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Is Global Warming Affecting Hurricanes?
Is Global Warming Affecting Hurricanes?
Kerry EmanuelMassachusetts 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
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
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
Population of Florida, 1790-2004
Source: Roger Pielke, Jr.
Total Adjusted Damage by Decade, in 10Total Adjusted Damage by Decade, in 101010 2004 U.S. Dollars2004 U.S. Dollars
Total Number of Landfall Events, by Category, Total Number of Landfall Events, by Category, 1870-20041870-2004
U.S. Hurricane Damage, 1900-2004,Adjusted U.S. Hurricane Damage, 1900-2004,Adjusted for Inflation, Wealth, and Populationfor Inflation, Wealth, and Population
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
Part II: Evidence Connecting Hurricane Activity to Tropical
Sea Surface Temperature
No Long-Term Trend in Global Frequency
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
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
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
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
Decadal Perspective:
What Environmental Factors Control Hurricane Power
Dissipation?
• Potential Intensity
• Wind Shear
• Low level environmental vorticity (“spin”)
Energy Production and Potential IntensityEnergy Production and Potential Intensity
Distribution of Entropy in Hurricane Inez, 1966
Source: Hawkins and Imbembo, 1976
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
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
Observed Potential Intensity
MDR Lower Stratospheric Temperature
Contributions to North Atlantic Potential Intensity
(Log of each contribution, minus long-term mean)
Contributions to North Atlantic Hurricane Power Dissipation:
(Log of each contribution, minus long-term mean)
Part III: What is Causing Changes in the Tropical Sea Surface
Temperature?
Atlantic Sea Surface Temperatures and
Surface Temperature
Aug-Oct Sea Surface Temperatures (at key latitudes)Aug-Oct HADCRU NH Surface Temperature
What is Controlling Northern Hemisphere Surface
Temperature?
Begin with Global Mean Surface Temperature
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
Natural Forcing have also Varied with Time
El Chichón PinatuboAgung Pelée
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:
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.
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.
Part IV: The FuturePart IV: The Future
Projections of Tropical Cyclone Activity:
Downscaling from Global Climate Models
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
Example: 200 Synthetic Tracks
Cumulative Distribution of Storm Lifetime Peak Wind Speed, with Sample of 2946
Synthetic Tracks
Use Daily Output from Climate Models to Derive Wind
Statistics, Thermodynamic State Needed by Synthetic Track
Technique
(but hold genesis PDF constant!)
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)
Results Using 2000 Atlantic and 2000 North Pacific Tracks from 5 Models:
Percent Increase in Basin Power Dissipation
Results Using 2000 Atlantic and 2000 North Pacific Tracks from 5 Models:
Percent Increase in Landfall Power Dissipation
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
• 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
Scientific Basis of the “Natural Cycles” Story
The Atlantic Multi-Decadal Oscillation (AMO)
Power Spectrum of North Atlantic Hurricane Frequency, 1851-2005
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
Variation with time of amplitude of third rotated EOF of the non-ENSO residual 1856-1991 de-trended SST data
Same, but showing global distribution. From Enfield et al., 1999
Source: Hadley Centre Global Surface Temperature Data
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
3000 Atlantic storms in the current climate
Boston
HURDAT: 28 events Method 2: 3000 events
U.S. Landfall Probability, by Category: Present Climate versus Warmed Climate