Presentation on theme: "Impacts of Climate Change on Tropical Cyclones"— Presentation transcript:
1Impacts of Climate Change on Tropical Cyclones Robert BrewerGeog 8901
2Tropical 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 SugiFebruary 21, 2010Nature Geoscience
3GoalInvestigate whether tropical cyclones have changed or will change in a warming climateHuge topic of discussion often leading to conflicting results
4ProblemsLarge 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.
5Main IssueRemains 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?
6SummaryHigh resolution models and theory suggest that tropical cyclone intensity will increase due to greenhouse warming.2-11% by 2100Existing studies also suggest a decrease in globally averaged frequency of tropical cyclonesDown 6-34%
7Summary 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 centerFor all cyclone parameters, modelling studies show large variations in projected changes of the individual basins. (Atlantic, Pacific, Indian, etc)
8Challenge to EveryoneDetermining whether observed changes in tropical cyclone activity exceeds the variability of natural causes.If so, attribute the changes to climate forcingsgreenhouse gasesaerosols
9Challenges Cont.Developing reliable projections that simulate the effect of local and remote climate factors on tropical cyclone activityIntensityFrequencyStorm track distribution
10Recent ConclusionsSea 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° FIntergovernmental Panel on Climate Change (IPCC) concluded that most of the global surface temperature increase is due to increasing anthropogenic greenhouse gas concentrations.
11Recent ConclusionsUS 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 gasesRecent decades have shown an increase in damages caused from tropical cyclones.Increase in population density in coastal regionsInfrastructure value increasesCan further warming, coupled with other environmental changes, have an effect on tropical cyclone activity?
12Purpose / Guidance Discuss issues related to tropical cyclones DetectionAttributionFuture projectionsProjection statements apply to the IPCC A1B scenario as of the late 21st century° C ( ° F) increase bym sea level rise (excluding changes in ice flow)New satellite intensity analysisImproved downscaling techniquesNew simulations with higher resolution global modelsFocus on Atlantic basin
13Warming SST Tropical Atlantic SST has increased rapidly Warming: Natural multidecadal variability or aerosol forcingClimate models suggest warming is not strongly influenced by greenhouse gas forcingConcluded 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
14Tropical Cyclone Frequency Looked at tropical Atlantic SST variabilityMultidecadal SST variability is evidentCauses: 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 centuryIncreases the plausibility of natural climatic variability in the tropical AtlanticMultidecadal SST variability complicates trend detection in the tropical AtlanticModel simulations indicate that the rise in observed SST arise from greenhouse gas warming
15Tropical Cyclone Frequency First noted that tropical storm frequency can be modelled by SST increases.Greatly reduced because of an estimated amount of storms unaccounted forLow reporting-ship track densityPre-satellite years (pre 1966)The trend in storm count in original data was large in part to storms lasting <2 daysSmaller increasing trend in storms lasting >2 days
16Tropical Cyclone Frequency Hurricane counts show a significant increase from 1800 to presentNo trend from presentOther studies infer a low-bias in Atlantic storm intensities ( )If corrected- possibly eliminates any long-term increasing trends in hurricane counts in the Atlantic basinUS landfalling tropical storms show no long-term increase.Basin wide major hurricane counts do show a significant increaseBlamed on unreliable climate-trend estimatation before airplane reconnaissance before 1944
17Tropical 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.
18Tropical Cyclone Frequency Projection Progress has been made in developing dynamical and statistical models in seasonal tropical cyclone frequencyGlobal coupled climate modelsHigh-resolution atmospheric models running over observed and projected SST distributionsRegional climate models used to downscale solutions from global coupled modelsNew statistical/dynamical techniques aimed at avoiding the limitations of intensity simulations in dynamical models
19Frequency projectionModels reproduce key aspects of observed tropical cyclone variabilityFrequency 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 creationConfidence in global and projections of tropical storms is starting to rise.Confidence in individual basins remains lowUncertainties of large-scale patterns of future tropical climate changeLack of agreement between model projections of patterns of tropical SST changesLimitations in downscaling strategies
20Frequency 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% globallyIncrease in vertical wind shear which inhibits atmospheric convectionFor individual basins there is much more uncertainty in tropical storm frequency projectionsChanges of up to ±50% or more are projected by various models
21Tropical 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 intensityRecords contain a large amount of heterogeneities which can lead to a trend in a shift of stronger stormsA 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
22Tropical 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 cyclonesNew satellite intensity data was designed to be more homogeneous but still carry uncertainties.Ex. Indian Ocean where satellite records are less consistentProjected intensity changes caused by the effects of greenhouse gas induced warming are smallGiven data limitations, detection of an intensity change of magnitude consistent with model projections caused by greenhouse warming is not possible at this timeOther factors relating to SST distribution such as vertical wind shear, reduce the ability to attribute intensity changes to greenhouse gas warming.
23Cyclone Intensity Projection Increase in mean maximum wind speed is expected of tropical cyclones with projected 21st century warming.Increases may not occur in all basinsStudies 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 centuryAt the individual basin scale, multimodal ensemble projections show a range of intensity changes from -1 to +9%.
24Cyclone Intensity Projection Clear that higher resolution models project a clear increase in frequency of more intense tropical cyclones.Intensity levels varies between modelsA 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.
25Intensity 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 observationsTropical 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
26Tropical Cyclone Rainfall Climate models are unanimous that the integrated water column in the tropics will increase as the atmosphere warms.Moisture convergence will be enhancedRainfall 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.
27Tropical 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 climateAll available studies (7) support substantial increase in storm-centered rainfall21st century projections of +3 to +37%Percent increase is very sensitive to the average radius considered in tropical cyclones.Larger sensitivities reported for smaller radiiSmaller sensitivities reported for larger radiiTypical 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
28Genesis, 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 OceanExplanation 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 classUnlikely 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)
29Genesis, Tracks, Duration, and Surge Flooding Sea level has risen globally by 0.17m (~6.7in) in the 20th centuryOwing to various factors, climate-change related and otherwiseDecrease in coastal wetlands and local variations in land arising from coastal development.Detectable increases in tropical storm surge flooding has not been established.
30Genesis, Tracks, Duration, and Surge Flooding Projections Low-level confindenceModel projections do not show large scale changes in these features.Tropical cyclone storm surge flooding is expected to increaseSea level riseIncreasing population density in coastal regions
31Progress Summary and Outlook Progress has been achieved from the previous World Meteorological Organization (WMO) expert team assessment on tropical cyclones and climate changeImprovementsNew analyses of global data on hurricane intensityImportant studies of data quality issues in Atlantic tropical cyclone recordsAffected conclusions about climate change detectionHigher resolution global modellingImproved simulations of global storm frequenciesProvides support for theoretical expectations for a globally averaged increase in tropical cyclone intensity and rainfallDynamical and statistical/dynamical downscaling tools have improved for tropical cyclone activity
32Progress Summary and Outlook Increased confidence concerning several aspects of cyclone activity projectionsTropical cyclone activity is likely to decrease or essentially remain the same in the futureProject a future increase in the globally averaged frequency of the strongest tropical cyclones
34OutlookAnthropogenic signals in past cyclone data can not be conclusively indentified at this timeSubstantial human influence on future tropical cyclones can not be ruled out but can arise from other mechanisms.Oceanic warmingSea level riseCirculation changesDependent on theoretical, observational and modelling studies to assess future climate changes in tropical cyclone activityCredible but have limitations
35Final Thoughts Societal impacts of topical cyclones are huge Further research is strongly needed in their understandingIncreasingly fine resolution models and new approaches for improving past tropical cyclone records are key to their future understandingProgressively reducing the amount of uncertainties provides hope in the understanding of causes of past changes, and future projections of tropical cyclone activity.