Scientists have been studying and reporting for the past 50 years and longer how earth’s atmosphere and oceans have been steadily warming at rates unprecedented in recent geologic history, rates that exceed any possible natural forces. The evidence is clear that global warming is primarily a direct result of the increase of greenhouse gases in the atmosphere from our burning coal and other fossil fuels. The slowly rising temperatures are having many often subtle but measurable impacts on Earth’s terrestrial and ocean ecosystems. These impacts of climate change will continue to increase for centuries unless carbon emissions are severely reduced.
One of the most well-known and most severe impacts is rise in sea level, caused by a combination of melting of land-based ice sheets and thermal expansion of the oceans. For Falmouth, as a coastal community already facing serious coastal issues such as erosion, storm-surge flooding, and salt-water intrusion into freshwater aquifers, the risk of sea-level rise and other global warming impacts deserves immediate attention from town leaders and the public alike.
Scientific projections of sea-level rise are imprecise due to a combination of lack of reliable data on likely future carbon emissions and especially the complexity of Earth’s systems that affect weather and ultimately global climate. As such, projections are usually given as a range of values over longer periods of time, say the next 50 years to the year 2100. As computer modeling has improved over the past three decades and more observations have been made, projections on sea-level rise have steadily increased. Records show that sea-level rise lags global temperature rise by decades. The global rise since 1880 was about 8 to 9 inches; however, since 1993, sea-level has risen about 3 inches, a 50 percent acceleration. For this region, sea level rise relative to the land since 1880 has been about 12 inches.
Current scientific projections are a minimum 3-foot rise in average global sea level by 2100 and possibly 6-foot or more, depending on rates of global warming and rates of ice melting across Greenland and Antarctica, the largest repositories of ice on earth. For Cape Cod and much of New England, “relative” sea-level rise will be even higher than average due to geologic conditions that the land is gradually sinking across the entire region, a result of “readjustment” of crustal rock that continues from the last glacial period that ended 20,000 years ago when globally sea level was about 400 feet lower than present.
Clearly awareness and planning for coastal resilience is needed to deal with long term sea-level rise, but a more immediate risk to Falmouth and other Cape communities is “nuisance coastal flooding.” This is extreme flooding of low elevation areas that occurs mostly during high spring tides in conjunction with winds from the southwest. Sea-level rise is causing high tides to reach higher ground and flood larger areas and the frequency and duration of flooding are increasing. Impacts from recurrent coastal flooding include overwhelmed stormwater drainage capacity, frequent road closures, and general deterioration and corrosion of infrastructure (e.g. bridges, sewer/septic systems, roads) not designed to withstand frequent inundation or salt-water exposure. Examples of these impacts are occurring along all the coast and I’ve witnessed increased frequency in the 17 years that I’ve lived in town and observed many significant changes to the coast. An example is the photo of the West Falmouth dock area inundated during such a nuisance high tide event. The second figure is a map showing flood zones associated with four intensities of tropical storms. For nuisance flooding vulnerability, the yellow areas of impacts of category 1 storms can be useful in assessing town-wide vulnerabilities.
Dr. Williams is senior scientist emeritus with the US Geological Survey in Woods Hole and on the Falmouth Water Stewards board of directors, who has a more than 40-year career studying coastal processes and sea-level rise impacts on coasts and wetlands.