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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 Sept. 16, 2008
A. Summary Statement
Two frequently asked questions on global warming and Atlantic hurricanes are the following:
i) Have humans already caused a discernible increase in Atlantic hurricane activity?ii) 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. My conclusions are:
i) It is premature to conclude that human activity--and particularly greenhouse warming--has already had a discernible impact on Atlantic hurricane activity.
ii) 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.
B. Statistical relationships between SSTs and Atlantic hurricanes
Figure: Atlantic Basin Sea Surface Temperature and Power Dissipation Index (Emanuel 2007). (more)
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 right). PDI is a combined measure of Atlantic hurricane frequency, intensity, and duration. Both Atlantic SSTs and PDI have risen sharply since the 1970s, and there is some evidence of higher PDI levels in recent years compared to the previous active Atlantic hurricane era in the 1950s and 60s. We note that Atlantic PDI is also well-correlated with other SST indices besides tropical Atlantic SST alone, such as an index of Atlantic SST relative to tropical mean SST (Swanson 2008, Fig. 1c),
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 indirect and based on statistical correlations. The indirect linkage of Atlantic PDI to global warming via the correlation of PDI with Atlantic SST suggests at least the possibility of an anthropogenic influence on Atlantic hurricanes. If the correlation between tropical Atlantic SSTs and hurricane activity based on observed data since the 1950s 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 yield very substantial increases in hurricane destructive potential--several times larger than the already large increase in PDI since the 1970s. On the other hand, extrapolation of the alternative statistical relationship shown by Swanson (2008, Fig. 1c), would not imply a large future increase in Atlantic hurricane activity due to greenhouse warming, at least as simulated by current models.C. Analysis of century-scale Atlantic tropical storm and hurricane records
Figure: Adjusted Atlantic Tropical Storm Counts (1878-2006). (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 hurricane activity, then the century scale increase in global and tropical
Atlantic SSTs since the late 1800s should have been accompanied by a long-term rising
trend 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,
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 right).
Thus the historical tropical storm count record does not provide
compelling evidence for a greenhouse warming induced long-term
increase.
Figure: Atlantic Basin Hurricane Counts (1851-2006). (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 left, 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 left, 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 left, 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.
D. Model assessments of greenhouse warming influence on Atlantic hurricanes
Figure: Simulated vs Observed Atlantic Hurricane Counts (1980-2006). (more)
Direct model simulations of hurricane activity under climate change scenarios
offer another perspective on the problem. Current climate models project that not only will
the tropical Atlantic SSTs warm dramatically during the 21st century, but also
that 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 of the latter two factors are detrimental for
hurricane development and intensification, while warmer SSTs favor development
and intensitification. To explore which effect might "win out", we have developed
a regional dynamical downscaling model for Atlantic hurricanes. 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 right).
This same model indicates that for the 21st century climate
change projected by current models, Atlantic hurricane and tropical storms
are substantially reduced in number, although the average intensity of the storms
that do occur increases by a few percent (Figure below). Comparing with previous studies, several other models that
use relatively high spatial resolution, as well as hurricane intensity theory,
indicate the following:
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.
Knutson and Tuleya (2004)
estimate 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. These
sensitivity estimates have considerable uncertainty, and
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.
There is less consensus among models on how Atlantic hurricane frequency or
PDI will change, but no model we have analyzed shows a sensitivity of Atlantic
hurricane PDI to greenhouse warming as large as the observed Atlantic PDI/SST relationship
in recent decades. Therefore there is little evidence from current models that
greenhouse warming leads to large increases in tropical storm numbers,
hurricane numbers, or PDI in the Atlantic. There is some indication
from models that the numbers of the most intense hurricanes may
increase even if the overall number of tropical storms and
hurricanes decreases (see Figure below and
Knutson and Tuleya (2004).
This important issue is currently being investigated further with our higher resolution
hurricane models.
Figure: Atlantic Hurricane Windspeeds: Observed, Simulated, and Late 21st Century Projection. The curves in the left panel show the number of storms in each intensity class during the 1980-2006 period. In the right panel the curves have been normalized to have the same area under each graph. (more)
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 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. The science is still highly uncertain on a number of these factors.
F. Synthesis and SummaryIn 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 hurricane numbers in the Atlantic.
Therefore, we 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 had a discernible impact on Atlantic hurricane activity.
Similarly, efforts to project future levels of Atlantic
hurricane activity using observed SST-PDI statistical relations derived from recent
decades should be treated as highly speculative at this stage.
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.
We note that such changes in hurricanes have not yet been unambiguously detected
in observations and attributed to greenhouse warming. This
situation contrasts with that for global mean temperature, where 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
Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions, Nature Geoscience, doi:10.1038/ngeo202 (Published online May 2008)
Frequency Asked Questions and Commentary on our recent Nature Geoscience study (Posted June 11, 2008)
On Estimates of Historical North Atlantic Tropical Cyclone Activity -- J. Climate (July 15, 2008 issue)
Simulation of the recent multidecadal increase of Atlantic hurricane activity -- BAMS (October 2007 issue)
Simulated Hurricane Animations Web Page
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 Previous GFDL Research on Global Warming and Hurricanes
Fig. 1. Comparison of simulated hurricane intensities. (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.
Fig. 2. Tropical storm simulations. (more)
Early Studies on Global Warming and Hurricanes
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.
Fig. 3. Sea surface temperatures and sea level pressure. (more)
A comprehensive idealized hurricane intensity modeling study by Knutson and Tuleya, published in Journal of Climate (2004) (download paper), 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 in the occurrence of highly destructive category-5 storms.