Glacial Melt and Ocean Warming Drive Sea
Level Upward
The average sea level around the world has risen a total of
222 millimeters (mm) since 1875, which means an annual rate
of 1.7 mm. Yet at the end of this long period, from 1993 to
2009, the sea level rose 3.0 mm per year—a much faster rate.
An estimated 30 percent of the sea level increase since 1993
is a result of warmer ocean temperatures that cause the
water to expand (thermal expansion). Another 55 percent of
the increase results from the melting of land-based ice,
mainly from glaciers and the Greenland and Antarctic ice
sheets. The other 15 percent of the rise is due to changes
in terrestrial freshwater dynamics, such as wetland drainage
and lowered water tables.
Ocean warming and land-based ice melt have happened in
tandem with other climatic changes during the last century.
These changes include rising atmospheric temperatures,
acidification of ocean waters, and changes in seasonal water
cycles—all of which are linked to a dramatic increase in
atmospheric greenhouse gases. Prior to the industrial
revolution, the atmospheric concentration of carbon
dioxide—a major greenhouse gas—was steady at around 280
parts per million (ppm).7 Since then, human
activities such as the burning of fossil fuels and land use
changes have boosted this concentration to over 385 ppm,
nearly a 38-percent increase.8
The world’s oceans absorb 80–90 percent of the excess
solar radiation trapped on Earth by greenhouse gases.9
But because the ocean’s mass is so much greater than the
atmosphere’s, the oceans warm at a slower rate. From 1969 to
2009, atmospheric temperatures rose 0.36 degrees Celsius
while the temperature in the upper ocean (the area down to
700 meters) rose 0.17 degrees.10 (See Figure 2.)
Read:
Glacial Melt and Ocean Warming Drive Sea Level Upward by
John Mulrow and Alexander Ochs, with Shakuntala Makhijani
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