Is Global Warming Affecting Hurricanes?

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

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Is Global Warming Affecting Hurricanes?

Kerry Emanuel

Massachusetts Institute of Technology

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  • Overview of hurricane risk

  • Evidence connecting hurricane activity to tropical sea surface temperature

  • The evidence for anthropogenic forcing of tropical ocean temperature

  • The future

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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

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Total U.S. Hurricane Damage by Decade, in 1010 2004 U.S. Dollars

Source: Roger Pielke, Jr.

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Population of Florida, 1790-2004

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Total Adjusted Damage by Decade, in 1010 2004 U.S. Dollars

Source: Roger Pielke, Jr.

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

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

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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

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

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No Long-Term Trend in Global Frequency

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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

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Power Dissipation Based on 3 Data Sets for the Western North Pacific(smoothed with a 1-3-4-3-1 filter)

Years included: 1949-2004

aircraft recon

Data Sources: NAVY/JTWC, Japan Meteorological Agency, UKMO/HADSST1, Jim Kossin, U. Wisconsin

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North Atlantic PDI and Sea Surface Temperatures

(Smoothed with a 1-3-4-3-1 filter)

Years included: 1970-2006

Scaled Temperature

Power Dissipation Index (PDI)


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Atlantic Sea Surface Temperatures and Storm Max PDI

(Smoothed with a 1-3-4-3-1 filter)

Years included: 1870-2006

Power Dissipation Index (PDI)

Scaled Temperature


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Decadal Perspective:

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What Environmental Factors Control Hurricane Power Dissipation?

  • Potential Intensity

  • Wind Shear

  • Low level environmental vorticity (“spin”)

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Energy Production and Potential Intensity

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Distribution of Entropy in Hurricane Inez, 1966

Source: Hawkins and Imbembo, 1976

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Maximum Theoretical Wind Speed, Vpot

Net outgoing radiation

Sea Surface Temperature

Incoming solar radiation

Ocean mixed layer entrainment

Temperature at top of storm

Surface Trade Wind speed

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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

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Observed Potential Intensity

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MDR Lower Stratospheric Temperature

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Contributions to North Atlantic Potential Intensity

(Log of each contribution, minus long-term mean)

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Contributions to North Atlantic Hurricane Power Dissipation:

(Log of each contribution, minus long-term mean)

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

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Atlantic Sea Surface Temperatures andSurface Temperature

Aug-Oct Sea Surface Temperatures (at key latitudes)

Aug-Oct HADCRU NH Surface Temperature

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What is Controlling Northern Hemisphere Surface Temperature?

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Begin with Global Mean

Surface Temperature

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Carbon Dioxide


Nitrous Oxide



Greenhouse Gases and Associated Radiative Forcing have been Increasing




Radiative forcing (Watts/square meter)












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Natural Forcing have also

Varied with Time

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El Chichón


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Northern hemisphere surface temperature (and late summer-early fall tropical Atlantic sea surface temperature) represents a linear combination of global warming and aerosol cooling

Hypothesis about Why the Northern Hemisphere Differs from the Globe:

Mann and Emanuel 2006

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Tropical Atlantic SST(blue), Global Mean Surface Temperature (red), Aerosol Forcing (aqua)

Global Mean Surface T


Aerosol forcing

Mann, M. E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. EOS, 87, 233-244.

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Best Fit Linear Combination of Global Warming and Aerosol Forcing (red) versus Tropical Atlantic SST (blue)


Global mean T

+ aerosol forcing

Mann, M. E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. EOS, 87, 233-244.

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Part IV: The Future

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Projections of Tropical Cyclone Activity: Downscaling from Global Climate Models

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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

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Example: 200 Synthetic Tracks

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

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

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Compare two simulations from IPCC set: 1. Last 20 years of 20th century simulations2. Years 2180-2200 of IPCC Scenario A1b (CO2 stabilized at 720 ppm)

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Results Using 2000 Atlantic and 2000 North Pacific Tracks from 5 Models:

Percent Increase in Basin Power Dissipation

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Results Using 2000 Atlantic and 2000 North Pacific Tracks from 5 Models:

Percent Increase in Landfall Power Dissipation

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  • 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

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  • 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

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Scientific Basis of the “Natural Cycles” Story

The Atlantic Multi-Decadal Oscillation (AMO)

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Power Spectrum of North Atlantic Hurricane Frequency, 1851-2005

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The AMO is a Pattern of Sea

Surface Temperature

High-latitude North Atlantic

“Main development region”

S. B. Goldenberg et al., 2001. Science, 293, 474-479

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

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

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Source: Hadley Centre Global Surface Temperature Data

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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)

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3000 Atlantic storms in the current climate

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HURDAT: 28 events Method 2: 3000 events

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

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