Download Global Warming and the Planetary Water Cycle

Global Warming and the Planetary Water Cycle
Ruth Curry
Research Specialist, Physical Oceanography
Woods Hole Oceanographic Institution
Rising greenhouse gas concentrations and global warming are altering the
processes that control the cycling of freshwater around the planet’s surface: evaporation,
precipitation, freezing and melting. In the 21st century, major issues confronting
humankind will almost certainly include sea level rise, freshwater resources, shifting
weather patterns and drought ― especially in the western U.S.
As our planet’s surface temperatures rose in the last half of the 20th century, it
became clear that the oceans were warming at a considerably faster rate than the
atmosphere [Levitus et al., 2001]. Climate research increasingly points to
anthropogenically elevated greenhouse gas concentrations, principally carbon dioxide, as
a major contributor to the observed warming [IPCC, 2001]. From ice core evidence, it is
known that carbon dioxide levels have fluctuated naturally over the past 400,000 years
―between 190 and 290 ppm― and global temperatures have warmed and cooled
synchronously resulting in glacial / interglacial cycles [Petit, et al, 1999]. Atmospheric
carbon dioxide levels are presently at 379 ppm with virtually all of the rise (from 290
ppm circa 1750) having taken place over the past 120 years [Etheridge, et al, 1998].
The planetary climate system is responding to this sharp rise in carbon dioxide in
more ways than just its temperature. There is ample evidence that the hydrologic engine
that cycles freshwater around the planet is changing, too. Although directly measuring
evaporation, precipitation, freezing and melting on global scales has not been practical
until very recently, the footprints of changes in these processes can be found in ocean
salinity distributions. Evaporation and freezing remove freshwater from the ocean, leave
salt behind, and raise salinity concentrations; while precipitation and melting both dilute
the salt content of seawater. The measurement record, which extends reliably back to the
1950s, reveals salinity increases at low latitudes in the Atlantic, Indian and South Pacific
Oceans that contrast sharply with decreases in the North Pacific Ocean and mid- to highlatitudes of both hemispheres [Boyer et al, 2005;Curry et al., 2003; Dickson et al., 2002;
Wong et al., 1999]. These global patterns suggest that an overall shift of Earth’s
freshwater balance is underway. Evaporation rates over warmer tropical and subtropical
oceans appear to have increased by ~10% in the last 20 years. In the Northern
Hemisphere, that extra water vapor is bypassing the mid-latitudes ― which have been
drying measurably since 1997 and producing severe drought conditions in the western
U.S. [Hoerling & Kumar, 2003]. The precipitation is instead falling in the high latitudes
across North America, Europe and Asia, elevating the rates of river runoff into the Arctic
Ocean [Serreze et al., 2000; Peterson et al., 2002].
As the global thermometer continues its upward trend, the picture emerging in the
last decade is of a climate system in transition, especially near Earth’s poles. Ice is
melting just about everywhere on the planet. Satellite pictures of Arctic sea ice provide
startling evidence of the thinning and shrinking that is occurring [Comiso et al. 2004].
Land ice and permafrost are in decline all around the Arctic [Dyurgerov and Carter,
2004; Zwally et al., 2002; Schiemeier, Nature,2004]. Greenland ― which stores enough
freshwater to raise global sea level by ~7 meters ― is now unquestionably losing mass.
Its summer melt area has increased by 15 –25% since the 1970s, its ice shelves are
beginning to break up, and melt water is percolating downward and lubricating the base
of its glaciers. The prognosis is that Greenland will take several centuries to melt
[R.Alley, personal communication]. But its release of freshwater will make sea level rise
a very significant issue in this century. At the opposite pole, the largest volume of land
ice ―Antarctica ―is still gaining mass. However, its ice shelves are becoming unstable
because of warming ocean temperatures. The disintegration of the Larson “A” and “B”
Ice Shelves changed notions of how fast and how completely ice shelves can break up.
The event also confirmed ideas that ice shelves control the rates of land ice movement
such that glacial surge results when an ice shelf is removed.
References
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