Categorized | Climate Change

Bringing Better Tools to the Study of Sea Level Rise

A thorny problem for climate change adaptation is figuring out how to sort through projections and uncertainties to make a better determination of what impacts to expect. Two studies – one newly published, the other just underway – show promise for a better understanding, and therefore a more informed response, to the problem of sea level rise and its impacts on coastal ecosystems in California.

One of the more challenging issues related to climate change is the extent and timing of future sea level rise. While there is broad consensus that ocean levels will rise this century, due to a combination of melting of land-based ice, and the thermal expansion of ocean waters, it is very difficult to estimate the rate of this rise: for instance the 2007 IPCC report authors decided that they didn’t have enough information to determine the effects of changes in polar ice sheets, so they left them out of their projections. Since that time, scientists have refined measures to estimate the climate change impacts on the rate of flow of large ice sheets. Thus, while the IPCC’s 2007 sea level rise estimation was eight inches to two feet by the end of the century, by 2009 the US Global Change Research Program projected 3-4 feet of average sea level rise worldwide.

However, the story is not as simple as that. Sea level rise will have different impacts in different areas, depending on whether the land is also undergoing a natural subsidence (as is the case in the Chesapeake Bay region, where the land is still settling after the last Ice Age), and also on the amount of sediment being carried to coastal areas by the region’s river systems (for instance, multiple upstream dams deprive the Mississippi delta of much-needed sediments). Thus, predicting the future of a given marsh, for instance, is a function of both sea level rise and the relative rate of natural sediment accretion or subsidence. In a new study published in PLoS One, researchers at the Point Reyes Bird Observatory combined the best-available knowledge about rates of sediment deposition with scenarios for sea level rise to ascertain possible futures for the San Francisco Bay. They found that under the worst-case scenario, with high sea level rise (5 feet) and low levels of accretion of sediment and organic matter, the San Francisco Bay could lose 93% of its existing marsh within 50 to 100 years (human activities have already reduced tidal marsh in the Bay by about 90% from its historic extent). However, the authors also note that this worst-case outcome can be avoided by “conserving adjacent uplands for marsh migration, redistributing dredged sediment to raise elevations, and concentrating restoration efforts in sediment-rich areas.” They have developed a planning tool to help conservation practitioners where and when these changes would take place, to better inform restoration and land acquisition decisions.

The second study, recently announced by the California and North Pacific Landscape Conservation Cooperatives, will examine the impacts of sea level rise at five National Wildlife Refuges from Humboldt Bay in northern California, to Tijuana Slough near San Diego. The study, the first of its kind to monitor SLR impacts in such detail over such a long stretch of coastline, will “develop high-resolution digital elevation models (DEMs); monitor water levels and tidal cycles to assess local-level inundation patterns; inventory vegetation species composition and relationship to elevation and tides; and quantify sensitive wildlife use at all five refuges.”

Coastal marshes are incredibly productive habitats that supply food and shelter to a array of birds and other wildlife, and serve as nurseries for many ecologically and commercially important species. Already greatly diminished by shoreline development, pollution and activities like dredging and ditching, these systems are also on the front lines of the impacts of climate change. These new tools may hold the key to helping managers and planners enable marshes to adapt to and move with the rising sea.

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- who has written 22 posts on dotWild.

Aimee Delach is a Senior Policy Analyst at Defenders of Wildlife. Aimee develops policies to help land managers and decision-makers incorporate climate change threats into efforts to protect wildlife and habitats.

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