Environment : Climate

Do changes in the formation and distribution of sea ice affect our global climate? Yes. The role of sea ice is greater than you might think.

Melting

The sun's rays strike the polar regions at a more grazing angle than over equatorial regions, where the rays strike at a more direct angle. The sun's angle is the primary reason why the polar regions are cold and the equatorial regions are warm.

Sea ice is nearly white, so most of the sunlight that hits the sea ice surface is reflected back into space; thus, it has a high albedo. High albedo helps keep the polar regions cold, because the sunlight reflected back into space does not warm the surface. When the climate changes enough to warm the Arctic and to melt sea ice, the polar regions have less of a reflective surface. More heat is absorbed, which causes more melting, which amplifies the warming. This cycle is known as a positive feedback loop that ultimately alters the circulation of the atmosphere.

Atmosphere and Ocean Circulation

The atmosphere and ocean act as "heat engines," always trying to restore a temperature balance by transporting heat it collects at the equator toward the poles. Our weather is a manifestation of this phenomenon. Low-pressure systems, such as storms, which can be especially strong in winter, are one of nature's best ways of transporting heat poleward by atmospheric circulation. The oceans, by contrast, tend to transport heat in a slower and less violent fashion. Changes in the amount of sea ice alter how cold the poles are, which could affect atmospheric and ocean circulation.

Diagram of thermohaline circulation
Diagram of thermohaline circulation.
Image courtesy of NASA GSFC.

Ocean currents transport heat from the equator to the poles through a heat- and saline-driven process called thermohaline circulation. Warm water moves from the equator northward along the ocean surface and eventually cools. As it cools, it becomes dense and heavy and sinks. This cold water then moves south along the lower part of the ocean and rises near the equator to complete the cycle. Like the atmospheric heat transport discussed earlier, this is a natural process that contributes to a proper temperature balance across the Earth. It also explains why Europe is relatively warm, because as northward flowing surface water in the Atlantic Ocean cools, heat is released to the atmosphere.

Thermohaline circulation can be disrupted if the ocean surface grows fresher. How might this happen? The biggest potential driver is melt from the Greenland Ice Sheet. Meltwater from Greenland melt water is fresh, which is less dense than the ocean's salt water and thus less prone to sink. So more Greenland melt water can prevent sinking and inhibit the thermohaline circulation.

Sea ice also has an effect on the circulation. Sea ice melt freshens the ocean surface and impedes thermohaline circulation. But a bigger factor is wintertime sea ice growth in the Labrador Sea and the northern Atlantic Ocean off the southeast coast of Greenland. Ice formation expels salt, so ice growth adds salt to the ocean surface, making ocean surface water denser and causing it to sink. By sinking, ocean water draws surfaces waters from the south to replace it. In other words, the sinking saline water "pulls" warmer surface waters from the south, like a conveyor belt, where those waters cool and sink. If sea ice growth is reduced in these regions, there will be less sinking, which will slow the conveyor belt.

Another, smaller effect of sea ice in thermohaline circulation is melt of sea ice that drifts out of the Arctic Ocean through Fram Strait into the North Atlantic. If there is more sea ice melt, it will freshen the ocean surface, just like adding freshwater from Greenland melt.

The Arctic Ocean itself is affected by freshwater, particularly through river runoff into the Arctic Ocean. Although it contains only about 1 percent of the global ocean's volume, the Arctic Ocean collects more than 10 percent of the world's river discharge. Thanks to increasing precipitation at high latitudes, driven by rising global temperatures, river discharge into the Arctic is increasing. According to the Arctic Report Card: Update for 2018, from 1976 to 2017, discharge of Eurasian rivers into the Arctic increased by an estimated 3.3 ± 1.6 percent per decade; discharge from North American rivers rose an estimated 3.3 ± 1.6 percent per decade.

Movement

Although the ocean is salty, the sea ice on top of the Arctic ocean is fresher. Multiyear ice is fresh enough to drink. Sea ice is fresh because sea ice expels salt into the water as it forms. When the ice moves south through the Fram Strait into the North Atlantic, it melts, creating a layer of fresher water over the ocean surface. This fresh water is less dense than salty water, so it tends to stay at the top of the ocean. This lower density discourages the normal process of sinking at high latitudes (poles) that supports thermohaline circulation, which makes it harder to move the warm water north from the equator. Strong evidence shows that this stagnation process happened over a period of several years in the late 1960s and early 1970s, when extra fresh water entered the North Atlantic and affected the climate of northern Europe. Scientists call this event the “Great Salinity Anomaly.”

Fram Strait
Fram Strait.
—Credit: Wikimedia Commons

While this process involves the transport of ice out of the Arctic, other processes are at work within the Arctic Ocean itself.

Heat Exchange

During winter, the Arctic's atmosphere is very cold. In comparison, the ocean is much warmer. The sea ice cover separates the two, preventing heat in the ocean from warming the overlying atmosphere. This insulating effect is another way that sea ice helps to keep the Arctic cold. But heat can escape rather efficiently from areas of thin ice and especially from leads and polynyas, small openings in the ice cover. Roughly half of the total exchange of heat between the Arctic Ocean and the atmosphere occurs through openings in the ice. With more leads and polynyas, or thinner ice, the sea ice cannot efficiently insulate the ocean from the atmosphere. The Arctic atmosphere then warms, which, in turn influences the global circulation of the atmosphere.

Last updated: 3 April 2020