The average temperature of the sea surface is about 20° C (68° F), but it ranges from more than 30° C (86° F) in warm tropical regions to less than 0°C at high latitudes. In most of the ocean, the water becomes colder with increasing depth. At 2000 meters, (6,560 feet) the global average temperature is about 2.5°C (36.5° F), and at some locations the ocean bottom temperature is less than 1°C (33.8° F).
The long-term warming of the oceans is strongest at the sea surface, temperature (SST) has increased by about 1°C over the past since 1910, or 0.1°C per decade. SST has been measured all over the world for more than a hundred years by ocean-going ships. Below the sea surface, historical measurements of temperature are far sparser, and the warming is more gradual, about 0.01°C per decade at 1,000 meters.
The long-term increase of SST and the warming over the whole water column are both important in the physics of climate. Sea surface temperature is an important factor because it controls the exchange of heat between the ocean and the atmosphere and in so doing, influences the temperatures experienced on land. Heat content averaged over the full water column is important because over 90 percent of the net energy being absorbed by the earth’s entire climate system – in the air, ocean and on land and ice – is stored in the oceans. The oceans have much greater capacity to store heat than the atmosphere, and ocean currents and mixing carry heat away from the sea surface into the deep ocean. Over the past 50 years, heat energy has been stored in the oceans at a rate of about ½ watt per square meter of surface area. This is equivalent to having each 10 meter-by-20 meter area on Earth continuously warmed by a 100-watt light bulb.
On time-scales of a decade or two, global SST fluctuates by about 0.2° C from year to year due to naturally-occurring climate phenomena such as El Niño and La Niña so the long-term warming trend is not evident in 10-year periods. That is the case in the most recent decade. Nevertheless, the warming over the whole water column occurs at a steadier rate than for SST, and indeed the heat content of the oceans has continued to rise in the past decade at the long-term rate of about ½ watt per square meter.
So it would take a few hundred years for SST to increase by a few more degrees Celsius if it continues at the same rate as the past century. Climate models predict that the rate of warming will increase. However, even the moderate warming that has occurred so far is having profound impacts on marine life, sea level, and on Earth’s water cycle. Sea level is rising by 2-3 centimeters per decade globally due to melting of glaciers and ice sheets and to thermal expansion of the seawater. Evaporation and rainfall have increased on a global basis as the warming ocean puts more water vapor into the atmosphere and more energy into global weather systems. Marine species are being displaced toward higher latitudes by the warming ocean, disrupting ecosystems. So while the rate of ocean temperature increase seems small, its effects on marine life, sea level, and the earth’s water cycle are the primary concerns for society.
—Dean Roemmich, physical oceanographer, Integrative Oceanography Division