Report

Simulated Increase of Hurricane Intensities in a CO2-Warmed Climate

Science  13 Feb 1998:
Vol. 279, Issue 5353, pp. 1018-1021
DOI: 10.1126/science.279.5353.1018

Abstract

Hurricanes can inflict catastrophic property damage and loss of human life. Thus, it is important to determine how the character of these powerful storms could change in response to greenhouse gas–induced global warming. The impact of climate warming on hurricane intensities was investigated with a regional, high-resolution, hurricane prediction model. In a case study, 51 western Pacific storm cases under present-day climate conditions were compared with 51 storm cases under high-CO2 conditions. More idealized experiments were also performed. The large-scale initial conditions were derived from a global climate model. For a sea surface temperature warming of about 2.2°C, the simulations yielded hurricanes that were more intense by 3 to 7 meters per second (5 to 12 percent) for wind speed and 7 to 20 millibars for central surface pressure.

Greenhouse gas–induced climate warming could affect hurricanes in a number of ways, including changing their intensity (1, 2), frequency (3-5), and locations of occurrence. Given the potential for catastrophic damage and loss of life from these storms, any such changes could have important societal consequences. In this study, we examine only the question of possible changes in storm intensity due to climate warming.

Theoretical models of hurricane intensity predict that the maximum potential intensity (MPI) of hurricanes will increase in a warmer climate (1, 2), although these techniques, which are based on thermodynamical considerations, contain many assumptions and caveats (2, 6, 7). Global climate models attempt to simulate the climate, including tropical storm–like features, by integrating dynamical and thermodynamical equations in three dimensions. To date, global models have provided suggestive, but not highly convincing, indications of increased hurricane intensities in a warmer climate (3, 4). However, the coarse resolution of these global models precludes their simulation of realistic hurricane structure. A 1995 assessment by the Intergovernmental Panel on Climate Change (8) concludes that “. . . it is not possible to say whether the . . . maximum intensity of tropical cyclones will change” because of increased greenhouse gas concentrations. In the present study, the relation between hurricane intensity and climate change was explored with a regional, high-resolution, hurricane prediction model. We focused on the northwest tropical Pacific region, where the strongest typhoons (the term used in the northwestern Pacific for hurricanes) are observed in the present climate.

In our case study approach, we selected 51 tropical storm cases from a control climate simulation of a global climate model and 51 cases from a high-CO2 climate simulation (9). The global model used was the Geophysical Fluid Dynamics Laboratory (GFDL) R30 coupled ocean-atmosphere climate model (10-12), which has resolution of about 2.25° latitude by 3.75° longitude. For the high-CO2 cases, we selected storms from years 70 to 120 of a +1%-per-year CO2 transient experiment, corresponding to CO2 increases ranging from a factor of 2.0 to 3.3. Tropical storm–like features (weaker and much broader than in real-world storms) have previously been analyzed in an R30 global atmospheric model very similar to that used here (5,13). The selected storm cases were then rerun as 5-day “forecast” experiments with the use of the high-resolution GFDL Hurricane Prediction System (14), which is currently used at the U.S. National Centers for Environmental Prediction (NCEP). This model has a maximum resolution in the storm region of 1/6° or about 18 km (15). Before beginning each hurricane model simulation, the crudely resolved global model storm (but not the background environment) was filtered from the global model fields and replaced by a more realistic initial vortex (16,17). This initial vortex replacement procedure is analogous to that used for operational hurricane prediction at NCEP. The storm intensity distributions of the control and high-CO2 case studies were then compared. Sea surface temperatures (SSTs) were held fixed during the hurricane model experiments. The SSTs and initial environmental conditions used for the regional hurricane model simulations were derived from the global climate model. The sensitivity of climate to CO2 concentrations in a hypothetical global version of the hurricane model could be different from that of the R30 global climate model, but is not investigated here.

The spatial distribution and magnitude of the wind speeds in the control cases (Fig. 1B) are fairly realistic in comparison to observed conditions (Fig. 1A). In particular, there is a decrease in maximum intensities over higher latitudes (with cooler SSTs), near the equator, and over land regions. One shortcoming of our simulations is that wind speeds in the strongest storms appear to be slightly underpredicted in the control cases (Fig.1B) as compared with actual observations (Fig. 1A). This underprediction of high wind speeds for intense storms is a known bias of the hurricane model, although the model nonetheless simulates surface pressure minima at least as low as the observed record (870 mb). The high-CO2 distribution (Fig. 1C) has more areas of very intense (>70 m/s) wind speeds than does the control distribution (Fig. 1B), which suggests a modest increase in maximum surface winds in response to CO2-induced warming.

Figure 1

Geographical distribution of the maximum surface wind speeds (in meters per second) observed during 1971–1992 (A) and simulated (B and C) for tropical storms in the northwest Pacific basin. Observations are from the Joint Typhoon Warning Center (Guam) as compiled by C. J. Neumann in 1993, available from the National Center for Atmospheric Research athttp://www.scd.ucar.edu/dss (ds824.1). The simulated distributions are based on 71 case studies each under control (B) and high-CO2 (C) conditions; results from 20 preliminary cases under each condition (9) were included in order to increase spatial coverage. Blank (white) regions denote areas where no tropical storms were reported during 1971–1992 (A) or none occurred in the case studies [(B) and (C)].

Comparison of the frequency distributions of the maximum surface wind speeds attained by each storm in the control and high-CO2case studies (Fig. 2) shows that the simulated maximum wind with the highest frequency of occurrence is about 5 m/s more intense in the high-CO2 case studies; the median of the high-CO2 wind speed distribution in Fig. 2 is 3.2 m/s higher than in the control. The Kolmogorov-Smirnov (KS) one-sided two-sample test (18) can be used to test whether values in one sample are statistically larger than those of a second independent sample, based on the cumulative distributions. According to this test (19), the tendency for the high-CO2storms shown in Fig. 2 to be more intense than the control storms is statistically significant at the 90% confidence level, with a probability of obtaining such a result by chance of 0.059. A comparison of the storm intensities simulated by the global climate model for these case studies (20) indicates that the high-CO2 cases were slightly more intense than the control cases, but the difference is not statistically significant according to the KS test.

Figure 2

Frequency distribution of maximum surface wind speeds obtained from the hurricane model in 51 case studies each from control (dashed line) and high-CO2 (solid line) conditions.

In terms of minimum surface pressure, there is considerable scatter among the storm cases (Fig. 3). In both the control and high-CO2 sets of storm cases, there are several relatively weak storms (>920 mb) even at high SSTs. The median value for the high-CO2 cases shown in Fig. 3 is lower (more intense) than the control by 6.6 mb. However, the overall pressure distribution for the high-CO2 cases is not significantly lower than the control distribution, according to the KS test. Nonetheless, the strongest storms occur in the high-CO2cases, with five storms intensifying to 860 mb or below, as compared to one storm in the control cases. Thus, the envelope of intensities appears to expand to include lower pressures (that is, higher storm intensities) for higher SSTs, as shown schematically by the dark curve. This result is consistent with theoretical calculations (1,2) suggesting an increase in the maximum attainable storm intensity in a CO2-warmed climate.

Figure 3

Scatter plot of minimum surface pressure versus local SST obtained from the hurricane model in 51 case studies each under control (circles) and high-CO2 (asterisks) conditions. The dark curve is drawn to schematically illustrate an expanding envelope of attainable surface pressures with increasing SST. The dashed line indicates the 860-mb level discussed in the text.

Application of the KS test to the available storm cases (21) at each hour of the 120-hour simulations (Fig.4A) indicated that the tendency for the high-CO2 storms to be more intense than the control storms is statistically significant, although not at all times during the 120-hour period. As a measure of the behavior of the more intense storms, we compared the value of the fifth lowest central pressure (∼90th percentile intensity) for each hour among the available storm cases for high CO2 and for the control (Fig. 4A). The high-CO2 curve generally lies below the control curve, and at times lies below the 95% confidence limit (22,23) for the control, indicating that the most intense storms in the high-CO2 case studies tend to be more intense than those in the control cases. A smaller and less statistically distinct change is seen in the medians of the central pressure distributions (20). Although the CO2-induced storm intensification is not statistically significant at all times, the sign of the intensity change indicates stronger storms in the warmer climate for virtually the entire 120-hour period. The increase in intensity of the fifth strongest (∼90th percentile intensity) storm is about 10 mb for surface pressure and 3 m/s (5%) for wind speed (20).

Figure 4

(A) Dark lines show the fifth strongest storm intensity for each hour under control (dashed) or high-CO2 (solid) conditions; shading depicts 95% confidence intervals for control conditions. Small circles (one to three rows) indicate periods when the high-CO2 distribution is significantly lower than the control distribution at the 0.1, 0.05, or 0.01 levels, respectively, according to a KS test. (B) Central surface pressure for control (dark dashed line) and high-CO2 (dark solid line) idealized experiments. Difference curves [light solid lines in (A) and (B)] are offset by +830 mb.

As a sensitivity test, the hurricane model simulations for all of the control and high-CO2 case studies were repeated without the use of the initial vortex replacement procedure. The results (20) show a somewhat stronger signal than that shown in Fig.4A, indicating that the increased storm intensity in the warmer climate suite is not likely to be an artifact of the vortex replacement procedure.

As an alternative to the case studies, a more idealized approach was used in which an initial storm was embedded in an otherwise uniform easterly flow (5 m/s). The SST, temperature, and moisture fields were derived (24) from area averages for the northwest tropical Pacific from the control and high-CO2runs of the climate model (from July through November, 8° to 26°N, 124° to 161°E). The increase in SST in the high-CO2climate was 2.2°C, compared with an increase of over 5°C in the upper troposphere. The surface pressure time series (Fig. 4B) indicate that the high-CO2 case is roughly 20 mb more intense than the control; the increase in maximum wind speeds (20) is about 7 m/s (12%). Typical changes of 15 to 20 mb were obtained with background easterly flows varying from 0 to 7.5 m/s (20). It has recently been suggested (2) that in a CO2-warmed climate, any intensification of hurricanes due to increased SST would be moderated by more stable lapse rates, such as those simulated in CO2-increase experiments using the global climate model. By design, our idealized and case study results include this moderating effect of a more stable tropospheric lapse rate (see above). Although the processes leading to more intense storms under high-CO2 conditions are not fully understood, we note that both the domain-averaged surface evaporation and the near-storm environmental convective available potential energy (CAPE) are enhanced in the high-CO2 cases, with the CAPE increasing despite the more stable tropospheric lapse rate under high CO2 conditions.

Our simulation results can be compared with theoretical estimates of the MPI of hurricanes that were obtained with the same time-mean thermodynamic profiles as our idealized simulations. Using Emanuel's method (6), we obtained an intensity increase of 23 mb and 10 mb, assuming thermodynamically reversible or pseudoadiabatic ascent of air parcels, respectively. With Holland's method (7), we obtained an intensity increase of 18 mb. Thus, the impact of CO2-induced warming on hurricane intensity as estimated with the theoretical methods is comparable to our simulation results.

Using both a case study and an idealized approach, we find that CO2-induced warming leads to more intense hurricanes (that is, typhoons) in the northwest Pacific basin. Our study does not address a number of important issues, such as the effect of the storm itself on the local SST, uncertainties in air-sea exchange processes (2, 25), sensitivity to model resolution or model physics, and applicability to other tropical cyclone basins. However, we are encouraged by the fact that with the present simulation approach, a reasonable spatial distribution and magnitude of storm intensities can be simulated for the northwest Pacific basin and that our CO2 sensitivity results are in reasonable agreement with calculations made with theoretical techniques (6,7).

  • * To whom correspondence should be addressed. E-mail: tk{at}gfdl.gov

REFERENCES AND NOTES

View Abstract

Subjects

Navigate This Article