Page 354 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

318 part II The Water, Weather, and Climate Systems
(a) Scientists drill into corals on the Clipperton Atoll, an uninhabited coral reef in the Pacific Ocean off the west coast of Central America.
(b) X-ray of core cross section shows each light/dark band indicates one year.
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◀Figure 11.14 Extraction and cross section of coral core sam- ples. [(a) Maris kazmers, nOaa Paleoclimatology Program (b) Thomas Felis, research Center Ocean Margins, Bremen/naSa.]
during this prolonged cold period. The climate record for the past 20000 years reveals the period of cold tempera- tures and little snow accumulation that occurred from the LGM to about 15000 years ago (Figure 11.15).
About 14000 years ago, average temperatures abruptly increased for several thousand years, and then dropped again during the colder period known as the Younger Dryas. The abrupt warming about 11700 years ago marked the end of the Pleistocene Epoch. Note in Figure 11.15 that snow accumulation is less during colder glacial periods. As we learned in Chapters 7 and 8, the capacity of cold air to absorb water vapour is less than for warm air, resulting in decreased snowfall during glacial periods even though a greater volume of ice is present over Earth’s surface.
From a.d. 800 to 1200, a number of climate prox- ies (tree rings, corals, and ice cores) show a mild cli- matic episode, now known as the Medieval Climate Anomaly (a period during which the Vikings settled Ice- land and coastal areas of Greenland). During this time, warmer temperatures—as warm or warmer than today— occurred in some regions whereas cooling occurred in other regions. The warmth over the North Atlantic region allowed a variety of crops to grow at higher latitudes in Europe, shifting settlement patterns northward. Scien- tists think that the cooling in some places during this time is linked to the cool La Niña phase of the ENSO phe- nomenon over the tropical Pacific.
From approximately a.d. 1250 through about 1850, temperatures cooled globally during a period known as the Little Ice Age. Winter ice was more ex- tensive in the North Atlantic Ocean, and ex- 0.35 panding glaciers in western Europe blocked many key mountain passes. During the cold- est years, snowlines in Europe lowered about 200 m in elevation. This was a 600-year span of somewhat inconsistent colder temperatures, a period that included many rapid, short-term climate fluctuations that lasted only decades and are probably related to volcanic activity and multiyear oscillations in global circula- tion patterns, specifically the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO)
Temperature
Last Glacial Maximum
1 cm
Younger Dryas
20 15 10 5 0 Time (thousands of years before present)
Medieval Climate Anomaly
Snow Accumulation
Little Ice Age
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◀Figure 11.15 The past 20 000 years of temperature and snow accumulation. evidence from greenland ice cores shows periods of colder tempera- tures occurring during the last glacial maximum and
0.10 the Younger Dryas, and an abrupt temperature rise occurring about 14 000 years ago and again about
12 000 years ago at the end of the Younger Dryas. although this graph uses ice-core data, these tempera- ture trends correlate with other climate proxy records. [From r.B. alley, “The younger Dryas cold interval as viewed from central greenland,” Quaternary Science Reviews 19 (January 2000): 213–226; available at www.ncdc.noaa .gov/paleo/pubs/alley2000/.]
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Temperature (˚C)
Snow accumulation (m·yr–1)