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