A study of how sea level rise and cyclone intensity will affect New York suggests the level of damage wrought by Hurricane Sandy is set to become a disturbingly frequent event. Disasters almost this large are now 20 times as likely as they would have been before humans started messing with the planet’s thermostat. Even more ominously, this impact has already occurred. Future climate changes will only make it worse.
In PNAS first author Andra Reed of Pennsylvania State University compared the flood risk from 850-1800 with that between 1970 and 2005. “The observational record of tropical cyclones in the North Atlantic basin is too short (A.D. 1851 to present) to accurately assess long-term trends in storm activity,” the authors write.
Instead the researchers modelled conditions for the millennium leading up to 1800 (the "pre-anthropogenic era") and compared them with models for 1970-2005 (the "anthropogenic era"). They found New York has had a double shot of vulnerability in recent years – sea levels have risen, and tropical cyclones have become more powerful. Consequently 2.25 meter (7.4 foot) floods have gone from being one-in-500-year events to something anticipated to come along every 24.4 years. Average flood heights have increased by 1.24 meters (4 feet).
“Tropical cyclones (TCs) and their associated storm surges are the costliest natural hazards to impact the U.S. Atlantic coast,” the paper notes. Hurricane Sandy destroyed 650,000 homes, brought America’s premier city to a standstill and is estimate to have caused $50 billion damage.
Deposits of seashells and sediments in southern New Jersey allowed the researchers a precise measure of sea level in the area, with a rise of 1.3 meters (4.3 feet) from a 1000 years ago. While much of this rise is natural, it has accelerated sharply as a result of recent emissions of greenhouse gasses.
Changes to TCs are harder to track. While cyclones are powered by warm water, of which there is much more these days, they are also disrupted by wind shear, which appears to be increasing as the world warms.
The interaction of these two variables may be reducing TC numbers, but increasing the frequency of the most extreme events, and causing them to peak further from the equator. While modelling of this component is less precise than sea level estimates, as the smaller component any errors here are unlikely to greatly change the overall picture.
The worst damage occurs when a cyclone coincides with a high tide, which contributes about 1.5m (five feet) to sea height in the area. However, many events are so long lasting they are almost certain to overlap with a high tide.
Tidal contributions aside, Sandy’s surge was 2.7 meters (9 feet), so events on that stage may still be once-in-a-lifetime, but the crucial point where the subway system floods may be expected far more frequently.
The study did not take into account the way land-use changes have affected storm impacts. Barriers, whether natural or man-made can reduce or redirect storm power, and replacing soil with concrete reduces the ground's capacity to absorb water and mitigate the damage.