Wednesday, January 18, 2017

Sea Level Rise - by National Geographic

Ocean levels are getting higher.  Why is this happening, and what can we do to stem the tide?

Families in Kiribati, especially those new to the island nation, are often forced to live in marginal areas, where flooding from high tides is increasing.  (Photograph Credit: Kennedy Warne) Click to Enlarge.
Core samples, tide gauge readings, and, most recently, satellite measurements tell us that over the past century, the Global Mean Sea Level (GMSL) has risen by 4 to 8 inches (10 to 20 centimeters).  However, the annual rate of rise over the past 20 years has been 0.13 inches (3.2 millimeters) a year, roughly twice the average speed of the preceding 80 years.

Over the past century, the burning of fossil fuels and other human and natural activities has released enormous amounts of heat-trapping gases into the atmosphere.  These emissions have caused the Earth's surface temperature to rise, and the oceans absorb about 80 percent of this additional heat.

The rise in sea levels is linked to three primary factors, all induced by this ongoing global climate change:
  • Thermal Expansion:  When water heats up, it expands.  About half of the past century's rise in sea level is attributable to warmer oceans simply occupying more space.
  • Melting Glaciers and Polar Ice Caps:  Large ice formations, like glaciers and the polar ice caps, naturally melt back a bit each summer.  In the winter, snows, primarily from evaporated seawater, are generally sufficient to balance out the melting.  Recently, though, persistently higher temperatures caused by global warming have led to greater-than-average summer melting as well as diminished snowfall due to later winters and earlier springs.  This imbalance results in a significant net gain in the ratio of runoff to ocean evaporation, causing sea levels to rise.
  • Ice Loss from Greenland and West Antarctica:  As with the glaciers and ice caps, increased heat is causing the massive ice sheets that cover Greenland and Antarctica to melt at an accelerated pace.  Scientists also believe meltwater from above and seawater from below is seeping beneath Greenland's and West Antarctica's ice sheets, effectively lubricating ice streams and causing them to move more quickly into the sea.  Higher sea temperatures are causing the massive ice shelves that extend out from Antarctica to melt from below, weaken, and break off.
When sea levels rise rapidly, as they have been doing, even a small increase can have devastating effects on coastal habitats.  As seawater reaches farther inland, it can cause destructive erosion, wetland flooding, aquifer and agricultural soil contamination, and lost habitat for fish, birds, and plants.

Read original at Sea Level Rise

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