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Impacts of Climate Change
on Tropical Cyclones
Robert Brewer
Geog 8901
Tropical Cyclones and Climate
Change
Thomas R. Knutson, John l. Mcbride,
Johnny Chan, Kerry Emanuel, Greg
Holland, Chris Landsea, Isaac Held,
James P. Kossin, A. K. Srivastava and
Masato Sugi
 February 21, 2010
 Nature Geoscience

Goal
Investigate whether tropical cyclones
have changed or will change in a
warming climate
 Huge topic of discussion often leading to
conflicting results

Problems
Large amplitude fluctuations in the
frequency and intensity in tropical cyclones
make it difficult to establish long-term
trends.
 These fluctuations also make it
complicated to detect their attribution to
rising levels of atmospheric greenhouse
gases.
 Availability and quality of global historical
records of tropical cyclones further deter
trend detection.

Main Issue
Remains uncertain if past changes in
tropical cyclone activity have exceeded the
variability from natural causes.
 Can rising global greenhouse gases make
an impact on tropical cyclone frequency
and intensity?

Summary

High resolution models and theory
suggest that tropical cyclone intensity
will increase due to greenhouse
warming.
 2-11% by 2100

Existing studies also suggest a
decrease in globally averaged frequency
of tropical cyclones
 Down 6-34%
Summary Cont.
New high resolution modelling studies
project an increase of intense tropical
cyclone frequency.
 Project increases on the order of 20% of
precipitation rate within 100 km (~62 mi)
of the storm center
 For all cyclone parameters, modelling
studies show large variations in
projected changes of the individual
basins. (Atlantic, Pacific, Indian, etc)

Challenge to Everyone
Determining whether observed changes
in tropical cyclone activity exceeds the
variability of natural causes.
 If so, attribute the changes to climate
forcings

 greenhouse gases
 aerosols
Challenges Cont.

Developing reliable projections that
simulate the effect of local and remote
climate factors on tropical cyclone
activity
 Intensity
 Frequency
 Storm track distribution
Recent Conclusions

Sea surface temperatures (SST) in regions
where tropical cyclone formations occur
have increased over the past several
decades.
 Several tenths of a degree (C)
 .1° C = .18° F

Intergovernmental Panel on Climate
Change (IPCC) concluded that most of the
global surface temperature increase is due
to increasing anthropogenic greenhouse
gas concentrations.
Recent Conclusions
US Climate Change Science Program
further concluded that the rise in SST in
hurricane formation regions can “very
likely” be blamed on the rise of humaninduced greenhouse gases
 Recent decades have shown an increase
in damages caused from tropical cyclones.

 Increase in population density in coastal regions
 Infrastructure value increases

Can further warming, coupled with other
environmental changes, have an effect on
tropical cyclone activity?
Purpose / Guidance

Discuss issues related to tropical cyclones
 Detection
 Attribution
 Future projections

Projection statements apply to the IPCC A1B
scenario as of the late 21st century
 1.7-4.4° C (3.06-7.92° F) increase by 2090-2099
 0.21-0.48 m sea level rise (excluding changes in ice flow)




New satellite intensity analysis
Improved downscaling techniques
New simulations with higher resolution global models
Focus on Atlantic basin
Warming SST





Tropical Atlantic SST has
increased rapidly
Warming: Natural
multidecadal variability or
aerosol forcing
Climate models suggest
warming is not strongly
influenced by greenhouse
gas forcing
Concluded increase in
power dissipation index
(hurricane activity) is due to
factors other than
greenhouse gas warming
*Noticed SST and power
dissipation show a very
weak correlation in the
northwest Pacific.
 # of category 4 and 5
typhoons show little relation
to increased SST
Tropical Cyclone
Frequency
Looked at tropical Atlantic SST
variability
 Multidecadal SST variability is
evident

 Causes: Climate variability and
radiative forcing changes?

African lake sediments indicate
rainfall variability before the 20th
century is at least as large as
that seen in the 21st century
 Increases the plausibility of natural
climatic variability in the tropical
Atlantic


Multidecadal SST variability
complicates trend detection in
the tropical Atlantic
Model simulations indicate that
the rise in observed SST arise
from greenhouse gas warming
Tropical Cyclone
Frequency


First noted that tropical storm
frequency can be modelled
by SST increases.
Greatly reduced because of
an estimated amount of
storms unaccounted for
 Low reporting-ship track
density
 Pre-satellite years (pre 1966)


The trend in storm count in
original data was large in part
to storms lasting <2 days
Smaller increasing trend in
storms lasting >2 days
Tropical Cyclone
Frequency

Hurricane counts show a
significant increase from 1800
to present
 No trend from 1850 -present

Other studies infer a low-bias in
Atlantic storm intensities (18501920)
 If corrected- possibly eliminates
any long-term increasing trends in
hurricane counts in the Atlantic
basin
US landfalling tropical storms
show no long-term increase.
 Basin wide major hurricane
counts do show a significant
increase

 Blamed on unreliable climate-
trend estimatation before airplane
reconnaissance before 1944
Tropical Cyclone Frequency

Considering past observational studies
and accounting for potential errors from
previous observational capabilities, it
remains uncertain whether or not
tropical cyclone frequency has
exceeded the variability expected
through natural causes.
Tropical Cyclone Frequency
Projection

Progress has been made in developing
dynamical and statistical models in
seasonal tropical cyclone frequency
 Global coupled climate models
 High-resolution atmospheric models running
over observed and projected SST distributions
 Regional climate models used to downscale
solutions from global coupled models
 New statistical/dynamical techniques aimed at
avoiding the limitations of intensity simulations
in dynamical models
Frequency
projection
Models reproduce key aspects of
observed tropical cyclone variability
 Frequency simulations are highly
dependent on the ability of global
coupled climate models (GCCMs) to
correctly simulate the changes in
large-scale conditions that affect
cyclone creation
 Confidence in global and
projections of tropical storms is
starting to rise.
 Confidence in individual basins
remains low

 Uncertainties of large-scale patterns of
future tropical climate change
 Lack of agreement between model
projections of patterns of tropical SST
changes
 Limitations in downscaling strategies
Frequency Projection Conclusion

Likely that the global mean tropical cyclone
frequency will either decrease or
essentially remain the same because of
greenhouse warming.
 Decrease of 6-34% globally
 Increase in vertical wind shear which inhibits
atmospheric convection

For individual basins there is much more
uncertainty in tropical storm frequency
projections
 Changes of up to ±50% or more are projected
by various models
Tropical Cyclone Intensity


Future surface warming and changes within the
thermodynamic properties of the tropical
atmosphere, as shown in projected climate models,
will lead to the increase in the limits in storm
intensities.
High-resolution models project not only and increase
in mean storm intensities, but also their frequency.
 These shifts are observed in the best track records of
tropical storm intensity
 Records contain a large amount of heterogeneities which
can lead to a trend in a shift of stronger storms

A global increase (x2) has been reported of high
intensity storms (Cat. 4 or 5) from 1975 to 2004.
 This is contested with concerns of data quality
Tropical Cyclone Intensity

Satellite based intensity estimations from 1981 show
that this best track data is indeed exaggerated.
 Do support an increase globally of the intensities of the
strongest tropical cyclones

New satellite intensity data was designed to be more
homogeneous but still carry uncertainties.
 Ex. Indian Ocean where satellite records are less
consistent



Projected intensity changes caused by the effects of
greenhouse gas induced warming are small
Given data limitations, detection of an intensity
change of magnitude consistent with model
projections caused by greenhouse warming is not
possible at this time
Other factors relating to SST distribution such as
vertical wind shear, reduce the ability to attribute
intensity changes to greenhouse gas warming.
Cyclone Intensity Projection

Increase in mean maximum wind speed is
expected of tropical cyclones with
projected 21st century warming.
 Increases may not occur in all basins

Studies based on potential intensity theory
and higher resolution models project mean
global maximum wind speed increases.
 +2 to +11% (+3 to +21% central pressure drop)
over the 21st century

At the individual basin scale, multimodal
ensemble projections show a range of
intensity changes from -1 to +9%.
Cyclone Intensity Projection






Clear that higher resolution models project a clear
increase in frequency of more intense tropical cyclones.
Intensity levels varies between models
A very small shift of storms towards higher intensities can
lead to a large fractional increase in the occurrence rate
of the strongest tropical storms.
A recent downscaling study using a high resolution model
projected an increase in Category 4 and 5 hurricanes in
the Atlantic over the 21st century.
Increased tropical cyclone intensity frequency is more
likely than not globally. (This may not occur in all tropical
regions)
Model projection results occur from a battle of increased
storm intensities versus decreased tropical storm
frequency.
Intensity Projection Conclusion

Further studies are needed to evaluate
model projections of tropical cyclone
intensities.
 Ex. comparing model simulations of the
interannual variability of intensities to
observations

Tropical cyclone intensities deserve more
attention (Category 3-5)
 Noted: Category 3-5 cyclones have accounted
for ~85% of US hurricane damage, despite only
representing 24% of US landfalling hurricanes
Tropical Cyclone Rainfall


Climate models are unanimous that the integrated
water column in the tropics will increase as the
atmosphere warms.
Moisture convergence will be enhanced
 Rainfall rates in systems (tropical cyclones) will be
increased. (speculation)


Storm wind intensities will increase which also will
aid moisture convergence.
Despite this speculation, tropical cyclone related
rainfall has not been established in existing studies.
 Increase in heavy precipitation events have been
observed and studied, but none of these events isolate
tropical cyclone precipitation rates.
Tropical Cyclone Rainfall
Projection

Tropical cyclone related rainfall rates are likely to
increase with greenhouse warming.
 Robust projection of rainfall in model simulations of
tropical cyclones in a warmer climate

All available studies (7) support substantial increase
in storm-centered rainfall
 21st century projections of +3 to +37%

Percent increase is very sensitive to the average
radius considered in tropical cyclones.
 Larger sensitivities reported for smaller radii
 Smaller sensitivities reported for larger radii
 Typical projected changes are about +20% within 100km
(~62 mi) of the storm center

“Average annual rainfall from tropical cyclones could
decrease if the decreased frequency of storms
exceeds that of increased rainfall rates in individual
storms” – not been quantified
Genesis, Tracks, Duration, and
Surge Flooding

No conclusive evidence of any observed changes in tropical
cyclone genesis, track, duration, and surge flooding exceed the
variability expected from natural causes.
 However – suggestions of storm track/ genesis location in the Atlantic
Ocean


Explanation for the lack of increasing trends in US and Gulf Coast landfalling
storms.
Century scale trend analysis of Atlantic tropical-cyclone-track
density show a decrease in density in the western part of the
basin and increase in the middle and eastern part.
 Increases in the eastern Atlantic are likely attributed to observing-system
changes according to recent studies of ship track density data and storm
occurrence by duration class
 Unlikely that the decrease in the western basin is attributable to the
same observing-system changes.

A long term decrease in average tropical storm duration has
been reported in the Atlantic basin.
 Upward trend of short duration storms (<2 days)
○ Interpreted (accredited to observing-system changes rather than climate
change)
 Little change in longer lived storms (>2 days)
Genesis, Tracks, Duration, and
Surge Flooding

Sea level has risen globally by 0.17m
(~6.7in) in the 20th century
 Owing to various factors, climate-change
related and otherwise
Decrease in coastal wetlands and local
variations in land arising from coastal
development.
 Detectable increases in tropical storm
surge flooding has not been established.

Genesis, Tracks, Duration, and
Surge Flooding Projections
Low-level confindence
 Model projections do not
show large scale
changes in these
features.
 Tropical cyclone storm
surge flooding is
expected to increase

 Sea level rise
 Increasing population
density in coastal regions
Progress Summary and Outlook


Progress has been achieved from the previous
World Meteorological Organization (WMO) expert
team assessment on tropical cyclones and climate
change
Improvements
 New analyses of global data on hurricane intensity
 Important studies of data quality issues in Atlantic tropical
cyclone records
○ Affected conclusions about climate change detection
 Higher resolution global modelling
○ Improved simulations of global storm frequencies
○ Provides support for theoretical expectations for a globally
averaged increase in tropical cyclone intensity and rainfall
 Dynamical and statistical/dynamical downscaling tools
have improved for tropical cyclone activity
Progress Summary and Outlook
Increased confidence concerning
several aspects of cyclone activity
projections
 Tropical cyclone activity is likely to
decrease or essentially remain the same
in the future
 Project a future increase in the globally
averaged frequency of the strongest
tropical cyclones

Outlook


Anthropogenic signals in past cyclone data
can not be conclusively indentified at this time
Substantial human influence on future tropical
cyclones can not be ruled out but can arise
from other mechanisms.
 Oceanic warming
 Sea level rise
 Circulation changes

Dependent on theoretical, observational and
modelling studies to assess future climate
changes in tropical cyclone activity
 Credible but have limitations
Final Thoughts




Societal impacts of topical cyclones are huge
Further research is strongly needed in their
understanding
Increasingly fine resolution models and new
approaches for improving past tropical cyclone
records are key to their future understanding
Progressively reducing the amount of
uncertainties provides hope in the
understanding of causes of past changes,
and future projections of tropical cyclone
activity.
Questions, Comments, or Concerns?