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Rising Temperatures, Atlantic Hurricanes, United States Forests and Carbon Sequestration: Another Negative Feedback Phenomenon That Reduces Global Warming
In a recent study of hurricane impacts on carbon sequestration by United States forests, McNulty (2002) determined that a single intense hurricane can convert the equivalent of 10% of the total annual carbon sequestered by US forests into "dead and downed biomass."  Hence, since intense hurricanes occur, on average, two out of every three years across the eastern part of the country, he further concluded that "hurricanes are a significant factor in reducing long-term carbon storage in US forests."

The tone of this conclusion is decidedly negative, implying that efforts to promote long-term carbon sequestration by US forests may not be as effective as proponents of that approach to fighting global warming have long assumed they would be.  Before such thinking is blindly accepted, however, one must determine if there are likely to be any changes in either the frequency or intensity of hurricanes making landfall along the US Atlantic Coast in a world that would need additional carbon sequestration, i.e., in a warming world.  Hence, we briefly explore this important question.

First, we consider how hurricane characteristics may have changed as the earth emerged from the global chill of the Little Ice Age and entered into what we could call the Modern Warm Period.  In a major analysis of Atlantic basin hurricane characteristics from 1944 to 1996, over which period climate alarmists claim it has substantially warmed, Landsea et al. (1999) found decreasing trends for (1) the total number of hurricanes, (2) the number of intense hurricanes, (3) the annual number of hurricane days, (4) the maximum wind speed of all hurricanes averaged over the course of a year, and (5) the highest wind speed associated with the strongest hurricane recorded in each year.  In addition, they report that the total number of Atlantic hurricanes making landfall in the United States decreased over the extended 1899-1996 time period - which fact has been subsequently reaffirmed by Easterling et al. (2000) - and that normalized trends in hurricane damage in the United States between 1925 and 1996 have decreased at a rate of 728 million US dollars per decade.

Moving further back in time, Elsner et al. (2000) studied major hurricane occurrences in Bermuda, Jamaica and Puerto Rico over the past three centuries.  Their data reveal that hurricanes at these locations occurred at far lower frequencies in the last half of the 20th century (the warmest period of the entire three hundred years) than they did in the preceding 250-year period.  Between 1701 and 1850, for example, major hurricane frequency was 2.77 times greater than it was from 1951 to 1998, while from 1851 to 1950 it was 2.15 times greater.  Consequently, as the earth has continued to recover from the coldness of the Little Ice Age (Esper et al., 2002), both the frequency and intensity of Atlantic basin hurricanes have continued to decline.

A second way of investigating this subject is to look at year-to-year fluctuations in the number of observed hurricanes in relation to the state of the El Niņo-Southern Oscillation phenomenon.  Wilson (1999), for example, examined Atlantic basin hurricane frequency over the period 1950 to 1998, finding that the probability of having three or more intense hurricanes during a warm El Niņo year was approximately 14%, while during a cool non-El Niņo year the figure jumped to 53%.  Likewise, in a study of Atlantic basin hurricane intensity over the period 1925 to 1997, Pielke and Landsea (1999) reported that average hurricane wind speeds during warmer El Niņo years were about 6 meters per second lower than during cooler La Niņa years.  In addition, they reported that hurricane damage during cooler La Niņa years was twice as great as during warmer El Niņo years (1.6 billion dollars per year for La Niņa conditions compared to 800 million dollars per year for El Niņo conditions).  Hence, these year-to-year variations also indicate that both the frequency and intensity of Atlantic basin hurricanes tend to decrease under warmer conditions.

In light of these several real-world observations, one can only conclude that, in a warming world, forests along the US Atlantic seaboard would likely experience fewer and fewer hurricanes as time progressed, while those hurricanes that did occur would likely become ever weaker as the years passed.  Consequently, there would be a significant long-term decline in the loss of previously sequestered carbon in eastern US forests as the world warmed, which would translate into a long-term increase in carbon sequestration that would leave less CO2 in the air to promote global warming.  Hence, by encouraging the development of US forests to sequester carbon as a means of fighting rising temperatures, one gets an extra dividend, so to speak, as nature actually amplifies the consequences of this important approach to removing CO2 from the atmosphere.

Dr. Sherwood B. Idso Dr. Craig D. Idso

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., 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.

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

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.

McNulty, S.G.  2002.  Hurricane impacts on US forest carbon sequestration.  Environmental Pollution 116: S17-S24.

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.

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.