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Global Warming and Hurricanes
Fig. 1. Comparison of simulated hurricane intensities. (more)
Overview
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 hurricane 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).
New Observational Studies
Recently, two studies have been published which argue that hurricane intensities may have already increased markedly in recent decades as the tropical oceans have warmed. Emanuel (Nature, Aug. 4, 2005) reports that a measure of the power dissipated by tropical cyclones (proportional to the cube of wind speeds accumulated over the North Atlantic and western North Pacific basins) has approximately doubled since about 1950, with most of the increase occurring over the past 30 years. According to Emanuel, increases in both intensity and duration of tropical cyclones have contributed to this apparent increase. Emanuel's power dissipation index (PDI) is strongly correlated with sea surface temperatures in these basins, which have increased markedly over the same period. For an update, see this discussion. In a second study, Webster et al. (Science, Sept. 16, 2005) report that the number of category 4 and 5 hurricanes has almost doubled globally over the past three decades. Although their analysis spans a shorter time period than Emanuel's, their results indicate that a substantial increase has occurred in all six tropical storm basins.
It should be noted that the rate of increase of hurricane intensities implied in Emanuel's results (per degree of SST warming) is much greater than that simulated in our future projections. We are not yet able to reconcile these large differences in apparent sensitivity of the tropical cyclone intensities. We speculate that these discrepancies could arise from three sources: i) possible overestimation of the observed intensity trends; ii) possible underestimation by our model of the sensitivity of tropical cyclone intensities to SST changes; or iii) possible influence of related environmental variables such as trends in atmospheric temperatures (lapse rates) and moisture. Further investigation is ongoing.
Fig. 2. Tropical storm simulations. (more)
Background and Previous Studies
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 the Feb 13, 1998 issue of Science, 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 (June 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)
The most recent and comprehensive study by Knutson and Tuleya, published in Journal of Climate in September 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 being published soon in the Journal of Climate. Our response, which will appear in the same issue, is available here.
An implication of these 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.