Hydrological Cycle

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THE HYDRoLoGIC CYCLE (water cycle) is a global, Sun-driven process. On Earth, water travels in a cyclic process, transported from water bodies to the atmosphere, then to land, and subsequently, to water bodies in the hydrologic cycle. Water is evaporated by the Sun, incorporated into clouds as water vapor, falls to the land and water bodies as rain, and from land it goes back to the water bodies through several hydro-logic processes, such as runoff and infiltration. The hydrologic cycle is a closed system for Earth, as the amount of water remains fixed throughout, but it may vary with its distribution geographically, temporally, and seasonally. Six major components of the hydro-logic cycle are precipitation, infiltration, evaporation, transpiration, surface runoff, and groundwater flow. Evaporation and transpiration together are known as evapotranspiration. The volumes of different components of hydrologic cycle are: 110,000 km.3 of precipitation onto land and 458,000 km.3 of precipitation on the ocean surfaces, with 502,800 km.3- of evaporation from oceans and 65,200 km.3 of evaporation from land, of which 42,600 km.3 is of river runoff, and 2,200 km.3 is of underground runoff.

Increasing atmospheric concentrations of greenhouse gases warmed the Earth's surface by nearly 1 degree F (0.6 degrees C) during the 20th century. It may continue in this century, leading to a higher sea-surface temperature. One instinctive consequence of a warmer ocean surface is a larger vapor pressure difference between the sea surface and the adjacent atmosphere. Therefore, there would be an increased evaporation rate and subsequent increase in the other components of the hydrologic cycle. Computer simulation models found that a global warming by 7.2 degrees F (4 degrees C) is expected to increase global precipitation by about 10 percent, and that rainfall intensity will greater than at present. Scientists, through models, found that the upper tropospheric water vapor amount will increase by 15 percent with each degree of atmospheric temperature rise. The global water vapor amount will increase by 7 percent with each degree of atmospheric temperature rise.

Conversely, to make matters worse, water vapor acts as a prominent greenhouse gas. Increased water vapor alters the climate feedback loop. With a rise in surface temperature the water vapor amount in the atmosphere increases. The additional water vapor absorbs additional radiated energy which would normally escape from Earth's surface to outer space, and it makes the Earth's surface even warmer. This somber picture is further complicated by important interactions between water vapor, clouds, atmospheric motion, and radiation from both the Sun and the Earth's surface.


Several studies have demonstrated that the hydrologic cycle has already intensified, with a distinct (measurable) amount due to an increases in earth surface temperature. The followings is some of the evidence:

Daily minimum nighttime temperatures have increased at twice the rate of daytime temperatures since 1950. Consequently, increased cloudiness and humidity at night as well as increased evaporative cooling during the daytime are encountered. That suggests that atmospheric water vapor amount has increased.

Radiosonde measurements from 1973-93, coupled with satellite data, suggest that there is a substantial increase in the level of water vapor in the atmosphere and the amount of precipitation in all regions of the Americas, except northern and eastern Canada.

There is a near-term collapse of the ocean's thermohaline circulation. More precipitation is falling in northern Europe, some parts of Canada, and northern Russia, but less to swathes of sub-Saharan Africa, southern India, and southeast Asia. A Canadian research team found with 75 years (1925-99) of rainfall data analysis through 14 powerful computer models that the Northern Hemisphere's mid-latitude (a region of 40-70 degrees N) received increased precipitation over the years. The models also showed that, in contrast, the Northern Hemisphere's tropics and subtropics (a region between the equator and 30 degrees N) became drier, while the Southern Hemisphere's similar regions became wetter.

More hurricanes, typhoons, ocean depressions, and floods are being experienced globally. The Americas, China, Japan, Bangladesh, and India are experiencing more than average of these. Flood frequencies and intensities in India and many other countries have increased.

South America and Central America are experiencing more frequent and more severe landslides. This is because the precipitation amount from a single event has increased drastically. Due to increase in monthly mean (average) temperature, precipitation amounts in the United States and Australia have increased. This is consistent with climate model predictions.

Glacial retreat is another example of a changing water cycle and has been extensive since 1850. It is clearly visible in the North Pole, Greenland, and Antarctica. It is happening as the loss of water from melting and sublimation is greater than the supply of water to glaciers from precipitation.

Other human activities are also altering the hydro-logic cycle. They include: agriculture, alteration of the chemical composition of the atmosphere, construction of dams, deforestation and afforestation, removal of groundwater from wells, water abstraction from rivers, and urbanization. These are also the cause of global warming. The increase in the amount of precipitation and reduction of the number of freezing days as a consequence of global warming are to some extent helping agriculture. However, the changes in the hydrologic cycle may have adverse consequences, including the ones described.


Due to the reduction in snowpack, many perennial rivers in the world are experiencing shortages of flowing water in summer months, and groundwater recharging has weakened. Consequently, agriculture suffers. Increased quick melting of snowpacks (due to the rise in surface temperature) in the northwestern United States and India causes summer floods. Increases in rainfall intensity may cause more soil erosion and less soil infiltration. This would create detrimental effects on agriculture. Many hydrologic structures may suffer damage, and may not withstand the higher precipitation intensity, because they were not designed for atypical storm events or rainfall intensities. Increased numbers of hurricanes, storms, and other activities as a consequence of changes in the hydrologic cycle associated with global warming are causing severe hardship to numerous people living on shorelines. Agriculture in the northern United States and southern Canada depends upon soil moisture conserved by snowpack on agricultural land. With less snowfall because of increased surface temperature, there may be less snow deposited on agricultural land, and agriculture will suffer. In summary, global warming causes and would cause the following visible changes in the hydrologic cycle: changes in total precipitation, changes in precipitation frequency and intensity, changes in flood and drought frequencies, and changes in tropical storm frequencies. It would also further amplify global warming, due to increased water vapor in the Earth's atmosphere.

SEE ALSO: Evaporation and Transpiration; Evaporation Feedbacks; Floods; Precipitation; Sea Level, Rising.

BIBILIOGRAPHY. American Geophysical Union, "Water Vapor in Climate System," www.agu.org (cited August 2007); Thomas Dunne and L.B. Leopold, Water in Environmental Planning (W.H. Freeman and Company, 1978); Cheryl Dybas, "Increase in Rainfall Variability Related to Global Climate Change," Earth Observatory, NASA (December 12, 2002); P. Groisman, et al., "Changes in the Probability of Heavy Precipitation: Important Indicators of Climatic Change," Climatic Change (v.42, 1999); J. Huang and H.M. van den Dool, "Monthly Precipitation-Temperature Relations and Temperature Prediction over the United States," Journal of Climate (v.6, 1993); M. Hulme, "Estimating Global Changes in Precipitation," Weather (v.50, 1995); United Nations -UNESCO "Water Day 2000," www.unesco.org (cited August 2007); U.S. Geologic Survey, "Glacier Retreat in Glacier National Park, Montana," www.usgs.gov (cited August 2007); Warren Viessman, Jr. and G.L. Lewis, Introduction to Hydrology (Prentice Hall Publishers, 2003).

SUDHANSHU SEKHAR PANDA Gainesville State College

Ice Ages

ENORMOUS ICE SHEETS hav, at times, spread across the globe, bringing with them a cold climate. One definition holds that the advance of glaciers to middle latitudes marks an ice age. Another, and perhaps complementary, definition holds that the spread of glaciers to encompass roughly one-third of the planet is the signature of an ice age. Glaciers have advanced and retreated from Earth's surface and from the ocean several times. The most exuberant estimate tallies 17 ice ages. A more conservative estimate is that Earth has endured the tumult of six ice ages. Scholars have debated not only the number of ice ages, but also their causes. The causes fall into three categories: astronomical, atmospheric, and terrestrial. The recognition of ice ages is recent. Only in the 1830s did Swiss zoologist Louis Agassiz, building on the work of predecessors, propose that parts of Europe, the Americas, and Asia had been covered by ice. As ice advanced and retreated, Agassiz asserted, it ground up rocks in its path and sometimes carried boulders a great distance. Agassiz thought in terms of a single ice age, but climatologists have increased this number in the 20th century.

The first ice age occurred, not thousands of years ago, but rather 2.6 billion years ago. The sun, com paratively young, was not burning at full strength, though this fact does not explain why the ice age had not begun earlier. A more satisfactory explanation focuses on carbon dioxide. The amount of carbon dioxide in the atmosphere diminished, lowering the capacity of the atmosphere to trap heat and, thereby, warm the Earth. As the amount of carbon dioxide diminished, temperatures fell low enough for ice to spread over parts of the primeval continents. The first ice age, lasting 300 million years, ended 2.3 billion years ago.


The end of the first ice age ushered in a warm climate that endured 1.3 billion years. This period was the longest interglacial, a warm interlude between two ice ages, in Earth's history. When it ended, ice returned in a trio of advances and retreats: one billion years ago, 750 million years ago, and 600 million years ago. Of these three, the ice age 750 million years ago may have been the largest, with ice near the Equator. Glaciers may then have covered half the Earth. Climatol-ogists group these three as the Upper Proterozoic Ice Age. One cause of it may have been an increase in the number of photosynthetic algae in the ocean. During photosynthesis, algae, as well as plants, absorb carbon dioxide and emit oxygen. Carbon dioxide is a green house gas, but oxygen is not. In absorbing carbon dioxide, the algae reduced the capacity of the atmosphere to trap heat.

A second cause of the Upper Proterozoic Ice Age may have been rapid continental drift, bringing the continents into high latitudes, where they acquired glaciers and subsequently drifted into the tropics where the glaciers disappeared. This explanation accounts for the fact that the Upper Proterozoic Ice Age was not a single event, but rather the aggregate of three glaciations. Still another cause may have been an exaggerated tilt to Earth's axis. The current tilt is 23.5 degrees, but the tilt during the Upper Proterozoic Ice Age may have been as large as 54 degrees, an orientation that would have given the Southern Hemisphere less sunlight than the Northern Hemisphere year round, prompting the growth of glaciers in the Southern Hemisphere.


The third glaciation of the Upper Proterozoic Ice Age lasted until 580 million years ago. A warm interlude held back the ice for 130 million years, but the recrudesce of glaciers 450 million years ago ushered in the Ordovician Ice Age. The concentration of carbon dioxide was 25 percent lower than it is today, reducing the capacity of the atmosphere to trap heat. This reduction, if it caused the Ordovician Ice Age, underscores that greenhouse gases play a large role in determining the climate. South America, South Africa, India, and Australia were then part of the super-continent Gondwanaland, much of which was above 60 degrees South. At this latitude, evaporation from the ocean formed clouds, which, in turn, released their moisture in the form of snow. Over time, the snow compacted into ice, giving Gondwanaland large expanses of ice. Parts of the continent drifted over the South Pole, ensuring that ice and snow did not melt at the highest latitudes. Ice covered the remainder of Africa, which was not then part of Gondwanaland. By 440 million years ago, North Africa had replaced Gondwanaland at the South Pole. Glaciers spread as far as what is now the Sahara Desert.

Glaciers retreated 425 million years ago. The periodicity of a 130 million year interglacial nearly repeated itself with an interglacial of 125 million years. The Permo-Caroniferous Ice Age, beginning 300 million years ago, was a time of mountain-building on the con tinents. As is true today, high elevations were home to ice and snow. Glaciers formed first in the mountains of Tasmania and Australia, lands then part of Gondwanaland. Parts of Angaraland, a super-continent of China and Siberia, were above 75 degrees North and so were cold enough for the formation of glaciers. Only Laurasia, a super-continent of North America, Greenland, and Europe west of the Ural Mountains, being between 15 degrees South and 40 degrees North, was near enough to the equator to be unsuited to the formation of glaciers. By 280 million years ago, the glaciers had reached their greatest extent. Forests, spreading in the tropics around this time, may have cooled Earth by absorbing carbon dioxide. Locking up water in ice, the glaciers reduced sea levels between 492-820 ft. (150-250 m.). Glaciers crept from the South Pole to 30 degrees South. By then, the continents had joined to form the super-continent Pangea. The formation of Pangea caused volcanoes to erupt, spewing debris and ash into the atmosphere. These airborne particles blocked out sunlight, cooling the climate and hastening the formation of glaciers. Lands once covered by warm seas dried out and became cold.


By 270 million years ago the glaciers were in retreat and by 250 million years ago only a few mountain glaciers remained. The ocean, swelled by water from melting glaciers, returned to its pre-glacial level. Dramatic as this transition from ice age to warm climate was, it was shorter than the two previous interglacials. Around 230 million years ago, the return of glaciers heralded the Permian Ice Age. This glaciation coincided with a mass extinction of both terrestrial and marine organisms. Around 180 million years ago, Pangea began to break up into South America, South Africa, India, Australia, and Antarctica, all of which retained glaciers.

The glaciers were again in retreat by 130 million years ago. The interglacial that chased away the glaciers lasted until one million years ago, when the Ceno-zoic Ice Age inaugurated a new cycle of advance and retreat. This most recent ice age has had several glaciations. Glaciations of 100,000 years have alternated with warm periods of 10,000 years. The most recent glaciation occurred 100,000 years ago. Glaciers spread to their greatest extent 18,000 years ago before retreating. Ice 10,000 ft. (3,048 m.) thick covered Canada,

Terrestrial Hydrological Cycle
Glaciers have advanced and retreated from Earth's surface and from the ocean several times; estimates of Ice Age occurrences range from six to 17. Their causes—astronomical, atmospheric, or terrestrial—have been debated by scholars.

Greenland, and northern Europe. The glaciers locked up 5 percent of Earth's water as ice. Sea level diminished, revealing the continental shelves as land. With its continental shelves exposed, Florida was twice as wide as it is today. Temperatures fell to 10 degrees F (5.5 degrees C) below current temperatures.

Colder temperatures reduced the rate of evaporation from the oceans. Rainfall decreased accordingly. The Cenozoic Ice Age was both cold and dry. Land not covered by snow and ice became desert. The glaciers held so much water as ice that the North Sea, the Baltic Sea, and the Bering Strait, bereft of water, did not exist. With ice 2 mi. (3.2 km.) thick in parts of North America and Eurasia, its melting released millions of gallons of water into the oceans. In North America, water flowed south through the Mississippi River as the glaciers retreated north. Once the glaciers reached the Great Lakes, water flowed not south, but rather east through the Lawrence River and into the Atlantic Ocean. The cold water stopped the warm water of the Gulf Stream from heating the Atlantic coast. Bereft of the Gulf Stream, North

America retained its glaciers until 10,000 years ago, when they shrank to their current size.

The Cenozoic Ice Age coincided with the origin of modern humans roughly 100,000 years ago. Modern humans arose in Africa, which was warm in contrast to the lands of the ice age. With minimal hair and abundant sweat glands, humans were adapted to warm climes. Nevertheless, they wandered into Europe about 40,000 year ago. There they encountered Neanderthals, a type of human that had lived in Europe for some 200,000 years. In contrast to modern humans, Neanderthals were adapted to the ice age. Their stout frame and short arms and legs minimized the body's surface area exposed to the cold and so conserved heat. In one of the great paradoxes in history, modern humans, adapted to a warm climate, thrived in Ice Age Europe, whereas Neanderthals adapted to the ice age went extinct.

The Cenozoic Ice Age also aided the migration of humans to new lands. The glaciers lowered sea level some 394 ft. (120 m.), revealing a land bridge between

Southeast Asia and Indonesia. To the north, a land bridge between Asia and North America allowed humans to colonize the New World. The end of the Cenozoic Ice Age coincided with the extinction of the saber-toothed cat, the wooly mammoth, the mastodon, the hippopotamus, and the horse in the Americas. Whether humans hunted them to extinction or whether they could not adapt to a warm climate remains open to question.


Around 1300 c.E., temperatures fell 2 degrees F (1 degree C), ushering in the Little Ice Age. The number of sun-spots decreased during the Little Ice Age, evidence that the sun emitted less heat than it does in warm periods. The number of sunspots reached its nadir 1645-1715, the coldest years of the Little Ice Age. From the outset, Europeans suffered under harsh conditions. Beset by cold, wet weather, their crops rotted in the fields. Famine, acute in 1315 and 1317, consigned some to starve and others to chronic hunger. Malnourished, the people of Eurasia were vulnerable to disease. One of the worst pandemics, the Black Death, struck Europe during the Little Ice Age, killing between one-third and half the population. The Norse of Greenland fared no better during the Little Ice Age. Able to farm the land in warm weather, the Norse struggled to eke out an existence in the cold of the Little Ice Age, and finally failed. In the New World, George Washington's Continental Army nearly disintegrated in the frigid winter at Valley Forge.

By 1850, the Little Ice Age had ended. Shedding its heat at the current rate, the Sun warmed Earth. Well-suited to this new warm period, humanity began its rapid ascent to the current population of more than 6 billion. Some climatologists predict that the current warmth, like warm climates of the past, will not last. The current interglacial has nearly run its course. Soon, ice will again cover vast areas of the world. Others, however, predict that the climate will become warmer, not colder, because of the increase in the concentration of carbon dioxide in the atmosphere. The ice caps and the ice on Greenland will melt. Mountains will lose their snow and ice. Coastal cities may flood. At the moment, no consensus exists whether Earth will or will not descend into another ice age.

sEE ALso: Climate; Glaciers, Retreating; Glaciology; Global Warming; Ice Albedo Feedback; Ice Component of Models; Weather.

BIBLIoGRAPHY. Windsor Chorlton, Ice Ages (Time Life Books, 1988); Jon Erickson, Ice Ages: Past and Future (TAB Books, 1990); Brian John, ed., The Winters of the World: Earth Under the Ice Ages (Wiley, 1979); Doug Macdougall, Frozen Earth: The Once and Future Story of Ice Ages (University of California Press, 2006).

Christopher Cumo Independent Scholar

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