In spring and summer, the Chesapeake Bay, due to seasonal changes in its upper and lower water density profiles, is a storehouse of methane-generating detritus in its cold, dense, oxygen-starved lower strata. As hurricane season swings into full gear in mid-summer, a strong storm can bring about the phenomenon know as wind set-up which pushes water level higher on the western shore of the bay and an upwelling of water from the lower layer, pulling methane from the underneath the pycnocline and bringing it to the surface.
|From Chesapeake Bay: Introduction to an Ecosystem, Chesapeake Bay Foundation, 1995.|
We already know that estuaries throughout the world contribute something like 3% of total methane to the atmosphere without the large pulses just described. In such an event, however, the Chesapeake Bay (the U.S.A.'s largest estuary) could almost double that figure, were all the sub-pycnocline gas bubbles to escape. Hurricanes and tropical storms have impacted my Maryland home near the Bay about once every three years, of late. It is worth considering that major storms in late summer, when methane buildup is at its peak, may cause methane bursts of this magnitude.
To grasp the magnitude of such an event, let's say just one-third of the Bay's methane inventory were to bubble out during a major storm in mid-August. That would be 47 times the amount released over a 100 day period in the Aliso Canyon natural gas leak! (Aliso Canyon incident emitted 0.1 Tg. - the largest such leak ever in the U.S. Based on annual total worldwide CH4 emissions of 469 Tg, a 1% increase from Chesapeake Bay would be 4.7 Tg.)
We cannot shift the burden of climate change to ocean estuaries and blame the Chesapeake Bay for emitting all these greenhouse gases. These episodes would not occur if the Chesapeake Bay were in a healthy condition in which the lower strata maintained adequate oxygen year-round. This is anthropogenic and the cause is mainly excess nutrients and uncontrolled runoff. The tendency of storm intensity to increase as global temperatures warm also gives this punishing sequence of events a positive feedback quality. To stop it, we need to not only think about how to reduce the concentration of atmospheric CO2, but also how to clean up the water draining into our major estuaries.