2 September 2010
Carbon dioxide releases by hurricanes are significant, but offset by ocean cooling and phytoplankton growth
A hurricane’s passage over warm ocean waters can drive a significant amount of carbon dioxide from the waves to the sky. The violent winds associated with a passing storm can dramatically increase the gas exchange between the ocean and the atmosphere, sending over a hundred million metric tons of the greenhouse gas into the atmosphere every year.
In an upcoming paper, Peisheng Huang and Jörg Imberger from the Centre for Water Research in Crawley, Australia used data collected during Hurricane Frances and a computer model to calculate how much carbon dioxide transitioned from ocean to air during the storm. They then used this information to produce an annual estimate of CO2 transfer due to hurricanes.
Huang and Imberger calculated that Hurricane Frances would have launched between 3.5 million and 10.4 million metric tons of carbon dioxide into the air. My own carbon footprint, which I figured out using the calculator over at The Nature Conservancy, is about 14 tons of CO2 per year. This means that in a span of three weeks, a single hurricane managed to toss as much carbon dioxide into the atmosphere as up to 740,000 apartment-dwelling, bus-riding people like me would manage in a year. That’s just under the population of Columbus, Ohio.
The researchers’ global estimate for the amount of carbon dioxide released by all hurricanes worked out to between 47 million and 141 million metric tons of CO2, which is about a quarter of the CO2 emitted in the United Kingdom in an entire year. (Or, about 2.4 percent of the United States’ yearly CO2 output.)
To figure this out, the researchers used data collected by floats which were dropped in the North Atlantic the day before hurricane Frances ploughed through in 2004. One was positioned 110 kilometres (68 miles) to the right of the storm track, another 55 km (34 mi) out where the winds would be strongest, and a third right in the storm’s path. Instruments on the floats measured the variables that help control the concentration of dissolved CO2, and the rate at which the gas is passed between air and water.
The ability of the oceans to store dissolved carbon dioxide gas depends on a number of factors, including water temperature, the amount of other carbon compounds in the water, its salinity, and its alkalinity (the ability of the water to neutralize any added acids). There are a constant series of reactions going on in the oceans between dissolved CO2 and other carbon compounds, and these interactions are the reason scientists worry about climate change’s potential to cause ocean acidification.
Huang and Imberger found that high-powered winds were the main cause for the large CO2 releases. When a hurricane goes by, the high winds boost vertical mixing within the water column, bringing cold water from below up to the surface. This upwelling causes the water temperature to drop between 1 and 6 degrees Celsius (1.8 to 10.8 degrees Fahrenheit), which makes the affected ocean region release about 4 percent of its dissolved CO2 for each degree Celsius the temperature drops. The increased mixing also brings nutrients up from the deeper waters below, causing changes in ocean chemistry which works to limit the gas’ escape. The intensity of the hurricane also plays a role – hurricanes that last longer and blow stronger extract more CO2 out of the ocean.
However, the researchers are uncertain about how important this process might be to global CO2 levels once the storm has waned. “The out-gassing of CO2 caused by the hurricane passage may be just a short-term event,” they write. “Long-term studies have indicated that there is no clear relationship between hurricane activities and the net air-sea CO2 fluxes on an annual basis.”
In a follow-up unpublished study, Huang and Imberger tried to estimate the long-term effects on CO2 exchange due to a hurricane’s passage. In the wake of a hurricane’s passage, there is often an explosion in phytoplankton populations as the increased vertical mixing brings nutrients up to the surface where the tiny plant-like organisms are better able to use them. The boost in activity by the phytoplankton quickly draws some of the CO2 back out of the air as the microorganisms use it to fuel their growth.
However, the researchers found that even though the phytoplankton undergoes a significant boom in concentration following the storm, this doesn’t impact the overall CO2 exchange rate much. The key factor, they found, is that surface waters along the storm track are left just a little bit colder after a hurricane blows through. These cold patches last more than a month and reduce the CO2 transfer between the ocean and the atmosphere while the water temperature recovers.
In the end, Imberger and Huang found that the amount of CO2 taken back up by the seas, or prevented from escaping thanks to the cold patches, more or less offsets the amount kicked out by the hurricane. Now if only I could trick some phytoplankton into growing along the path my bus takes to work…
P. Huang, & J. Imberger (2010). Variation of pCO2 in ocean surface water in response to the passage of a hurricane J. Geophys. Res. : 10.1029/2010JC006185
– Colin Schultz, AGU science writer