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| NOAA > NESDIS > NCDC > NOAA Paleoclimatology > Abrupt Home > The Story > Mechanisms | |
Mechanisms that Can Cause Abrupt Climate ChangeExample 1: Glacial-Interglacial
transitions
Example 2: Thermohaline Circulation Example 3: Vegetation Feedbacks Example 2: Thermohaline CirculationThe 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.
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.
Next: Example 3: Vegetation Feedbacks |
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