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FROM VOLUME 11, NUMBER 9, SEPTEMBER 1998

NEWS...
Feature of the Month: Hurricane CO₂

Item #d98sep33

A wave of attention swept through the media during September after an announcement that hurricanes put large amounts of CO₂ into the atmosphere (see the Bates, Knap, and Michaels article in Prof. Pubs/Of General Interest in this Sept. 1998 issue). The media attention was encouraged by the facts that the hurricane season was at its peak and that the subtropical Atlantic Ocean was quite productive this year, at one time hosting four tropical storms.

The original report appeared in the Letters to Nature section of that journal and related measurements taken at the Bermuda Atlantic Time-Series site over which hurricane Felix passed in August 1995. Two other hurricanes passed 200 to 250 km west of the sampling area, but produced no changes in the measured variables “due to their distance from Bermuda.” Routine measurements of sea-surface temperature, salinity, pCO₂ of the air and seawater, total (air and seawater) pCO₂, and mixed-layer depth were presented in the paper along with calculated values of the seawater-air CO₂ flux. All of these measurements covered the period from February to October of 1995. For the days immediately preceding Felix, the direct measurements were supplemented with satellite observations.

With the passage of Felix, the measurements showed: a significant and rapid decrease in the sea-surface temperature, no observable change in the salinity (although a gap of about 15 days in the data hinders interpretation), a large and sudden decrease in seawater pCO₂, no observable change in the atmospheric pCO₂ (although, again, a multiday gap in the collection of data may obscure a short-term variation), a continuation of a four-month slow decline in total CO₂, and a slight increase in mixed-layer depth that built up in the days preceding the storm and reverted quickly after its passing. Indeed, the data presented indicate that atmospheric pCO₂ declined remarkably smoothly from the beginning of April to the end of August at the Bermuda station. Heavy sampling rates during the days immediately after Felix revealed no perturbations that were any larger than other variations in the record during the preceding months. Although the authors assumed that the seawater CO₂ was emitted from the ocean into the air, the air-CO₂ record seems to raise the question, “Where did all the CO₂ go?”

The BBC reported “Scientists have discovered that hurricanes ... contribute to global warming by transferring carbon dioxide from the ocean to the atmosphere. Climate models indicate that global warming will make hurricanes more frequent, so this newly-discovered process could make climate change accelerate once it is started. The reason for this ... is that the winds inside a hurricane are so violent they literally pull carbon dioxide from the sea’s surface. Once in the atmosphere, it adds to global warming. So the more hurricanes there are, the more there will be.” (BBC News, “Vicious Circles,” Sept. 2, 1998; Internet: http://news.bbc.co.uk/hi/english/sci/tech/newsid_163000/163610.stm).

This statement is a logical extension of the facts presented in the Nature paper. Other publications made the same connections: “... by encouraging global warming, hurricanes could in turn be spawning more hurricanes” [New Scientist 160, 12 (Sept. 5, 1998)]. National Public Radio echoed the same sentiment on its Sept. 2 edition of “All Things Considered,” but, as NPR’s Richard Harris commented, this is a broad-sweeping conclusion to draw from measurements of one storm.

The Christian Science Monitor (“How Hurricanes May Add to Global Warming” by R. C. Cowen; Internet: http://www.csmonitor.com/durable/1998/09/03/f-p4s1.htm) also noted the hurricane¾global warming connection. To its credit, the CSM article displayed a healthy dose of skeptical caution. The Sept. 3 edition of The Boston Globe carried an AP story (“Hurricanes May Add to Global Warming” by J. B. Verrengia; Internet: http://www.boston.com/dailynews/wirehtml/246/Hurricanes_may_add_to_global_warmin.shtml) that errone-ously reported “the Bermuda researchers measured the increased carbon dioxide levels generated by three hurricanes” and gushed “hurricanes hurl large amounts of carbon dioxide into the atmosphere as their winds furiously churn the ocean over hundreds of square miles.” However, it did go on to say “the researchers could not precisely determine the contribution that hurricanes might make to global climate change,” and it closed with a statement that put the issue in even closer perspective: “Hurricanes are powerful, but short-lived and localized events, and that makes them but one small factor in global climate calculations.”

The authors of the Nature paper, although extrapolating from a small sample and an isolated event to a large area and broad time frame, cautiously couched their conclusions in terms of changes in carbon flux. Their statement of the significance of the observations reads: “Although we expect that changes in surface-to-deep mixing remains the primary control of ocean uptake of CO₂ over multi-year, decadal timescales, CO₂ fluxes due to hurricanes provide an additional secondary feedback mechanism that is not accounted for in present global carbon cycle and climate models.”