is global warming affecting hurricanes? kerry emanuel massachusetts institute of technology
TRANSCRIPT
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Is Global Warming Affecting Hurricanes?
Is Global Warming Affecting Hurricanes?
Kerry EmanuelMassachusetts Institute of Technology
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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
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Hurricane RiskHurricane 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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Source: Roger Pielke, Jr.
Total U.S. Hurricane Damage by Decade, in 10Total U.S. Hurricane Damage by Decade, in 101010 2004 U.S. Dollars2004 U.S. Dollars
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Population of Florida, 1790-2004
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Source: Roger Pielke, Jr.
Total Adjusted Damage by Decade, in 10Total Adjusted Damage by Decade, in 101010 2004 U.S. Dollars2004 U.S. Dollars
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Total Number of Landfall Events, by Category, Total Number of Landfall Events, by Category, 1870-20041870-2004
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U.S. Hurricane Damage, 1900-2004,Adjusted U.S. Hurricane Damage, 1900-2004,Adjusted for Inflation, Wealth, and Populationfor Inflation, Wealth, and Population
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Summary of U.S. Hurricane Summary of U.S. Hurricane Damage Statistics: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
0
3maxPDI V dt
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)
aircraft recon
Data Sources: NAVY/JTWC, Japan Meteorological Agency, UKMO/HADSST1, Jim Kossin, U. Wisconsin
Years included: 1949-2004
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North Atlantic PDI and Sea Surface Temperatures
(Smoothed with a 1-3-4-3-1 filter)P
ow
er D
issi
pat
ion
Ind
ex (
PD
I)
Sca
led
Tem
per
atu
re
Years included: 1970-2006
Data Sources: NOAA/TPC, UKMO/HADSST1
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Atlantic Sea Surface Temperatures and Storm Max PDI
(Smoothed with a 1-3-4-3-1 filter)
Sca
led
Tem
per
atu
re
Po
wer
Dis
sip
atio
n In
dex
(P
DI)
Years included: 1870-2006
Data Sources: NOAA/TPC, UKMO/HADSST1
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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 IntensityEnergy 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
2
| |entrains o
poto D s
F F FT TV
T C
V
Net outgoing radiation
Surface Trade Wind speed
Ocean mixed layer entrainment
Sea Surface Temperature
Temperature at top of storm
Incoming solar radiation
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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 and
Surface 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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Greenhouse Gases and Associated Radiative Forcing have been Increasing
Total LLGHG
Carbon DioxideMethane
Nitrous OxideHalocarbons
1750 1800 1850 1900 1950 2000
Year
3
2.5
1
0.5
0
Rad
iati
ve f
orc
ing
(W
atts
/sq
uar
e m
eter
)
2
1.5
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Natural Forcing have also Varied with Time
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El Chichón PinatuboAgung Pelée
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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
Mann and Emanuel 2006
Hypothesis about Why the Northern Hemisphere Differs from the Globe:
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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.
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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.
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Part IV: The FuturePart 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 Using Physics to Improve Hurricane Risk AssessmentHurricane 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 simulations
2. 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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SummarySummary
• 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
“Main development region”
High-latitude North Atlantic
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)
A
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3000 Atlantic storms in the current climate
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Boston
HURDAT: 28 events Method 2: 3000 events
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U.S. Landfall Probability, by Category: Present Climate versus Warmed Climate