“Could future greenhouse gas-induced global warming push the Earth’s climate system into an unstable mode, triggering a catastrophic meltdown of the polar ice sheets?”—Vivien Gornitz
The follow post is from Vivien Gornitz, author of Rising Seas: Past, Present, Future.
Superstorm Sandy, although a rare and freakish event today, was a rough taste of what may await us as ocean levels continue to rise. New York City is no stranger to tropical cyclones, in spite of its northerly location. The surge from a hurricane in 1821 reached 13 feet in 1 hour and flooded parts of lower Manhattan as far north as Canal Street. In 1893, another hurricane submerged southern Brooklyn and Queens, erasing a small barrier island off the Rockaways. During the twentieth century, the “Long Island Express” (1938), hurricane Donna (1960), and the weaker hurricane Gloria (1985) created extensive damage on nearby Long Island and in New Jersey. Even winter nor’easters, such as one in December, 1992, flooded low-lying neighborhoods and seriously disrupted ground and air transportation. But the fury and destructiveness of Sandy topped these all, aided by the historic 1.4 foot rise in sea level since the mid-19th century.
At least eight times during the last million years, vast ice sheets blanketed much of the Northern Hemisphere and subsequently retreated. Both sea level and greenhouse gas concentrations fell during the ice ages and rose again as the ice sheets shrank. Sea level climbed 13 to 20 feet higher than present during the last warm interglacial period, 125,000 years ago, but then dropped 394 feet (120 meters) at the peak of the last ice age, around 20,000 years ago. Once the ice sheets began their retreat, sea level rose rapidly and climbed still faster in several episodic spurts. After the ice melted, the sea reached nearly its present height by 7,000 to 6,000 years ago, fluctuating at most by a few feet since then.
Climate skeptics like to point to past wide variations in climate and global sea level as proof that we are merely experiencing yet another natural variation. Anthropogenic atmospheric greenhouse gases are heating the Earth. Carbon dioxide (394 parts per million in 2012, http://www.esrl.noaa.gov/gmd/ccgg/trends/) approaches levels last experienced in the balmier Pliocene epoch, around 3 million years ago, when sea levels stood over 66 feet (20 meters) higher than present. Temperatures now reach 1.0 F (0.6 C) above the mid-twentieth century average, with the nine warmest years in the 132-year record occurring since 2000 (http://giss.nasa.gov/research/news/20130115/). The climatic effects are most pronounced near the poles and on lofty mountaintops.
The pace of sea level rise has quickened since the late 19th century, averaging around 1.7-1.8 mm/yr throughout most of the 20th century, climbing even faster to 3 mm/yr within the last 20 years. This closely parallels the rising trajectory of global temperature. Sea level is likely to rise even higher as the Earth’s climate warms, due to thermal expansion of the oceans and melting of glaciers and ice sheets, which have receded dramatically in many places especially within the last few decades. Coastal cities, ports, and wetlands would be threatened with more frequent and severe flooding, increased beach erosion, and saltwater encroachment into coastal lagoons, streams, and aquifers.
Could future greenhouse gas-induced global warming push the Earth’s climate system into an unstable mode, triggering a catastrophic meltdown of the polar ice sheets? Would major coastal cities with millions of inhabitants, like New York City, Los Angeles, London, Tokyo, Shanghai, Hong Kong, Mumbai (Bombay), Bangkok, to name just a few, be inundated and rendered uninhabitable? As of 2005, around 40 million city dwellers worldwide were vulnerable to the 1-in-100 year flood (i.e., a coastal flood having a 1 percent chance of occurring in any given year). In the United States (excluding the Great Lakes), 8.4 million people (or approximately 3 percent of the 2000 population) live within the 1-in-100 year flood zone. Even today, many coastal residents face flood hazards from major storms, such as Sandy. As sea level rises, the surges unleashed by large cyclones will reach higher and penetrate further inland. In addition, the strength, if not frequency, of such extreme meteorological events could also magnify in a warmer world.
Recent observations of Greenland and the West Antarctic Ice Sheet raise concerns for the future. Satellites detect a thinning of parts of the Greenland Ice Sheet particularly at lower elevations, and glaciers are disgorging ice into the ocean more rapidly. Even parts of the West Antarctic Ice Sheet are showing some signs of thinning. The Greenland and West Antarctic Ice sheets, if melted completely, contain enough ice to raise sea level by 23 and 10 feet (7 and 3m), respectively. It would take a global temperature rise of only 1.4-6F (0.8-3.2ºC) to destabilize Greenland irreversibly. These temperatures lie within the range of several future climate projections for the 21st century. Nonetheless, any significant meltdown would take many centuries. Furthermore, even with possible future accelerated discharge from the West Antarctic Ice Sheet, it highly unlikely that future annual rates of sea level rise would exceed those of the major post-glacial meltwater pulses.
While a major destabilization of the polar ice sheets appears unlikely within the next century or two, it cannot be ruled out entirely. Therefore, a certain sense of urgency motivates us to learn more about the causes and consequences of sea level rise and begin preparations for the future. Rising Seas: Past Present, Future addresses these issues and enables us to comprehend why sea level rise truly matters. The recent climate and sea level trends reinforce the book’s conclusions that we are moving toward an increasingly warmer and, quite likely, a more aquatic planet. We do well to take heed and begin preparations to stem the oncoming tide.