uncertainties

1. Climate change: facts and uncertainties Prof. Gerbrand Komen(ex-) Director Climate Research KNMI 20 November 2008 KNGMG Conference Climatechange facts - uncertainties - myths

3. . . complex turbulent geophysical flow . .

4. Variability • Many mechanism with characteristic patterns and characteristic time scales • Example: North Atlantic Oscillation • NAO Index - measuresair pressure difference between Iceland and the Acores

5. . . manyfeedbacks and forcings . .

6. Scientificapproach • Use all available observations • Quantify processes, transports, interactions and feedbacks

7. Observations: many time scales • Natural archives (tree rings, ice cores, oceanic sediments, etc): • Instrumental observations (20th century) • Longer time scales provide important perspective • Because of the limited time I will focus on the past century

8. Instrumentalobservations

9. Surface stations in the Global Observing System (GOS) of the Word Meteorological Organisation (www.wmo.ch)

10. Upper air observations inthe Global Observing System (GOS) of the World Meteorological Organisation (www.wmo.ch)

11. Voluntary Observing Ships in GOS

12. In the year 2007 : 383 ppm (37% above pre-industrial)

13. Example: annual mean precipitation

14. Earth System Models 1975 1985 1992 1997 Atmosphere Atmosphere Atmosphere Atmosphere Atmosphere Atmosphere Land surface Land surface Land surface Land surface Land surface Ocean & sea-ice Ocean & sea-ice Ocean & sea-ice Ocean & sea-ice The Mt.OfficeHadley Centre Sulphate aerosol Sulphate aerosol Sulphate aerosol Non-sulphate aerosol Non-sulphate aerosol Carbon cycle Carbon cycle Atmospheric chemistry • Concertedefforts in Climate Model Diagnosis and Intercomparison http://www-pcmdi.llnl.gov/

15. Observations Multi-model average Average annual precipitation

16. Global mean temperature in the 20th century model simulations vs observations

17. Climate is alwayschanging • Natural causes: chaos, sun, ocean, orbital variations, volcanic dust . . • How does one establish a human influence: Greenhouse gases, Aerosols, Land surface change . .

18. Attribution Observations • Forcing with all known forcings • Forcing with natural forcings only Compare model simulations with observations All forcing Solar + volcanic

19. Sources of uncertainty • Complex en unique system: • Observations • Model limitations • Limitedunderstanding • Limitedpredictability

20. ‘Quantifying’ uncertainties • Most warming over the past 50 years is verylikely due to anthropogenic GHG increases. Part of the variation can be accounted for by natural causes. • Δ T in 2100 (relative to 1990) is likely to lie between 1.1 en 6.4 °C Note: These probabilities involve a certain amount of (subjective) expert judgement

21. IPCC: "Robust findings, key uncertainties" Robust: Global warming1, increase CO21, human influence2, expected future rise of mean temperature3, . . Key uncertainties: Magnitude of expected change, trends in extremes, regional change, feedbacks, role aerosols, Greenland (sea level rise!), . . Basedonobservations Basedonobservation and interpretationwith the help of models Basedon Greenhouse gas theory and models

22. IPCC, possibleresponses Robust • Impacts on human society and biosphere • Reduction of vulnerability requires more adaptation - but there are limits to adaptability • Mitigation is possible Key uncertainties • Costs: Unmitigated change, adaptation, mitigation • Best possible development path (energy innovation speed, adaptive capacity, governance, . .)

23. Taxonomy of actors in the climate debate Pure scientists Honest broker Alarmists Sceptics

24. Concludingremarks • Climate science will never provide absolute certainty • A description of current understanding is rather complex (see summary in my summary) • This complex message is often deformed in the media and in the public debate • Values play an important role in decision making • Science should proceed as usual Thank you !

25. The end