1. The Impact of Global Warming on Hurricanes Do we know yet? Alex Ruane 09/29/05

2. Overview Other important mechanisms for hurricane intensification The difference between intensity and damage The Atlantic Multidecadal Oscillation (AMO) – Kerr (2000) – Delworth and Mann (2000) The current state of scientific consensus – IPCC TAR (2001) – IPCC controversy? – Emmanuel (2005) – Webster et al. (2005) Stan?

3. Mechanisms For Hurricane Intensification Sea surface temperatures are not the only mechanism for hurricane intensification – Hurricanes are steered by upper level winds and temperatures – Need low-shear environment – Many natural cycles affect these factors El Niño Southern Oscillation (ENSO) North Atlantic Oscillation (NAO) Quasi-biennial Oscillation (QBO) SSTs have cycles of their own

4. The Atlantic Multidecadal Oscillation Kerr (2000) identified a large- amplitude natural oscillation in Atlantic SSTs on ~60-year period – Matches ~80-year oscillation seen in Greenland ice cores – Several tenths of a degree amplitude on same scale as global warming signal since 1860 (~0.6 ºC) – Was in high phase in 1920s and 1930s, low for 1970s and 1980s Delworth and Mann (2000) found the AMO in model simulations and proxy data – Derives from oscillation in the Thermohaline Circulation – Feeds into global circulation

5. Metrics of Hurricane Alterations Gray (2005) and Pielke et al. (2005) both point out problems with looking at damages to U.S. – Hurricanes have remained East of U.S. in recent years Luck finally caught up with us this year – Damages skewed by shifts in population density and affluence – Public memory very unreliable We had 2 category 4 storms strike New Orleans and Galveston 6 weeks apart in 1915 1970s-1995 was below normal hurricanes Better to look at metrics of storm intensity that are independent of the luck of landfall location – Record is poor for this type of storm before WWII – Better in Atlantic 1940s – 1960s due to aircraft measurements – Good global coverage began in 1970s with satellite measurements

6. Current State of Scientific Consensus? IPCC TAR (2001) – Suggest there is not enough data for definitive statements – Definite cycles in activity, not much global consistency – No trend in the peak intensity of the strongest storm each year Knutson and Tuleya (2004) simulate a 6% increase in maximum surface wind over 80 years Dr. Christopher Landsea resigned from IPCC in January, 2005, over perceived politicization of the tropical cyclone portion of the IPCC Dr. William Gray (Colorado State University), who issues the best-known Atlantic basin seasonal hurricane predictions: "I mean, there's almost an equation you can write the degree to which you believe global warming is causing major hurricanes to increase is inversely proportional to your knowledge about these storms.“ Impossible to claim a specific storm wouldn’t have occurred without Global Warming

7. New Papers – Emmanuel (2005) Used a Power Dissipation Index to suggest SSTs and hurricane strength are correlated and rising significantly – Unable to actually calculate temporal and spatial integral, so relied on maximum wind Short and incomplete data set – Multiple empirical adjustments and assumptions made to data Polynomial fit to 1970-2000 data used to adjust pre-1970 data downwards to correct for shift in data collection techniques in N. Atlantic 2 adjustments to pre-1973 data were made for W. Pacific Can’t separate intensities from number/duration – These adjustments correct high-phase AMO data with low-phase AMO data Atlantic W. Pacific Atlantic and W. Pacific

8. New Papers – Webster et al. (2005) Shows that more intense hurricanes are making up a larger percentage of hurricanes today than 30 years ago – Using accurate data from satellite era Does it go back far enough? Can’t yet claim that this is a global warming signal, as opposed to a natural cycle No change in maximum intensities

9. Conclusions Sea Surface Temperatures are only part of the story The Atlantic Multidecadal Oscillation accounts for significant variation in SST Metrics matter Emmanuel (2005) and Webster et al. (2005) suggest a trend, but the data cannot separate global warming from natural variability – Variations are larger than predicted Not enough evidence or time to alter scientific consensus