sábado, 25 de agosto de 2018


SEA LEVEL RISE. Fredric M. Menger, Emory University, Atlanta, GA, USA

In a previous blog (T4), Clifford A. Bunton wrote a piece entitled, “Why I Believe That Global Warming Is Real.” Few people today question global warming as a source of deadly hurricanes, drought, floods, forest fires, heat waves, and (the subject the present essay) sea level rise. Most scientists ascribe these events to climate changes that are human-driven, i.e. a worldwide release of carbon dioxide into the atmosphere from automobiles, industrial plants, and agriculture. Experts have estimated that burning one gallon of gasoline translates into adding 400 gallons of water volume to the ocean, an amazing statistic (if accurate).

In the 20th century, ocean levels rose by a global average of about 14 cm due mainly to the melting of Arctic and Antarctic ice. Projections for a sea level rise by the year 2100 vary widely from 0.2 meters (0.7 feet) to as high as 2.0 meters (6.6 feet). Estimating the prospects for sea-level change is a complex business aside from assessing the details of future climate behavior. For example, all models must take into consideration that ice-melting is autocatalytic. Thus, as ice melts, the exposed bare earth absorbs more of the sun’s heat than does the original ice, thereby accelerating the subsequent rate of warming. Similarly, as the Arctic’s permafrost (sub-surface ice that in the past never melted) melts with warmer temperatures, methane is released, and increased warming ensues. Since water becomes less dense as it warms, this alone increases sea level. And the Antarctic and Greenland ice sheets are so massive that the resulting gravitational effects “pull in” the nearby ocean; this collected water is released into the ocean body as the ice sheets disappear. The earth’s rotation, the sinking of certain cities (e.g. Jakarta), the local sea-bed topography, and the danger of earthquakes all fuel uncertainty in flood predictions for any given city.

Muir Glacier at Glacier Bay National Park and Preserve in Alaska is among the many worldwide that are disappearing. Muir, left, as seen in August 1941, and photographed in August 2004.CreditW. Field; B. Molnia/U.S.G.S., via Glacier Photograph Collection

Despite the uncertainty, it seems clear that without corrective action humans will face grave difficulties in the future. To illustrate the dangers, I will focus on Mumbai, India, a so-called mega-city with a metropolitan area of >21 million people located on the Arabian Sea. When the Portuguese settled the region in the 16th century, they occupied only a group of coastal islands. But after the British took over in 1661 they interconnected the islands to create a peninsula. Thus, much of Mumbai is built on top of landfill, only slightly above sea level, such that a small increase in sea level could promote devastating floods. Sea water is not the only danger. Mumbai receives 85 inches of rain a year, mainly during the June-September monsoon season, turning streets into rivers and paralyzing India’s financial capital (as happened last year when a 33 cm of rain fell in a single day).

What to do about this? Since sea rise seems like a more distant threat, at least relative to more urgent social needs, governments are hesitant to spend the huge sums necessary to remedy the situation. In any case, even a massive sea wall, protecting Mumbai from the ocean west of it, is questionable because the wall would retain fresh water entering from the east side. Other strategies include ceasing to pave over water-absorbent soils, improving storm drains, and restoring protective dunes and mangrove trees. Perhaps the best solution is to take the tenets of the Paris Climate Agreement seriously. The agreement acknowledges that the threat of climate change is "urgent and potentially irreversible," and can only be addressed through "the widest possible cooperation by all countries" and "deep reductions in global emissions." Unfortunately, the U.S. has pulled out of the accord, but the hope still remains that the world will strive to limit its CO2 output by replacing coal-fired power plants with sun and wind technology.

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