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Hurricanes and Oceanic Carbon Sequestration: Another Negative Feedback Process to Slow Global Warming
In the summer of 1995, Bates et al. (1998) were in the midst of an ambitious program of making more than 50 one- to five-day cruises to collect meteorological and ocean surface physical and chemical data in the Sargasso Sea approximately 25 to 45 km southwest of Bermuda, when hurricane Felix passed directly over their measurement area.  From their "before and after" measurements, the scientists determined that Felix had an enormous impact on the local environment.  Most importantly, from our perspective, they calculated that the hurricane's strong winds had been responsible for an enhanced sea-to-air flux of CO2 that was somewhere in the range of 40 to 135 mmol CO2 m-2.  Adding the effects of the strong winds of two other hurricanes that passed nearby in September increased these numbers to 61 to 182 mmol CO2 m-2, or to nearly 45% of the total summertime loss of CO2 from this region of the sea.

Impressed with the magnitude of this phenomenon, the scientists wondered how important it was to the global carbon cycle, within which the world's oceans serve as a net sink for CO2 over the course of a year.  Hence, they turned to the Global Tropical Extra-tropical Cyclone Climatic Atlas of the U.S. National Climatic Data Center for information about the intensity, track and duration of all such storms of record, after which they calculated the annual impact of the totality of hurricanes and tropical storms that occurred each year from 1983 to 1992 on the ocean-atmosphere exchange of CO2.

What did they find?  Bates et al. said their calculations suggested that "hurricanes and tropical storms in the latitudinal band 40°S to 40°N should contribute to the ocean-to-atmosphere flux of CO2 by between 0.042 and 0.509 Pg C year-1."  One way to get a feel for the significance of these numbers is to compare them to estimates of the atmosphere-to-ocean - or oppositely-directed - net global carbon flux of 0.290 to 1.340 Pg C year-1.  Doing so, one finds that the presence of hurricanes provides an effective counterforce to something on the order of 14 to 38% of this important global impetus for oceanic carbon sequestration.

What do these findings imply about the future?  Obviously, it depends upon how earth's climate may change and how that change will affect hurricane characteristics such as frequency and intensity.  Taking the climate-alarmist path in this instance, Bates et al. say "there may be a modest 10-20% increase in tropical cyclone intensity" as a consequence of what is nearly universally predicted for the climate of the planet, i.e., global warming; and if such were to occur, the phenomenon they elucidated would, as they say, "increase the importance of the ocean-to-atmosphere CO2 flux of hurricanes."  But if the opposite were to occur, we could validly expect that (1) the sea-to-air flux of CO2 would be reduced, (2) less CO2 would accumulate in the atmosphere, and (3) there would be a smaller CO2-induced impetus for warming than would otherwise exist, which chain of events completes an important negative feedback loop between the initial increase in temperature and the warming-driven decrease in the forcing that is characteristically claimed to be responsible for the initial rise in temperature, i.e., the atmospheric CO2 concentration.

Which of these two outcomes of future global warming - more and stronger hurricanes, or fewer and weaker hurricanes - is more likely to be true?  There is really only one way to find out; and that is to discover how hurricane frequency and intensity have changed in the past in response to global warming.

With respect to hurricane frequency, there is little evidence of real-world hurricanes occurring more frequently during real-world periods of greater, as opposed to lesser, warmth.  However, several scientific studies describe just the opposite behavior, i.e., reductions in hurricane frequencies in response to global warming: Boose et al. (2001), Easterling et al. (2000), Elsner and Bossak (2001), Elsner et al. (2000), Landsea et al. (1999), Liu and Fearn (2000), Liu et al. (2001), Muller and Stone (2001), Parisi and Lund (2000), Singh et al. (2000, 2001), and Wilson (1999).

With respect to hurricane intensity, much the same is true, although the data are more sketchy.  Again, for example, there is little evidence of real-world hurricanes intensifying during real-world periods of greater, as opposed to lesser, warmth; but there are some indications of reductions in hurricane intensity during warmer periods: Cerveny and Balling (1998), Landsea et al. (1999), and Pielke and Landsea (1999).

In view of these real-world observations, we think it more likely than not that if the world were to warm in the future - for whatever reason - there would be a modest reduction in the frequency of occurrence of hurricanes and tropical storms, as well as a small decrease in their intensities, especially in the Atlantic basin, to which most of the cited empirical studies specifically apply.  Consequently, these changes would tend to keep more carbon in the world's oceans and, therefore, reduce the rate at which CO2 is accumulating in the atmosphere, which would then complete the negative feedback loop and provide a natural brake upon the warming that was responsible for the changes in hurricane characteristics.

Dr. Sherwood B. Idso Dr. Keith E. Idso

Reference
Bates, N.R., Knap, A.H. and Michaels, A.F.  1998.  Contribution of hurricanes to local and global estimates of air-sea exchange of CO2Nature 395: 58-61.

Boose, E.R., Chamberlin, K.E. and Foster, D.R.  2001.  Landscape and regional impacts of hurricanes in New England.  Ecological Monographs 71: 27-48.

Cerveny, R.S. and Balling Jr., R.C.  1998.  Weekly cycles of air pollutants, precipitation and tropical cyclones in the coastal NW Atlantic region.  Nature 394: 561-563.

Easterling, D.R., Evans, J.L., Groisman, P.Ya., Karl, T.R., Kunkel, K.E. and Ambenje, P.  2000.  Observed variability and trends in extreme climate events: A brief review.  Bulletin of the American Meteorological Society 81: 417-425.

Elsner, J.B. and Bossak, B.H.  2001.  Secular changes to the ENSO-U.S. hurricane relationship.  Geophysical Research Letters 28: 4123-4126.

Elsner, J.B., Liu, K.-b. and Kocher, B.  2000.  Spatial variations in major U.S. hurricane activity: Statistics and a physical mechanism.  Journal of Climate 13: 2293-2305.

Landsea, C.N., Pielke Jr., R.A., Mestas-Nuņez, A.M. and Knaff, J.A.  1999.  Atlantic basin hurricanes: indices of climatic changes.  Climatic Change 42: 89-129.

Liu, K.-b. and Fearn, M.L.  2000.  Reconstruction of prehistoric landfall frequencies of catastrophic hurricanes in northwestern Florida from lake sediment records.  Quaternary Research 54: 238-245.

Liu, K.-b., Shen, C. and Louie, K.-s.  2001.  A 1,000-year history of typhoon landfalls in Guangdong, southern China, reconstructed from Chinese historical documentary records.  Annals of the Association of American Geographers 91: 453-464.

Muller, R.A. and Stone, G.W.  2001.  A climatology of tropical storm and hurricane strikes to enhance vulnerability prediction for the southeast U.S. coast.  Journal of Coastal Research 17: 949-956.

Parisi, F. and Lund, R.  2000.  Seasonality and return periods of landfalling Atlantic basin hurricanes.  Australian & New Zealand Journal of Statistics 42: 271-282.

Pielke Jr., R.A. and Landsea, C.N.  1999.  La Niņa, El Niņo, and Atlantic hurricane damages in the United States.  Bulletin of the American Meteorological Society 80: 2027-2033.

Singh, O.P., Ali Khan, T.M. and Rahman, S.  2000.  Changes in the frequency of tropical cyclones over the North Indian Ocean.  Meteorology and Atmospheric Physics 75: 11-20.

Singh, O.P., Ali Kahn, T.M. and Rahman, S.  2001.  Has the frequency of intense tropical cyclones increased in the North Indian Ocean?  Current Science 80: 575-580.

Wilson, R.M.  1999.  Statistical aspects of major (intense) hurricanes in the Atlantic basin during the past 49 hurricane seasons (1950-1998): Implications for the current season.  Geophysical Research Letters 26: 2957-2960.