1. Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

2. Program Overview of hurricane risk Evidence connecting hurricane activity to tropical sea surface temperature The evidence for anthropogenic forcing of tropical ocean temperature The future

3. Hurricane Risk Tropical cyclones account for the bulk of natural catastrophe U.S. insurance losses Losses vary roughly as the cube of the maximum wind speed Katrina caused > 1300 deaths and > $130 billion in damage

4. Source: Roger Pielke, Jr. Total U.S. Hurricane Damage by Decade, in 10 10 2004 U.S. Dollars

5. Population of Florida, 1790-2004

6. Source: Roger Pielke, Jr. Total Adjusted Damage by Decade, in 10 10 2004 U.S. Dollars

7. Total Number of Landfall Events, by Category, 1870-2004

8. U.S. Hurricane Damage, 1900-2004,Adjusted for Inflation, Wealth, and Population

9. Summary of U.S. Hurricane Damage Statistics: >50% of all normalized damage caused by top 8 events, all category 3, 4 and 5 >90% of all damage caused by storms of category 3 and greater Category 3,4 and 5 events are only 13% of total landfalling events; only 30 since 1870 Landfalling storm statistics are grossly inadequate for assessing hurricane risk

10. Part II: Evidence Connecting Hurricane Activity to Tropical Sea Surface Temperature

11. No Long-Term Trend in Global Frequency

12. Intensity Metric: The Power Dissipation Index A measure of the total frictional dissipation of kinetic energy in the hurricane boundary layer over the lifetime of the storm

13. Power Dissipation Based on 3 Data Sets for the Western North Pacific (smoothed with a 1-3-4-3-1 filter) aircraft recon Data Sources: NAVY/JTWC, Japan Meteorological Agency, UKMO/HADSST1, Jim Kossin, U. Wisconsin Years included: 1949-2004

14. North Atlantic PDI and Sea Surface Temperatures (Smoothed with a 1-3-4-3-1 filter) Power Dissipation Index (PDI) Scaled Temperature Years included: 1970-2006 Data Sources: NOAA/TPC, UKMO/HADSST1

15. Atlantic Sea Surface Temperatures and Storm Max PDI (Smoothed with a 1-3-4-3-1 filter) Scaled Temperature Power Dissipation Index (PDI) Years included: 1870-2006 Data Sources: NOAA/TPC, UKMO/HADSST1

16. Decadal Perspective:

17. What Environmental Factors Control Hurricane Power Dissipation? Potential Intensity Wind Shear Low level environmental vorticity (“spin”)

18. Energy Production and Potential Intensity

19. Distribution of Entropy in Hurricane Inez, 1966 Source: Hawkins and Imbembo, 1976

20. Maximum Theoretical Wind Speed, V pot Net outgoing radiation Surface Trade Wind speed Ocean mixed layer entrainment Sea Surface Temperature Temperature at top of storm Incoming solar radiation

21. Potential intensity and SST can be changed by: Changing solar and infrared radiation Changing ocean mixed layer entrainment Changing mean surface wind speed Also, Potential Intensity (but NOT SST) can be changed by changing the storm top temperature

22. Observed Potential Intensity

23. MDR Lower Stratospheric Temperature

24. Contributions to North Atlantic Potential Intensity (Log of each contribution, minus long-term mean)

25. Contributions to North Atlantic Hurricane Power Dissipation: (Log of each contribution, minus long-term mean)

26. Part III: What is Causing Changes in the Tropical Sea Surface Temperature?

27. Atlantic Sea Surface Temperatures and Surface Temperature Aug-Oct Sea Surface Temperatures (at key latitudes) Aug-Oct HADCRU NH Surface Temperature

28. What is Controlling Northern Hemisphere Surface Temperature?

29. Begin with Global Mean Surface Temperature

30. Greenhouse Gases and Associated Radiative Forcing have been Increasing Total LLGHG Carbon Dioxide Methane Nitrous Oxide Halocarbons 175018001850190019502000 Year 3 2.5 1 0.5 0 Radiative forcing (Watts/square meter) 2 1.5

31. Natural Forcing have also Varied with Time

32. El Chichón Pinatubo AgungPelée

33. Northern hemisphere surface temperature (and late summer-early fall tropical Atlantic sea surface temperature) represents a linear combination of global warming and aerosol cooling Mann and Emanuel 2006 Hypothesis about Why the Northern Hemisphere Differs from the Globe:

34. Tropical Atlantic SST(blue), Global Mean Surface Temperature (red), Aerosol Forcing (aqua) Global Mean Surface T MDR SST Aerosol forcing Mann, M. E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. EOS, 87, 233-244.

35. Best Fit Linear Combination of Global Warming and Aerosol Forcing (red) versus Tropical Atlantic SST (blue) MDR SST Global mean T + aerosol forcing Mann, M. E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. EOS, 87, 233-244.

36. Part IV: The Future

37. Projections of Tropical Cyclone Activity: Downscaling from Global Climate Models

38. Using Physics to Improve Hurricane Risk Assessment Generate very large number of synthetic storm tracks consistent with the general circulation of the atmosphere in a given climate Run a coupled ocean-atmosphere model of hurricane intensity along each track to generate wind fields

39. Example: 200 Synthetic Tracks

40. Cumulative Distribution of Storm Lifetime Peak Wind Speed, with Sample of 2946 Synthetic Tracks

41. Use Daily Output from Climate Models to Derive Wind Statistics, Thermodynamic State Needed by Synthetic Track Technique (but hold genesis PDF constant!)

42. Compare two simulations from IPCC set: 1. Last 20 years of 20 th century simulations 2. Years 2180-2200 of IPCC Scenario A1b (CO 2 stabilized at 720 ppm)

43. Results Using 2000 Atlantic and 2000 North Pacific Tracks from 5 Models: Percent Increase in Basin Power Dissipation

44. Results Using 2000 Atlantic and 2000 North Pacific Tracks from 5 Models: Percent Increase in Landfall Power Dissipation

45. Summary Atlantic TC frequency, intensity and duration are co-varying with tropical Atlantic SST Changes in tropical cyclone power are driven by changing potential intensity, wind shear, and “spin” of the low-level winds

46. Changes in tropical North Atlantic sea temperature mirror changes in northern hemispheric temperature and are probably driven by a combination of cooling by volcanoes and air pollution, and warming by greenhouse gases Long-term risk assessments must account for climate change

47. Scientific Basis of the “Natural Cycles” Story The Atlantic Multi-Decadal Oscillation (AMO)

48. Power Spectrum of North Atlantic Hurricane Frequency, 1851-2005

49. The AMO is a Pattern of Sea Surface Temperature “Main development region” High-latitude North Atlantic S. B. Goldenberg et al., 2001. Science, 293, 474-479

50. Variation with time of amplitude of third rotated EOF of the non-ENSO residual 1856-1991 de-trended SST data

51. Same, but showing global distribution. From Enfield et al., 1999

52. Source: Hadley Centre Global Surface Temperature Data

53. De-trended Aug-Oct Northern Hemisphere Surface Temperature (Hadley Centre Global Surface Temperature Data) Variation with Time of the Strength of the AMO (Goldenberg et al. 2001) A

54. 3000 Atlantic storms in the current climate

55. Boston HURDAT: 28 events Method 2: 3000 events

56. U.S. Landfall Probability, by Category: Present Climate versus Warmed Climate