Wednesday, July 6, 2016

Bay Burps

Laura Lapham, et. al., authors of the paper referenced in my previous post state that the methane bubbles suspected to be lurking under the pycnocline are 85% oxidized by the time the layers of the Chesapeake Bay mix in autumn. This leaves 15% of the methane free to circulate into the atmosphere when the final mixing occurs.

Termed "fall turnover," this mixing sometimes occurs as quickly as overnight when air temperatures drop and wind increases wave action. A quick fall turnover, like a tropical storm surge, can result in sudden bursts of methane from the bay. At 15% of the inventory, the amount available is only about half of the hypothetical storm release noted previously. That is only 23 Aliso Canyon incidents spread around the bay, happening overnight (vice 100 days).

Turnover usually does not occur all at once everywhere in the bay, so chances of this much of a release are small, but with wild weather swings from climate change, it is more likely that all the local turnovers will still add up to something close to turnover en masse.

Measurements of surface methane releases showed that there is significant flux into the atmosphere from the bay even when there is no major storm or turnover. What my educated guess adds up to is about (23 + 47) = 70 Aliso Canyon equivalents every 3 years or, on average, 7/3 Tg = 2.33 Tg CH4 annually contributed by the Chesapeake Bay, which is about 6 times the amount spewed annually from the city of Los Angeles.

We've long known that estuaries emit methane disproportionately more than their relative geographic extents, but the inclusion of heretofore unnoticed sub-pycnocline burps makes the Chesapeake Bay, and other polluted water bodies with similar methane-storing mechanisms, even larger concerns in the fight against global warming.
Photo by Jason R. Berg

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