1. Impacts of Climate Change on Tropical Cyclones
Robert Brewer
Geog 8901

2. 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

3. Goal
Investigate whether tropical cyclones have changed or will change in a warming climate
Huge topic of discussion often leading to conflicting results

4. 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.

5. 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?

6. 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%

7. 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)

8. 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

9. Challenges Cont.
Developing reliable projections that simulate the effect of local and remote climate factors on tropical cyclone activity
Intensity
Frequency
Storm track distribution

10. 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.

11. 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 human-induced 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?

12. 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
° C ( ° F) increase by
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

13. 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

14. 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

15. 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

16. Tropical Cyclone Frequency
Hurricane counts show a significant increase from 1800 to present
No trend from present
Other studies infer a low-bias in Atlantic storm intensities ( )
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

17. 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.

18. 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

19. 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

20. 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

21. 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

22. 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.

23. 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%.

24. 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.

25. 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

26. 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.

27. 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

28. 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)

29. 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.

30. 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

31. 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

32. 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

34. 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

35. 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.

36. Questions, Comments, or Concerns?