Environmental Engineering Reference
In-Depth Information
Accelerated Sea Level Rise
A major concern related to climate change is rising sea level associated
to the melting of ice-sheet, land ice, and thermal expansion of the ocean
(Webb III et al. 1993, Wigley and Raper 1993). During the last 100 years,
eustatic sea level has risen was about 1.5 to 2 mm per yr (Miller and Douglas
2004), but local rates of relative sea level rise are highly variable, due to
regional differences in groundwater and oil withdrawal, compaction of
muddy soils, subsidence, isostatic rebound, and tectonic uplift. The rate of
rise is predicted to accelerate in the 21st century, but the actual amount of
increase is diffi cult to tell precisely. While predictions based on empirical
relationships between temperature and sea level suggest rates of sea level
rise of 1 m or more in the 21st century (Rahmstorf 2007), IPCC projections
span from 9 to 88 cm rise by 2100. Model averages range more narrowly
from 28 to 43 cm above the global sea level at the beginning of the century
(IPCC 2007), but uncertainties for regional predictions are about 50% greater
than for the global average.
There has been considerable discussion as to how coastal wetlands will
develop in the future under climate enhanced sea level rise (Reed 1990,
Simas et al. 2001). Early studies (Boorman et al. 1998, Titus 1987) predicted
the large-scale loss of coastal wetlands as a consequence of sea level rise
exceeding sediment supply (Temmerman et al. 2004), or the infl uence of
sea-level rise on marsh productivity (Morris et al. 2002). However, there is
some evidence to suggest that, at some locations, the geomorphic response
of salt marshes is not sediment limited. Many temperate salt marshes built
from allochthonous sediment show a signifi cant excess of vertical sediment
accretion relative to sea level rise (French 2006, Stupples and Plater 2007). In
the Mississippi Delta, accretion rates greater than 10 mm year
−1
have been
measured where there is suffi cient sediment input from the river (Cahoon
et al. 1995, Conner and Day Jr. 1991, Day et al. 2000, Hatton et al. 1983),
and mangroves in many estuaries in northern Australia tolerated sea level
rise of 8-10 mm per year in the early Holocene (Woodroffe 1995). These
accretion rates are higher than most projections, and suggest that coastal
wetlands can persist at a given location, in spite of high rates of sea level
rise, if there is suffi cient mineral and organic soil formation.
Nevertheless, human activities alter the ability of wetlands to accrete
both at local and regional scales, and enhanced sea level rise has led to
signifi cant changes on coastal systems, mainly associated with salinity
intrusion in estuaries and altered sediment transport. There are numerous
examples of detrimental effects of accelerated sea level rise on coastal
wetlands around the world, including Chesapeake Bay, the Mississippi
Delta and other Atlantic estuaries in North America (Day et al. 2007, Day et
al. 2003, Hackney and Cleary 1987, Stevenson et al. 1985), Rhone, Ganges,
