Prominent Cooling Events

One of the most studied ice ages is that of the last one, when 20,000 years ago the global temperature was 9°F (5°C) lower than it is today and wooly mammoths and saber-toothed tigers roamed the present-day New York City area. During this period, commonly known as the Ice Age, one-third of the planet's land surface was covered with ice. In North America, an imposing ice sheet up to two miles (3.2 km) thick covered Canada and reached as far south as southern Illinois. Ice sheets also covered Greenland, northern Asia, and northern Europe.

Scientists, from professionals at the USGS to glacial geomorpholo-gists at universities, study the landforms left behind to piece the trail of ice advances together. They also deduce past conditions through the analysis of biological evidence. Fossil remains have been discovered of species such as reindeer and wolverines—cold climate species—in areas that today would be much to warm and inhospitable for their survival. By radiometrically dating these specimens, climatologists and paleontologists, such as those at the USGS and NOAA, can determine when the colder climate conditions existed. Examples of this include musk ox fossils that have been found as far south as Mexico. Mastodon fossils have been located off the coast of present-day New Jersey in the Atlantic Ocean; walrus once existed off the coast of present-day Virginia.

The La Brea tar pits in Los Angeles are another area where Ice Age life has been preserved and has been radiometrically dated, giving scientists at the Natural History Museum of Los Angeles and from universities useful data for reconstructing past climate. This type of proxy evidence (using related physical phenomena as an indicator of something else) offers proof that during the Ice Age, conditions were cooler much farther south, and ocean levels were significantly lower.

Since the Ice Age, there have been some prominent cooling events. One of the most well known is that of the Younger Dryas event, which occurred around 11,000 years ago. The Younger Dryas was a brief but intense return to cold climate conditions, almost leading to reglaciation, and lasted for about 1,000 years. This weather event serves as a good example of how rapidly climate can oscillate. Its beginning was abrupt; in just 20 years atmospheric temperature dropped 12°F (7°C). Just as dramatically, at the end of the thousand years, the atmospheric temperature returned to its previous state.

Initially, there were two theories that suggested its cause: a major influx of icebergs deposited into the ocean from a disintegrating Arctic ice shelf or shifts in the patterns of orographic (mountain) winds in response to the retreat of the ice sheets. These theories had major flaws, however, because they could not explain how the event could influence such a large geographical area and how it could start and stop so rapidly. A subsequent proposal for the cause was that a diversion of the meltwater occurred between the Mississippi and St. Lawrence Rivers. This theory was developed by Wallace Broecker of the Lamont-Doherty Earth Observatory in New York. The meltwater coming off of the massive Laurentide Ice Sheet drained down the Mississippi and into the Gulf of Mexico. Then, through a sequence of events, it was gradually diverted to eastern outlets via the St. Lawrence. The last of these "eastern diversions" documented was when the Lake Agassiz drainage was routed across Canada into the St. Lawrence estuary. At this critical point, all the drainage was now shifted to the east. The eventual impact of this was to shut down the deepwater circulation in the North Atlantic. Based on this model, as glaciers were retreating, their meltwater sank to the bottom of the ocean and triggered the "conveyor belt" of oceanic circulation. Because the oceanic conveyor system serves to transport heat to the north, it caused the remaining ice sheets to melt faster. The Younger Dryas was first documented in northern Europe, through the discovery of proxy evidence found in marine cores, pollen samples, and terrestrial plants. There are some problems with this model, as well. It does not adequately explain the same effect in the Pacific Ocean or the rapid fluctuations that occurred in Greenland. Because of this, the "Heinrich events" have also been suggested as part of the cause.

The Heinrich events were caused by a multitude of icebergs calving and entering the Atlantic. Iceberg calving is the process by which pieces of ice break away from the terminus of a glacier that ends in a body of water or from the edge of a floating ice shelf that ends in the ocean. Once they enter the water, the pieces are called icebergs. This would have caused a sudden sea-level rise of many feet (m) and lowered the elevation of the Laurentide Ice Sheet. This would then have caused the Northern Hemisphere to warm. Increased snow accumulation would have allowed the ice sheet to return to its former elevation and begin a new cold phase called the Younger Dryas. The extent of the Younger Dryas was global, not just regional.

Another sudden decrease in global temperature occurred around 8,200 years ago (circa 6200 b.c.e.) and lasted 200-400 years. This cold interval, however, was not as severe as the Younger Dryas. Scientists have discovered evidence for this cooling phase in ice cores retrieved from Greenland and in sedimentary records from the temporal and tropical regions in the Atlantic. This episode was global in nature and affected sea level worldwide.

It is possible that this episode was caused by the introduction of massive amounts of meltwater still flowing off the Laurentide Ice Sheet of northeastern North America and flowing into the North Atlantic. Another possibility was the sudden draining into the North Atlantic of glacial Lake Ojibway. This huge influx of fresh meltwater once again negatively impacted the Gulf Stream and the thermohaline circulation of heat transfer in the North Atlantic, causing an abrupt climatic cooling. In temperate zones, temperatures dropped 9-11°F (5-6°C); in the Tropics, 5°F (3°C).

Conditions became cooler and drier; documented evidence also exists that the changes in climate were severe enough that they impacted early human settlements. Areas in East Africa experienced 500 years of drought as a result. This climatic condition lasted approximately 400 years before climate reverted back to normal.

The next notable cooling episode is referred to as the "Little Ice Age," which lasted from about 1350 to 1850 c.E. This was characterized by the advance of glaciers worldwide and unusually cold winters in North America, Europe, and Asia. Scientists suggest the cause for this cold interval was low solar activity and an increase in volcanism. From 1645 to 1715, there was a documented interval of low solar activity referred to as the Maunder minimum. It has been suggested that the episode was directly linked to the cold temperatures of the Little Ice Age. The Earth's volcanoes were extremely active during this time period, also, and some believe it is possible that the addition of ash and sulfuric acid particles to the atmosphere may have contributed to the lower temperatures.

The cooling during this episode was not as extreme as in previous ones—roughly only 1.67°F (1°C). It is also not clear that the effects were global. Cold winters and the advance of glaciers show up in records for Europe and North America, where agricultural practices were negatively impacted. Around 1850, the Little Ice Age came to an end, possibly owing to human activity and global warming.

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