Mechanisms that Can Cause Abrupt Climate Change

Example 2: Thermohaline Circulation

The Younger Dryas is one of the best-studied examples of abrupt change.  As the Earth's climate was changing from a cold glacial to a warmer interglacial period about 14,500 years ago, temperatures in the Northern Hemisphere rapidly returned to near-glacial conditions.  This period is called the Younger Dryas, named after a flower (Dryas octopetala) that grows in cold conditions and became common in Europe during this time.  Its end, about 11,500 years ago, was particularly abrupt.  In Greenland, temperatures rose 10º C in a decade (Figure 6).  Scientists have found evidence in paleoclimate records that the Younger Dryas was caused by a change in the ocean thermohaline circulation.

What is the thermohaline circulation?

The thermohaline circulation is a global ocean circulation. It is driven by geographic differences in the density of sea water, which are controlled by temperature (thermal) and salinity (haline). In the North Atlantic this circulation transports warm and salty water from the tropics to the north. There, during the winter, the water cools and releases heat to the atmosphere, warming the North Atlantic region (Figure 7). Once the water loses heat, it becomes cooler and more dense, sinking into the deep ocean. This newly formed deep water (purple) then flows slowly (~0.1 m/s) south, and rises to mid-depths around Antarctica, where it joins the Antarctic circumpolar current. The deep water around Antarctica flows northward into the Indian and Pacific Oceans, returns to the surface, and eventually flows back into the Atlantic.

Based on evidence from marine sediment cores and computer models, scientists think that the thermohaline circulation has several stable modes. Today, the circulation is similar to that described above. At certain times in the past, such as the Younger Dryas, the volume of water in the thermohaline circulation was reduced and the sites of deep water formation in the North Atlantic shifted southward. These changes have significant effects on climate in the North Atlantic region.

Figure 7. Often called a conveyor belt because of its northward transport at the surface, and southward return flow in the abyss in the Atlantic, the ocean thermohaline circulation system is a slow, three-dimensional pattern of flow involving the surface and deep oceans around the world. From CLIVAR.

Why did the thermohaline circulation change abruptly?

The Younger Dryas occurred during the transition from the last glacial period into the present interglacial (the Holocene). During this time, the continental ice sheets were rapidly melting. A pulse of this meltwater flowing into the North Atlantic (Figure 8) reduced the salinity and density of the surface ocean, causing a reduction in the rate of deepwater formation. As deepwater formation slowed, less warm water flowed north from the tropics and the North Atlantic became colder. Eventually, the meltwater flux slowed and other changes occurred, causing deepwater formation to increase. See Model of Abrupt Change in the Thermohaline.

Figure 8. The Younger Dryas in the GISP2 ice core (Cuffey and Clow (1997) and Alley (2000)) and reconstructed freshwater flux into the North Atlantic (Clark et al., 2001). Freshwater fluxes to the North Atlantic from melting glaciers and ice caps were high throughout the Younger Dryas interval. In addition, scientists think that there was a period of particularly high freshwater flux about 13,000 years ago, not shown in this figure, that resulted from a large discharge of freshwater from a glacial lake in North America.

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