If you were to spend a day sitting in Battery Park in Manhattan watching the ocean, you would see ships in the Upper Bay rise and fall with the daily tides. But if you could compare those ships with the trend over the last few decades, there would be no doubt that the ships have been gradually rising along with the water in the bay. A significant portion of this rise can be attributed to how the climate has been changing since the industrial revolution and how that has been affecting the oceans.
100 Million People at Risk
In a study recently released in the journal Nature Geoscience [Ref 1], a group of U.S. National Oceanic and Atmospheric Administration (NOAA) scientists predicted that the continued emission of greenhouse gases (GHG) at rates equal to or greater than the present-day value will result in sea-level changes that will be particularly hazardous to the east coast of North America. With this region being populated by approximately 100 million people and comprising a significant part of the U.S. GDP, any increased risk to the eastern seaboard infrastructure and economy could have broad implications.
We’re Already Above the Threshold
Using global climate models to compare the impact of various emission rates on the Atlantic and Pacific oceans, the study found that GHG emission rates of at least 5 GtC/yr (gigatonnes of carbon per year) results in a greater warming of the oceans that subsequently weakens a key ocean current that transfers heat from the equator to the North Atlantic. For perspective, the average global CO2 emissions rate from 2002 to 2011 due to fossil-fuel combustion and cement production was roughly 8 GtC/yr [Ref 1].
Thermal Expansion IS NOT Uniform
The weakened oceanic circulation slows the rate of heat movement within and out of the Atlantic Ocean, resulting in the warming of the interior of the ocean. Since water expands when heated, the warmer ocean leads to higher rates of sea level rise: a process known as thermal expansion. For several reasons, including the location of currents and geographical variations in water density within the oceans, the changes in sea level due to thermal expansion are not uniform across the ocean. This new study predicts that this process will result in the eastern coast of North America, from the Davis Strait between Greenland and Baffin Island down through the mid-Atlantic, experiencing some of the highest rates of sea level rise anywhere in the world by the end of the 21st century.
How Does Gravity Effect Sea Level?
Critically, the study’s east coast vulnerability conclusions may be conservative. A major effect that was not included in the NOAA study, which focused on the effects of thermal expansion within the oceans, is the melting of the Greenland and Antarctic ice sheets. Melting of these large ice sheets produces a very unique, and counterintuitive, pattern of sea level change [Ref 2]. When an ice sheet melts, the ice that is removed from land and water is added to the ocean. The reduced mass of the remaining ice results in a lower gravitational attraction between the ice and the water surrounding it.
With a weaker pull on the adjacent ocean, sea levels will actually be lower than the global average (and even fall) near the melting ice sheet. Additionally, the surface of the Earth in the vicinity of the melting region responds to the decreased weight of the ice by popping up, similar to how a ship rebounds when it is unloaded. Since sea level is defined as the depth from the ocean surface to the ocean floor, a higher sea floor near the ice sheets represents a sea level fall. Conversely, in regions far away from the melting ice sheets, the rate of sea level rise will be higher than the global average as the water is displaced. This could have dire consequences for the east coast of North America. As an example, melting of the West Antarctic Ice Sheet, an ice sheet for which the stability is unclear but which contains the equivalent of 5 m of global sea level rise, can lead to sea level changes along the east coast that are 25% greater than the global average [Ref 2].
Why the Insurance Industry is Very Concerned
The potential costs of sea level rise are huge. One risk is inundation, which describes the gradual encroachment of the oceans into areas that were previously above the water line. Assuming a continuation in the present-day state of emissions, estimates of the value of U.S. coastal property to be continuously underwater by 2100 is between $238 billion and $507 billion [Ref 3]. Low-lying Florida alone will likely see $53 billion to $208 billion (present-day USD) in destroyed property.
But inundation is not the only risk. Higher sea levels increase the reach and subsequent damage of storm surges or flooding that can occur during hurricanes or other severe coastal storms. Estimates have the New York and New Jersey shores, two states hit hard by Hurricane Sandy in 2012, experiencing 11% to 27% and 14% to 36% increases in flood storm damage by 2030 [Ref 3].
Impacts Could Be Far Worse
It is important to appreciate that both the storm surge and inundation damage estimates were made prior to these recent NOAA predictions of increased rise along the east coast. As such, they could represent underestimates of the potential cost. Significant reductions in GHG emissions must be made to reduce the hazard described by the NOAA predictions and other sea level estimates. If not addressed, the financial and societal costs, particularly to those living along the eastern coastline of North America could be catastrophic.
Ref 3 – http://climateprospectus.org/
Eric Morrow is a climate scientist and professional engineer with 14 years of experience in quantitative and statistical analysis, project management and collaboration with international project teams.