Largescale Salinity Changes

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According to the Woods Hole Oceanographic Institution, tropical ocean waters have become much saltier over the past 40 years, while the polar waters have become much fresher. In addition to this, there is increased warming at Earth's surface, which has increased evaporation over the low-latitude (equatorial) oceans. When water is evaporated, it picks up freshwater, leaving the salt behind. Carried by Earth's circulation patterns, the freshwater moves toward the polar regions where it is deposited as precipitation, adding still more freshwater to those extreme latitudes.

Global warming may be one of the processes that is contributing to this shift. A study conducted by Ruth Curry (at Woods Hole), Bob Dickson (of the Centre for Environment, Fisheries, and Aquaculture Sciences, in the United Kingdom), and Igor Yashayaev (at the Bedford Institute of Oceanography, in Nova Scotia, Canada) suggests that an acceleration of Earth's global water cycle can cause far-reaching effects, leading to droughts, floods, and storms. In addition to the current melting of polar glaciers, the influence of global warming enhances the pro cess by rapidly adding more water, which causes the polar regions to become fresher. The long-term concern is the disruption of the Atlantic Ocean conveyor; if this major ocean circulation is altered, it will trigger other climate changes (see chapter 6 for a more in-depth discussion).

During their study, the researchers collected ocean water samples over a broad area—from Greenland to the southern tip of South America. They concluded that water masses in the Atlantic have been actively changing to this equatorial-salty/polar-fresh pattern over the past 50 years. Since 1990, the process has accelerated, corresponding to the same time interval of the 10 warmest years of recorded temperatures on Earth. Net evaporation in the Tropics is currently estimated at 5 to 10 percent, leading scientists to conclude that the loss of freshwater from the Tropics and the subsequent increase of freshwater to the polar regions are happening faster than the natural circulation of the ocean can keep up with. According to GISS, based on data collected from the Mediterranean Sea and Pacific and Indian Oceans, similar trends are occurring there as well, suggesting that the water cycle is being affected globally. These conditions could cause significant climate changes to occur on timescales of just decades, not centuries or longer.

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