1. Climate Change & The Probability of Extreme Events Brian Hoskins Royal Society Research Professor & Professor of Meteorology University of Reading Department of Meteorology

2. My position: a Sceptical Alarmist Overwhelming evidence based on theory, observations and models that we are perturbing the climate system in a very significant way There may be a joker in the pack but nearly all the ones we can think of would make things worse Natural variability could lead to a cooler decade, less tropical cyclones… but the problem would not have gone away Decisions for the next decades have to be made now Uncertainty in knowledge, construction of models, observation of initial states, specified fields (e.g. greenhouse gases) and inherent in the chaotic nature of the atmosphere-ocean system Consensus does not mean correct My confidence in the current ability to give most regional projections is low Many quote/use the (extreme) regional results & likely impacts with unwarranted confidence Sceptical questioning & probing is essential in climate science as in all science

3. Changes in Greenhouse gases in the atmosphere from Ice-Core and Modern Data IPCC (2007)

4. Since 1970, rise in:Decrease in:  Global surface temperatures NH Snow extent  Tropospheric temperatures Arctic sea ice  Global ocean temperatures Glaciers  Global sea level Cold temperatures  Water vapour  Rainfall intensity  Precipitation in extratropics  Hurricane intensity  Drought  Extreme high temperatures  Heat waves IPCC: “Global Warming is unequivocal”

5. Drawn by Tim Osborn using published data Estimates of Northern Hemisphere Temperature for the Past 1000 Years

6. Understanding and Attributing Climate Change

7. Atmospheric Water Vapour Clausius-Clapeyron; the water holding capacity of the atmosphere goes up at about 7% per degree Celsius increase in temperature. Models all suggest that this will occur with greenhouse gas warming. Observations show that it is happening at the surface and in lower atmosphere: 0.55C since 1970 over global oceans and 4% more water vapor. This means more moisture available to enhance the greenhouse warming, enhance the growth of storms and produce heavier rain.

8. Regions of disproportionate changes in heavy (95 th ) and very heavy (99 th ) precipitation Proportion of heavy rainfalls: more land areas with increases than decreases

9. Medium strength (20-40hPa) Intense (>40hPa) Changes in the track density of mid- latitude storms; 1979/03-1958/78 Hoskins & Hodges (2006)

10. N. Atlantic hurricane record best after 1944 with aircraft surveillance. Global number and percentage of intense hurricanes is increasing North Atlantic hurricanes have increased with SSTs SST (1944-2005)

11. Reasons for Confidence in Model Projections Models built on basic physics General consistency of globally averaged T response from simplest to most complex Success in forecast/hindcast of weather, seasonal climate, impact of Pinatubo, past century Simulation of phenomena such as El Niňo, storms Reasons for Lack of Confidence in Model Projections Underestimation of natural variability? E.g. 1940s Uncertainty in forcing used for past century, e.g. solar, aerosols Just starting to have interactive atmospheric chemistry & carbon cycle Uncertainty in cloud behaviour, aerosol effects, solar variability,… Poor representation of some phenomena particularly on smaller scales

12. IPCC (2007) Surface Temperature Projections 2020s & 2090s relative to 1980-99 Global mean 2020s 2090s

13. Changes in Temperature Extremes

14. Climate Extremes 2081-2100 annual global mean 2m temperature 95% 12 13 14 15 16 17 18 19 °C Weisheimer, A. & T.N.Palmer (2005)

15. Probability of exceeding 95% ile DJF temperature 2081-2100 (A1B, A2, B1) Extremely Warm Dec-Feb

16. Daily maximum temperature change due to CO2 doubling Hottest day of summer Average day of summer Hadley Centre Doubled CO 2 : Changes in probability distributions for summer day temperatures in Southern England

17. Stippled areas are where more than 90% of the models agree in the sign of the change Precipitation increases very likely in high latitudes Decreases likely in most subtropical land regions This continues the observed patterns in recent trends Fig. SPM-6 Projected patterns of end of 21 st century change (%) in precipitation for one scenario Dec-FebJune-Aug

18. Extremely Wet Dec-Feb Frequency of exceeding the 95%ile precipitation in DJF 2081-2100

19. Extremely Dry June-Aug Frequency of falling below the 5%ile precipitation in JJA 2081-2100

20. Daily rainfall change due to CO2 doubling Wettest day of winter Average wet day of winter Doubled CO 2 : Changes in probability distributions for winter wet days in Southern England Hadley Centre

21. Daily rainfall change due to CO2 doubling Wettest day of summer Average wet day of summer Doubled CO 2 : Changes in probability distributions for summer wet days in Southern England Hadley Centre

22. IPCC 2007 projections of 21 st century sea level rise

23. Melt descending into a moulin, a vertical shaft carrying water to ice sheet base. Source: Roger Braithwaite, University of Manchester (UK) Surface Melt on Greenland

24. Climate change & Midlatitude Cyclones/Storm-Tracks Reduced pole-equator temperature contrast More moisture in warmer air Same storms will transport more energy polewards. W W W N America Eurasia N Africa Greenland N Atlantic

25. Conclusions on midlatitude cyclones? Less in general? More in the NE Atlantic? Less in the Mediterranean? More strong ones?

26. Heat wavesVery likely Heavy rain eventsVery likely Drought areasLikely Intense tropical cyclonesLikely Extreme sea levelLikely IPCC Projections for some extreme weather events Working Group 1 Working Group 2 Impacts on Agriculture Water resources Human health Industry/settlement/society

27. Mechanisms for extreme changes? Large dynamical ice sheet loss: Greenland & West Antarctic Reduced carbon absorption/emission: soil, vegetation, ocean Methane emission from melting tundra, peat, clathrates Rapid change in the circulation of the atmosphere/ocean: reduction in the Atlantic northward heat transport frequency or nature of ENSO Asian monsoon circulation summer European blocking nature or location of winter storm-track nature or location of tropical cyclones Complex dynamical system behaviour