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The Impact of Global Warming on Hurricanes

The Impact of Global Warming on Hurricanes. Do we know yet? Alex Ruane 09/29/05. Overview. Other important mechanisms for hurricane intensification The difference between intensity and damage The Atlantic Multidecadal Oscillation (AMO) Kerr (2000) Delworth and Mann (2000)

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The Impact of Global Warming on Hurricanes

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  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 Atlantic • 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 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

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