Causes Of Abrupt Climate Change

One of the overwhelming causes of abrupt climate change that scientists focus upon is the sudden shutdown or start-up of the meridional over turning circulation (MOC). Also commonly referred to as the thermohaline circulation, or great ocean conveyor belt, it is a complex network of ocean currents in the Atlantic. The Gulf Stream, which is the current that transports a significant amount of heat northward from Earth's equatorial region toward western Europe, helping to warm its climate, is part of the circulation. In fact, if it were not for the Gulf Stream, the North Atlantic and Europe would be 8°F (5°C) cooler, especially in the winter. If this extensive current were to shut down, it would negatively impact the entire ocean-atmospheric system and cause adverse effects worldwide in not only ocean circulation but also the jet stream in the atmosphere that drives storm systems.

Based on evidence retrieved from ice cores in Greenland, scientists have determined that the MOC has been shut down in the past and that every time it has been shut down, an abrupt climate change has occurred. The chief mechanism for shutting down the great ocean conveyor belt is the addition of freshwater. In the circulation process, the high-density water sinks and the low-density water rises. Salty water is denser than warm water; therefore, combining these two principles, a place where cold, salty water exists is where the water sinks vertically in the ocean.

In the tropical regions of the Atlantic Ocean where there is a tremendous amount of heat, large amounts of water get evaporated. This area contains warm, salty water that flows westward toward North America, travels along the east coast of the United States, and then heads northeast toward Europe as the Gulf Stream current. Once it reaches Greenland, it cools and sinks to the bottom of the ocean. At this point, the current heads south again and travels toward Antarctica and around to the Pacific Ocean. A water molecule travels through the entire circuit of the great ocean conveyor belt in about 1,000 years.

Abrupt climate change could occur if freshwater were added near Greenland where the Gulf Stream cools and sinks. Freshwater can be added through the melting of glaciers, a situation that is becoming more probable with global warming. If too much freshwater is added, the water is no longer dense enough to sink, which then causes the circulation to slow down or stop all together. If the circulation stops, an abrupt climate change will be triggered. One of the unknowns in this case is how much freshwater would have to be added in order to turn off the conveyor belt. The circulation was shut off during the Younger Dryas event and then suddenly restarted after 1,100 years. Even though scientists have developed computer models to study the phenomenon, they have not been able to duplicate the start-up or shutdown processes, meaning they still have much to learn about the natural process itself.

Today, as Earth heats up with global warming, there could be an increase in precipitation in some areas, as well as a melting of freshwater ice in the Arctic Ocean. Scientists are concerned this would dilute the Gulf Stream. According to the Environmental Literacy Council, over the past 40 years, salinity measurements within the North Atlantic have shown that the area has been decreasing in its level of salinity. They caution that if it continues and the Gulf Stream were to eventually come to a halt, western Europe and eastern North America would cool up to 8°F (5°C)—about the same difference as the global average temperature difference between today and the Ice Age.

Another cause of abrupt climate change centers on the cryosphere. This involves the interaction of the incoming solar radiation and the ice cover on Earth's surface. When large areas on Earth's surface are covered with ice, much of the incoming sunlight is reflected back into space, a property called "albedo." Scientists think this may cause a positive feedback situation for an abrupt cooling. As solar radiation is reflected back into space, it lowers the temperature on Earth, which encourages more snow and ice to form, which increases the albedo, continuing to accelerate the cooling of the climate.

A third mechanism of triggering climate change involves methane hydrates. Methane is a greenhouse gas, but in comparison to carbon dioxide, it is a much more powerful absorber of heat. Because of this, even small releases of methane in the atmosphere could have enormous effects on Earth's climate. Methane hydrates are solid compounds of methane and water and are mainly located in the Arctic tundra and in marine sediment. It is believed that release of methane hydrates in the past have caused abrupt climate changes. Today, there is concern that if there is an increase in temperature and the Arctic ice continues to melt, the methane hydrates (currently buried under the Arctic tundra) would be released into the atmosphere, heat it quickly, and cause an abrupt climate change.

Top: When snow and ice cover the planet, the albedo is high. As incoming solar radiation reaches the Earth's surface, it is reflected off the ice and snow and sent back into the Earth's atmosphere, keeping the Earth's surface cool. This can serve as a positive feedback, encouraging more snow and ice to form in the cool environment. Bottom: If the ice on the surface of the Earth begins to melt or becomes covered with soot and other pollutants, it exposes darker surfaces. The incoming solar radiation heats up the darker surface, causing more snow and ice to melt. This starts a cycle of increased melting, referred to as negative feedback. (NASA)

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