If, like me, you thought sea level rise was as simple as ice melting and the ocean rising, like a five pound block of party ice melting into warm kiddies pool, then you need a rethink about sea level rise. Even though the ocean is rising overall, complex forces cause sea levels to rise in some places and drop in others. Yet out of a bewildering array of factors, scientists have determined that Global Mean Sea Level (GMSL) rise is picking up speed.
Dr. Carling Hay, a leading researcher in the field, presented results by a team of experts that evokes a sobering view of what the future may hold. She presented her report at the American Geophysical Union (AGU) Annual Meeting in San Francisco last week.
Dr. Carling explained that our old idea of ice melting and the seas rising uniformly is wrong. Big ice sheets like those in Greenland exert gravitational attraction on the oceans nearby, and when an ice sheet melts away its reduced gravity causes the nearby ocean waters to drop and sea levels farther away to rise. Every large ice store has its own “signature” of such water loss and gravitational effects, and sea levels both locally and across the planet change unevenly. Even so, such “static effects” are only one of a number of factors that affect sea levels in a given locale. Dynamic effects like the water’s thermal expansion, atmospheric process, ocean currents, and other factors can all contribute to uneven local effects. Thus sea levels can rise faster, slower, or sometimes even go down for a given island chain or coastline, depending on a host of factors. Still, the overall trend for most of the world is decidedly up.
Earlier estimates of sea level rise were based on sparse tidal measurements that started in the mid-19th century. Until recently records did not cover enough locations across the globe nor did researchers have a sufficient theoretical understand of the problem’s complexity to get an accurate picture of what was happening. In the last 20 years measurements have improved dramatically with the advent of satellites that measure the exact altitude of ocean surfaces. With such measurements, plus other refined techniques, various theories and estimates of sea level rise have come into strong agreement. Better data and better models have led Dr. Hay’s research team to find that GMSL rise for the period 1900 to 1990 was about 1.2 (plus or minus 0.2) mm/yr, a revision downward of about 20% from earlier scientific estimates. Things change rather dramatically, however, beginning around 1990. During the period 1990 to 2010 there has been an marked acceleration, with seas rising at an average of 3.0 mm (plus or minus 0.7 mm) per year. Coastal communities in Alaska, Pacific Ocean island nations, and other places are already experiencing dramatic effects from rising seas and warming temperatures. An acceleration of rising waters with the magnitude shown by the new data may portend dramatic new problems soon.
Authors of the research cited in Carling Hay’s presentation include:
Carling Hay: Rutgers University New Brunswick and Harvard University
Eric Morrow: Rutgers University New Brunswick
Robert Kopp: Rutgers University New Brunswick
Jerry Mitrovica: Harvard University