Temperature And Sea Level Rise

The freshening of the high-latitude (generally from 45-60 degrees N) North Atlantic creates an exception to the overall trend of warming in the Atlantic, because water from the melting ice is cold. When examined over their entirety, the energy content in the form of heat in the world's oceans is increasing, at least since the mid-20th century. This increase in heat may be greater for the Atlantic than other oceans. Temperature increases have been measured at the surface and to depths of 9,843 ft. (3,000 m.). A warming atmosphere caused by anthropogenic greenhouse gases is widely considered to be the cause of increasing ocean temperatures.

Heat from the air at the surface of the ocean is transferred to the water, raising sea surface temperatures. Heat in the surface water is mixed downward in the summer. During winter, some heat moves by convection from deeper water to the surface and then is released to the atmosphere. Elevated sea surface temperature during winter weakens this convection, thereby trapping heat in the ocean. Factors other than greenhouse gases, such as changes in solar radiation or cosmic rays, have been proposed to explain the rising temperatures. Measured changes in these phenomena appear inadequate to explain the extent to which the heat content of the ocean and atmosphere has increased.

Both increasing ocean heat content and melting ice are contributing to sea-level rise in the Atlantic Ocean and elsewhere. Sea-level change is not uniform worldwide, with sea levels far exceeding the mean rise at some locations, and drops in levels at some others. The overall trend of rising sea levels, however, is apparent. Based largely on tidal gauge readings, this rise was from about 0.06-0.08 in. (1.5-2 mm.) per year in the 20th century. Part of this, about 0.02 in. (0.5 mm.), occurred through expansion of the water caused by heating (thermal expansion). The remainder is an actual change in the mass of the oceans caused by influx of water from melting glaciers, ice caps, and the Antarctic and Greenland ice sheets.

Scientists have determined that glaciers and ice caps around the world are melting and are responsible for most of the changes witnessed in oceans' mass. Ice caps are larger than glaciers, but larger still are the world's two ice sheets. These huge bodies of ice have the greatest potential to raise sea level, holding enough water to raise sea levels 230 ft. (70 m.). Fortunately, there is no indication that a complete melting will occur. The Greenland Ice Sheet is, however, losing mass, largely from its periphery, and there is concern that the rate of discharge of glaciers from the ice sheet into the surrounding ocean is accelerating.

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