Tuesday, May 12, 2009

Hurricanes might contribute to global warming

In a large-scale study published this week in Proceedings of the National Academy of Sciences, Hongcheng Zeng and colleagues show that hurricane damage can diminish a forest’s ability to absorb carbon dioxide from the atmosphere. Their results suggest that an increase in hurricane frequency due to global warming may further amplify global warming.

The annual amount of carbon dioxide a forest absorbs from the atmosphere is determined by the ratio of tree growth to tree mortality each year. When hurricanes cause extensive tree mortality, not only are there fewer trees in the forest to absorb greenhouse gases, but these tree die-offs also emit carbon dioxide, thus potentially warming the climate.

Using field measurements, satellite image analyses, and empirical models to evaluate forest and carbon cycle impacts of hurricanes, the researchers established that an average of 97 million trees have been affected each year for the past 150 years over the continental United States, resulting in a 53-million ton annual biomass loss and an average carbon release of 25 million tons per year. Over the period of 1980–1990, released CO2 potentially offset carbon absorption by forest trees by 9–18% over the entire United States. Impacts on forests were primarily located in Gulf Coast areas such as southern Texas, Louisiana, and Florida, but significant impacts also occurred in eastern North Carolina.

These results have important implications for evaluating positive feedback loops between global warming and environmental change.

Zenga, H., J. Q. Chambers, R. I. Negrón-Juárez, G. C. Hurtt, D. B. Baker, and M. D. Powell. (2009). Impacts of tropical cyclones on U.S. forest tree mortality and carbon flux from 1851 to 2000. PNAS, 106 (19), 7888-7892. DOI:10.1073/pnas.0808914106

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