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Global Warming and Hurricanes

An Overview of Current Research Results

1. Has Global Warming Affected Atlantic Hurricane Activity?

Thomas R. Knutson
Geophysical Fluid Dynamics Laboratory/NOAA
Sept. 3, 2008; Last Revised Jan. 26, 2010

A. Summary Statement

Two frequently asked questions on global warming and Atlantic hurricanes are the following:

  • Have humans already caused a detectable increase in Atlantic hurricane activity?
  • What changes in Atlantic hurricane activity are expected for the late 21st century, given the pronounced global warming scenarios from current IPCC models?

In this review, I address these questions in the context of published research findings. I will first present my main conclusions and then follow with some background discussion of the research that leads to these conclusions. My main conclusions are:

  • It is premature to conclude that human activity--and particularly greenhouse warming--has already had a detectable impact on Atlantic hurricane activity. However, human activity may have already caused substantial changes that are either below the 'detection threshhold' or are not properly modeled yet (e.g., aerosol effects).
  • Anthropogenic warming over the next century will likely cause Atlantic hurricanes to be more intense (by a few percent on average) and have substantially higher rainfall rates than present-day hurricanes.
  • Anthropogenic warming over the next century more likely than not will lead to greater numbers of very intense Atlantic hurricanes, despite a decrease in the overall numbers of hurricanes.
B. Statistical relationships between SSTs and Atlantic hurricanes

Likelihood Statements

The terminology here for likelihood statements follows the conventions used in the IPCC AR4, i.e., for the assessed likelihood, using my judgement, of an outcome or result:

  • Very likely: > 90%,
  • Likely: > 66%
  • More Likely Than Not: > 50%

Figure 1. Atlantic hurricane power dissipation index (PDI, green) and tropical Atlantic SST (blue) from Emanuel 2007. (more info)

Observed records of Atlantic hurricane activity (e.g. Emanuel 2007.) show a strong correlation, on multi-year time-scales, between local tropical Atlantic sea surface temperatures (SSTs) and the Power Dissipation Index (PDI) (Figure 1). PDI is an aggregate measure of Atlantic hurricane activity, combining frequency, intensity, and duration of hurricanes in a single index. Both Atlantic SSTs and PDI have risen sharply since the 1970s, and there is some evidence that PDI levels in recent years are higher than in the previous active Atlantic hurricane era in the 1950s and 60s.

Model-based climate change detection/attribution studies have linked increasing tropical Atlantic SSTs to increasing greenhouse gases, but the link between increasing greenhouse gases and hurricane PDI or frequency has been based on statistical correlations. The statistical linkage of Atlantic hurricane PDI to and Atlantic SST in Figure 1 suggests at least the possibility of a large anthropogenic influence on Atlantic hurricanes. If the correlation between tropical Atlantic SSTs and hurricane activity shown in Figure 1 is used to infer future changes in Atlantic hurricane activity, the implications are sobering: the large increases in tropical Atlantic SSTs projected for the late 21st century would imply very substantial increases in hurricane destructive potential--roughly a 300% increase in the PDI by 2100 (Figure 2 a).

Figure 2. Two different statistical models and projections of Atlantic hurricane activity.  Adapted from Vecchi et al. 2008). (more)

On the other hand, Swanson (2008) and others have noted that Atlantic hurricane power dissipation is also well-correlated with other SST indices besides tropical Atlantic SST alone, and in particular with indices of Atlantic SST relative to tropical mean SST (e.g., Figure 2b from Vecchi et al. 2008). This is in fact a crucial distinction, because the statistical relationship between Atlantic hurricanes and local Atlantic SST shown in the upper panel of Figure 2 would imply a very large increases in Atlantic hurricane activity (PDI) due to 21st century greenhouse warming, while the statistical relationship between the PDI and the alternative relative SST measure shown in the lower panel of Figure 2 would imply only modest changes of Atlantic hurricane activity (PDI) with greenhouse warming. In the latter case, the alternative relative SST measure in the lower panel does not change very much over the long term in global warming projections from climate models, because the warming projected for the tropical Atlantic in the models is not very different from that projected for the tropics as a whole.

A key question then is: Which of the two future Atlantic hurricane scenarios inferred from the statistical relations in Figure 2 is more likely? To try to gain insight on this question, we have first attempted to go beyond the ~50 year historical record of Atlantic hurricanes and SST to examine even longer records of Atlantic tropical storm activity and second to examine dynamical models of Atlantic hurricane activity under global warming conditions. These separate approaches are discussed below.

C. Analysis of century-scale Atlantic tropical storm and hurricane records

Figure 3 (more)

To gain more insight on this problem, we have attempted to analyze much longer (> 100 yr) records of Atlantic hurricane activity. If greenhouse warming causes a substantial increase in Atlantic hurricane activity, then the century scale increase in tropical Atlantic SSTs since the late 1800s should have produced a long-term rise in measures of Atlantic hurricanes activity.

Existing records of past Atlantic tropical storm numbers (1878 to present) in fact do show a pronounced upward trend, which is also correlated with rising SSTs (see Figs. 1 and 9 of Vecchi and Knutson 2008). However, the density of reporting ship traffic over the Atlantic was relatively sparse during the early decades of this record, such that if storms from the modern era (post 1965) had hypothetically occurred during those earlier decades, a substantial number would likely not have been directly observed by the ship-based "observing network of opportunity." We find that, after adjusting for such an estimated number of missing storms, there is a small nominally positive upward trend in tropical storm occurrence from 1878-2006. But statistical tests reveal that this trend is so small, relative to the variability in the series, that it is not significantly distinguishable from zero (Figure 3). In addition, a new study by Landsea et al. (2010) notes that the rising trend in Atlantic tropical storm counts is almost entirely due to increases in short-duration (<2 day) storms alone. Such short-lived storms were particularly likely to have been overlooked in the earlier parts of the record, as they would have had less opportunity for chance encounters with ship traffic. In short, the historical tropical storm count record does not provide compelling evidence for a substantial greenhouse warming induced long-term increase.

Figure 4 (more)

If we instead consider Atlantic basin hurricanes, rather than all Atlantic tropical storms, the result is similar: the reported numbers of hurricanes were sufficiently high during the 1860s-1880s that again there is no significant positive trend in numbers beginning from that era (Figure 4, black curve, from CCSP 3.3 (2008) ). This is without any adjustment for "missing hurricanes".

The evidence for an upward trend is even weaker if we look at U.S. landfalling hurricanes, which even show a slight negative trend beginning from 1900 or from the late 1800s (Figure 4, blue curve). Hurricane landfalling frequency is much less common than basin-wide occurrence, meaning that the U.S. landfalling hurricane record, while more reliable than the basin-wide record, suffers from degraded signal-to-noise characteristics for assessing trends.

While major hurricanes (Figure 4, red curve) show more evidence of a rising trend from the late 1800s, the major hurricane data are considered even less reliable than the other two records in the early parts of the record. Category 4-5 hurricanes show a pronounced increase since the mid-1940s (Bender et al., 2010) but again, we consider that these data need to be carefully assessed for data inhomogeneity problems before such trends can be accepted as reliable.

D. Model assessments of greenhouse warming influence on Atlantic hurricanes

Direct model simulations of hurricane activity under climate change scenarios offer another perspective on the problem. We have developed a regional dynamical downscaling model for Atlantic hurricanes and tested it by comparing with observed hurricane activity since 1980. This model, when forced with observed sea surface temperatures and atmospheric conditions, can reproduce the observed rise in hurricane counts between 1980 and 2006, along with much of the interannual variability (Figure 5).

Figure 5 (more)

Turning to future climate projections, current climate models suggest that tropical Atlantic SSTs will warm dramatically during the 21st century, and that upper tropospheric temperatures will warm even more than SSTs. Furthermore, most of the models project increasing levels of vertical wind shear over parts of the western tropical Atlantic (see Vecchi and Soden 2007). Both the increased warming of the upper troposphere relative to the surface and the increased vertical wind shear are detrimental factors for hurricane development and intensification, while warmer SSTs favor development and intensitification. To explore which effect of these effects might "win out", we can run experiments with our regional downscaling model.

Our regional model projects that Atlantic hurricane and tropical storms are substantially reduced in number, for the average 21st century climate change projected by current models, but have higher rainfall rates, particularly near the storm center. The average intensity of the storms that do occur increases by a few percent (Figure 6), in general agreement with previous studies using other relatively high resolution models, as well as with hurricane potential intensity theory (Emanuel 1987).

Figure 6 (more)

Earlier, Knutson and Tuleya (2004) estimated the rough order of magnitude of the hurricane sensitivity to be about 4% per deg C SST warming for maximum intensities and about 12% per deg C for near-storm (100 km radius) rainfall rates (see also Knutson and Tuleya (2008) abstract here). These sensitivity estimates have considerable uncertainty, as CCSP 3.3 (2008), gives an estimated range of 1-8% per deg C SST warming for hurricane intensity, and 6-18% per deg C for near-storm rainfall rates.

These existing studies lead us to conclude that it is likely that greenhouse warming will cause hurricanes in the coming century to be more intense on average and have higher rainfall rates than present-day hurricanes.

Turning now to the important question of the frequency of very intense hurricanes, the regional model of Knutson et al. (2008) has an important limitation in that it does not simulate such very intense hurricanes. For example, the maximum surface wind in the simulated hurricanes from that model is less than 50 m/s (which is borderline category 3 hurricane intensity). Furthermore, the idealized study of Knutson and Tuleya (2004) assumed the existence of hurricanes and then simulated how intense they would become. Thus, that study could not address the important question of the frequency of intense hurricanes.

In our latest study, just published in Science, we have tried to address both of these limitations by letting the regional model of Knutson et al. (2008) provide the overall storm frequency information, and then downscaling each individual storm from the regional model study into the GFDL hurricane prediction system. The GFDL hurricane model (with a grid spacing as fine as 9 km) is able to simulate the frequency, intensity, and structure of the more intense hurricanes, such as category 3-5 storms, much more realistically than the regional (18 km grid) model.

Using this additional downscaling step, the new GFDL hurricane model study is able to reproduce some important historical characteristics of very intense hurricanes, include the wind speed distribution and the change of this distribution between active and inactive decadal periods of hurricane activity (Fig. 1 of the new study). The model also supports the notion of a decrease in the overall number of hurricanes with projected 21st century climate warming. However, the study also projects approximately a doubling of the frequency of very intense (category 4-5) hurricanes in the basin by the end of the 21st century, using an 18-model average climate change projection to drive higher resolution "downscaling" models. The largest increase is projected to occur north of the Main Development Region, in the western part of the basin (Fig. 7). In a related calculation, four individual climate model projections were downscaled using the same framework, and three of the four projected an increase in category 4-5 hurricanes, while one of the four models showed a decrease. Thus, not all global climate model 21st century projections imply a future increase in Atlantic category 4-5 hurricane numbers, according to our model. While the 18-model ensemble result is probably more reliable than individual model results, each of the individual model results can be viewed as at least plausible at this time.

Figure 7. (more)

Returning to the issue of future projections of aggregate activity (PDI, as in Fig. 2), while there remains a lack of consensus among various studies on how Atlantic hurricane PDI will change, no model we have analyzed shows a sensitivity of Atlantic hurricane PDI to greenhouse warming as large as that implied by the observed Atlantic PDI/local SST relationship shown in Figures 1 and 2 (top). In other words, there is little evidence from current dynamical models that 21st century climate warming will lead to large (~300%) increases in tropical storm numbers, hurricane numbers, or PDI in the Atlantic. There is some indication from high resolution models of substantial (~100%) increases in the numbers of the most intense hurricanes even if the overall number of tropical storms and hurricanes decreases. In our Science study, we estimate that the effect of increasing category 4-5 storms outweighs the reduction in overall hurricane numbers such that we project (very roughly) a 30% increase in potential damage in the basin by 2100. This estimate does not include the influence of future sea level rise or other factors such as coastal development or changes in building practices.

Finally, one can ask whether the change in Category 4-5 hurricanes projected by our model is already detectable in the Atlantic hurricane records. Owing to the large interannual to decadal variability of SST and hurricane activity in the basin, we estimate that detection of this projected anthropogenic influence on hurricanes should not be expected for a number of decades. While there is a large rising trend since the mid 1940's in category 4-5 numbers in the Atlantic, our view is that these data are not reliable for trend calculations, until they have been further assessed for data homogeneity problems, such as those due to changing observing practices.

E. Other possible human influences on Atlantic hurricane climate

Apart from greenhouse warming, other human influences conceivably could have contributed to recent observed increases in Atlantic hurricanes. For example, Mann and Emanuel (2006) hypothesize that a reduction in aerosol-induced cooling over the Atlantic in recent decades may have contributed to the enhanced warming of the tropical North Atlantic, relative to global mean temperature. However, the cause or causes of the recent enhanced warming of the Atlantic, relative to other tropical basins, remains highly uncertain. A number of anthropogenic and natural factors (e.g., aerosols, greenhouse gases, volcanic activity, solar variability, and internal climate variability) must be considered as potential contributors, and the science remains highly uncertain in these areas.

F. Synthesis and Summary

In summary, neither our model projections for the 21st century nor our analyses of trends in Atlantic hurricane and tropical storm counts over the past 120+ yr support the notion that greenhouse gas-induced warming leads to large increases in either tropical storm or overall hurricane numbers in the Atlantic. A new modeling study projects a large (~100%) increase in Atlantic category 4-5 hurricanes over the 21st century, but we estimate that this increase may not be detectable until the latter half of the century.

Therefore, I conclude that despite statistical correlations between SST and Atlantic hurricane activity in recent decades, it is premature to conclude that human activity--and particularly greenhouse warming--has already caused a detectable change in Atlantic hurricane activity. ("Detectable" here means the change is large enough to be distinguishable from the variability due to natural causes.) However, human activity may have already caused some some substantial changes that are still below the "detection threshold", or are not properly modeled yet (e.g., aerosol effects on regional climate).

I also conclude that it is likely that climate warming will cause hurricanes in the coming century to be more intense on average and have higher rainfall rates than present-day hurricanes. In my view, it is more likely than not that the numbers of very intense (category 4 and 5) hurricanes will increase by a substantial fraction in the Atlantic.

The relatively conservative confidence levels attached to these projections, and the lack of a claim of detectable anthropogenic influence at this time contrasts with the situation for other climate metrics, such as global mean temperature. In the case of global mean surface temperature, the IPCC 4th Assessment Report (2007) presents a strong body of scientific evidence that most of the global warming observed over the past half century is very likely due to human-caused greenhouse gas emissions.


2. Recent GFDL Papers, Commentary, & Animations on Global Warming and Hurricanes


3. WMO/IWTC 2006 Statement on Tropical Cyclones and Climate Change

Given the high degree of interest in the possible relationship between climate change and tropical cyclones (including hurricanes and typhoons), a new summary statement on the topic has been developed by the global community of tropical cyclone researchers and forecasters as represented at the 6th International Workshop on Tropical Cyclones of the World Meteorological Organization (November 2006). A more comprehensive statement was also developed at the workshop.

The summary statement notes the following: "The surfaces of most tropical oceans have warmed by 0.25-0.5 degree Celsius during the past several decades. The Intergovernmental Panel on Climate Change (IPCC) considers that the likely primary cause of the rise in global mean surface temperature in the past 50 years is the increase in greenhouse gas concentrations....

...Some recent scientific articles have reported a large increase in tropical cyclone energy, numbers, and wind-speeds in some regions during the last few decades in association with warmer sea surface temperatures. Other studies report that changes in observational techniques and instrumentation are responsible for these increases."

Consensus statements by the workshop participants

"1. Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point.

2. No individual tropical cyclone can be directly attributed to climate change.

3. The recent increase in societal impact from tropical cyclones has been largely caused by rising concentrations of population and infrastructure in coastal regions.

4. Tropical cyclone wind-speed monitoring has changed dramatically over the last few decades leading to difficulties in determining accurate trends.

5. There is an observed multi-decadal variability of tropical cyclones in some regions whose causes, whether natural, anthropogenic or a combination, are currently being debated. This variability makes detecting any long-term trends in tropical cyclone activity difficult.

6. It is likely that some increase in tropical cyclone peak wind-speed and rainfall will occur if the climate continues to warm. Model studies and theory project a 3-5% increase in wind-speed per degree Celsius increase of tropical sea surface temperatures.

7. There is an inconsistency between the small changes in wind-speed projected by theory and modeling versus large changes reported by some observational studies.

8. Although recent climate model simulations project a decrease or no change in global tropical cyclone numbers in a warmer climate there is low confidence in this projection. In addition, it is unknown how tropical cyclone tracks or areas of impact will change in the future.

9. Large regional variations exist in methods used to monitor tropical cyclones. Also, most regions have no measurements by instrumented aircraft. These significant limitations will continue to make detection of trends difficult.

10. If the projected rise in sea level due to global warming occurs, then the vulnerability to tropical cyclone storm surge flooding would increase."

The full texts of the summary statement and comprehensive statement should be consulted for more details and context.

**** END OF MATERIAL RELATED TO THE WMO/IWTC STATEMENT *****


4. An Overview of Earlier GFDL Research on Global Warming and Hurricanes

Figure 1 (more)

The strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the earth's climate is warmed by increasing levels of greenhouse gases in the atmosphere. Although we cannot say at present whether more or fewer hurricanes will occur in the future with global warming, the hurricanes that do occur near the end of the 21st century are expected to be stronger and have significantly more intense rainfall than under present day climate conditions. This expectation (Figure 1) is based on an anticipated enhancement of energy available to the storms due to higher tropical sea surface temperatures.

The results shown in Figure 1 are based on a simulation study carried out by Thomas R. Knutson and Robert E. Tuleya at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). In this study hurricanes were simulated for a climate warming as projected to occur with a substantial build-up of atmospheric CO2. An increase of intensity of about one-half category on the Saffir-Simpson scale was simulated for an 80 year build-up of atmospheric CO2 at 1%/yr (compounded). For hurricane wind speeds, our model shows a sensitivity of about 4% per degree Celsius increase in tropical sea surface temperatures, with a larger percentage increase in near-storm rainfall.

Early Studies on Global Warming and Hurricanes

Figure 2 (more)

An increase in the upper-limit intensity of hurricanes with global warming was suggested on theoretical grounds by M.I.T. Professor Kerry Emanuel in 1987. In the late 1990s, Knutson, Tuleya, and Kurihara at GFDL/NOAA began simulating samples of hurricanes from both the present-day climate and from a greenhouse-gas warmed climate. This was done by "telescoping-in" on coarsely resolved tropical storms in GFDL's global climate model using the high-resolution GFDL hurricane prediction model (Figure 2). A research report describing this work was published in Science (1998), with a more detailed paper in Climate Dynamics (1999, vol. 15). All of these studies, as well as our more recent ones, include the moderating effect of atmospheric stabilization aloft under high CO2 conditions, rather than simply increasing the sea surface temperature alone.

In a follow-up study, which appeared in the Journal of Climate (2001), NOAA scientists Knutson and Tuleya teamed up with Isaac Ginis and Weixing Shen of the University of Rhode Island to explore the climate warming/ hurricane intensity issue using hurricane model coupled to a full ocean model. The coupled model was used to simulate the "cool SST wake" generated by the hurricanes as they moved over the simulated ocean (Figure 3). The model simulations including this additional feedback still showed a similar percentage increase of hurricane intensity under warm climate conditions as the original model without ocean coupling.

Figure 3 (more)

A comprehensive idealized hurricane intensity modeling study by Knutson and Tuleya, published in Journal of Climate (2004), confirms the general conclusions of previous studies but makes them more robust by using future climate projections from nine different global climate models and four different versions of the GFDL hurricane model. The GFDL hurricane model used for the study is an enhanced resolution version of the model used to predict hurricanes operationally at NOAA's National Centers for Environmental Prediction. According to this latest study, an 80 year build-up of atmospheric CO2 at 1%/yr (compounded) leads to roughly a one-half category increase in potential hurricane intensity on the Saffir-Simpson scale and an 18% increase in precipitation near the hurricane core. A 1%/yr CO2 increase is an idealized scenario of future climate forcing. As noted by the Intergovernmental Panel on Climate Change (IPCC), there is considerable uncertainty in projections of future radiative forcing of earth's climate. A criticism of our paper by Michaels et al. is responded to here.

An implication of the GFDL studies is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse-gas induced warming may lead to an increasing risk globally in the occurrence of highly destructive category-5 storms.