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

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

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Page 1: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Is Global Warming Affecting Hurricanes?

Is Global Warming Affecting Hurricanes?

Kerry EmanuelMassachusetts Institute of Technology

Page 2: 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

Page 3: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 4: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 5: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Population of Florida, 1790-2004

Page 6: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Source: Roger Pielke, Jr.

Total Adjusted Damage by Decade, in 10Total Adjusted Damage by Decade, in 101010 2004 U.S. Dollars2004 U.S. Dollars

Page 7: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Total Number of Landfall Events, by Category, Total Number of Landfall Events, by Category, 1870-20041870-2004

Page 8: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 9: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 10: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Part II: Evidence Connecting Hurricane Activity to Tropical

Sea Surface Temperature

Page 11: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

No Long-Term Trend in Global Frequency

Page 12: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 13: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 14: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 15: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 16: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Decadal Perspective:

Page 17: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

What Environmental Factors Control Hurricane Power

Dissipation?

• Potential Intensity

• Wind Shear

• Low level environmental vorticity (“spin”)

Page 18: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Energy Production and Potential IntensityEnergy Production and Potential Intensity

Page 19: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Distribution of Entropy in Hurricane Inez, 1966

Source: Hawkins and Imbembo, 1976

Page 20: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 21: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 22: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Observed Potential Intensity

Page 23: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

MDR Lower Stratospheric Temperature

Page 24: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Contributions to North Atlantic Potential Intensity

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

Page 25: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Contributions to North Atlantic Hurricane Power Dissipation:

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

Page 26: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Temperature?

Page 27: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Atlantic Sea Surface Temperatures and

Surface Temperature

Aug-Oct Sea Surface Temperatures (at key latitudes)Aug-Oct HADCRU NH Surface Temperature

Page 28: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

What is Controlling Northern Hemisphere Surface

Temperature?

Page 29: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Begin with Global Mean Surface Temperature

Page 30: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 31: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Natural Forcing have also Varied with Time

Page 32: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

El Chichón PinatuboAgung Pelée

Page 33: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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:

Page 34: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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.

Page 35: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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.

Page 36: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Part IV: The FuturePart IV: The Future

Page 37: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Projections of Tropical Cyclone Activity:

Downscaling from Global Climate Models

Page 38: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 39: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Example: 200 Synthetic Tracks

Page 40: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Synthetic Tracks

Page 41: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Use Daily Output from Climate Models to Derive Wind

Statistics, Thermodynamic State Needed by Synthetic Track

Technique

(but hold genesis PDF constant!)

Page 42: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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)

Page 43: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Percent Increase in Basin Power Dissipation

Page 44: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Percent Increase in Landfall Power Dissipation

Page 45: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 46: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

• 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

Page 47: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Scientific Basis of the “Natural Cycles” Story

The Atlantic Multi-Decadal Oscillation (AMO)

Page 48: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Power Spectrum of North Atlantic Hurricane Frequency, 1851-2005

Page 49: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 50: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 51: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 52: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Source: Hadley Centre Global Surface Temperature Data

Page 53: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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

Page 54: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

3000 Atlantic storms in the current climate

Page 55: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

Boston

HURDAT: 28 events Method 2: 3000 events

Page 56: Is Global Warming Affecting Hurricanes? Kerry Emanuel Massachusetts Institute of Technology

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