How Fast Can Climate Change

Understanding how fast climate can shift from a warm period to a cold, or cold to a warm, is difficult. The record of climate indicators is incomplete and difficult to interpret. Only 18,000 years ago the planet was in the midst of a major glacial interval, and since then global average temperatures have risen 16°F (10°C) and are still rising, perhaps at a recently accelerated rate from human contributions to the atmosphere. still, recent climate work is revealing that there are some abrupt transitions in the slow warming, in which there are major shifts in some component of the climate, where the shift may happen on scales of 10 years or fewer.

one of these abrupt transitions seems to affect the circulation pattern in the North Atlantic Ocean, where the ocean currents formed one of two different stable patterns or modes, with abrupt transitions occurring when one mode switches to the other. In the present pattern the warm waters of the Gulf Stream come out of the Gulf of Mexico and flow along the eastern seaboard of the united states, part of the British Isles, to the Norwegian Sea. This warm current is largely responsible for the mild climate of the British Isles and northern Europe. In the second mode the northern extension of the Gulf Stream is weakened by a reduction in salinity of surface waters from sources at high latitudes in the North Atlantic. The fresher water has a source in increased melting from the polar ice shelf, Greenland, and northern glaciers. With less salt, seawater is less dense and less able to sink during normal wintertime cooling.

Studies of past switches in the circulation modes of the North Atlantic reveal that the transition from one mode of circulation to the other can occur over a period of only five to 10 years. These abrupt transitions are apparently linked to increase in the release of icebergs and freshwater from continental glaciers, which upon melting contribute large volumes of freshwater into the North Atlantic, systematically reducing the salinity. The Gulf Stream presently seems on the verge of failure, or of switching modes from mode 1 to 2, and historical records show that this switch can be very rapid. If this predicted switch occurs, northern Europe and the United Kingdom may experience a significant and dramatic cooling of their climate, instead of the warming many fear.

See also atmosphere; carbon cycle; climate; global warming; greenhouse effect; ice ages; meteorology; Milankovitch cycles; paleo-climatology; sea-level rise; thermohaline circulation.


Ahrens, C. D. Meteorology Today: An Introduction to Weather, Climate, and the Environment. 6th ed. Pacific Grove, Calif.: Brooks/Cole, 2000. Dawson, A. G. Ice Age Earth, London: Routledge, 1992. Douglas, B., M. Kearney, and S. Leatherman. Sea-Level Rise: History and Consequence. San Diego, Calif.: Academic Press, International Geophysics Series 75, 2000.

Intergovernmental Panel on Climate Change 2007. Climate Change 2007: The Physical Science Basis. Contributions of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller. Cambridge: Cambridge University Press,

2007. Also available online. URL: http://www.ipcc. ch/index.htm. Accessed October 10, 2008.

National Aeronautic and Space Administration (NASA). "Earth Observatory." Available online. URL: http:// Accessed October 9,

2008, updated daily.

U.S. Environmental Protection Agency. Climate Change homepage. Available online. URL: http://www.epa. gov/climatechange/. Updated September 9, 2008.

Cloud, Preston (1912-1991) American Historical Geologist, Geobiologist Preston Ercelle Cloud Jr. was an eminent geobiologist and paleontologist who contributed important observations and interpretations that led to greater understanding of the evolution of the atmosphere, oceans, and crust of the Earth, and most important, to understanding the evolution of life on the planet.

Born in West Upton, Massachusetts, on September 26, 1912, as a child Cloud moved to Waynesboro, Pennsylvania, where he developed a keen sense of the outdoors and the rolling hills of the Appalachians. He joined the U.S. Navy from 1930 to 1933, then enrolled in George Washington University in Washington, D.C., where he cultivated contacts at the

National Museum of Natural History. As an undergraduate student Cloud developed a solid knowledge of paleontology, learning much especially about brachiopods from the collections at the National Museum in Washington, D.C.

Preston Cloud continued his education at Yale University and received a Ph.D. in 1940 for a study of Paleozoic brachiopods. From there he moved to Missouri School of Mines in Rolla, but then returned to Yale as a Research Fellow from 1941 to 1942. During World War II Cloud was called to duty with the U.S. Geological Survey, where he worked with the wartime strategic minerals program, first mapping manganese deposits in Maine, then investigating bauxite in Alabama. After this Cloud studied the Ellenburger Limestone—an important oil reservoir—from the Lower Paleozoic section of Texas, and made accurate descriptions of the stratigraphic and paleontologic relationships in this unit.

In 1946 Cloud took a position as an assistant professor of paleontology at Harvard University but in 1948 returned to the U.S. Geological Survey to map parts of Saipan Island in the Mariana Islands in the Pacific. This work led him to publish many papers on modern carbonate and coral reef systems, including his landmark works on evolution, in which he proposed that complex, multicellular organisms evolved from many different ancestors about 700 million years ago. Through his studies of geochemi-cal processes Cloud linked the rapid evolution of these species to a change in atmospheric chemistry in which the oxygen levels in the atmosphere climbed rapidly, helping the organisms expand into available ecological niches. Cloud was promoted to chief paleontologist with the U.S. Geological Survey from 1949 to 1959, and the department grew dramatically under his guidance.

After he resigned as head paleontologist at the survey, Cloud studied the continental shelves and coastal zone, expanding the knowledge of these regions dramatically and leading to oil exploration on the continental shelves. He then accepted a job as chairman of the Department of Geology and Geophysics at the University of Minnesota, and organized a new multidisciplinary approach to earth sciences by forming the School of Earth Sciences. While at Minnesota Cloud concentrated on the Precambrian and the first 86 percent of Earth history, the origin and development of life, and studied Precambrian outcrops from around the world in this context. Cloud became world-famous for his studies of Precambrian carbonate rocks and their fossil assemblages, which consist mostly of stromatolites, and ideas about the origin and evolution of life.

In 1965 Cloud moved to the University of California, Los Angeles, then in 1968 he moved to the Santa Barbara campus. In 1979 he retired but remained active in publishing books on life on the planet, and was also active on campus. Preston Cloud emphasized complex interrelationships among biological, chemical, and physical processes throughout Earth history. His work expanded beyond the realm of rocks and fossils, and he wrote about the limits of the planet for sustaining the exploding human population. He recognized that limited material, food, and energy resources with the expanding human activities could lead the planet into disaster. One of his most famous works in this field was his Oasis in Space. Cloud was elected a member of the Academy of Sciences and was active for 30 years. In 1976 Preston Cloud was awarded the Penrose Medal by the Geological Society of America, and in 1977 he was awarded the Charles Doolittle Walcott Medal by the National Academy of Sciences. The Preston Cloud Laboratory at the University of California, Santa Barbara, is dedicated to the study of pre-Phanerozoic life on Earth.

See also historical geology; paleontology, sedimentary rock, sedimentation.


Cloud, Preston. "Life, Time, History and Earth Resources."

Terra Cognita 8 (1988): 211. -. Oasis in Space: Earth History from the Beginning.

New York: W. W. Norton, 1988. -. "Aspects of Proterozoic Biogeology." Geological

Society of America Memoir 161 (1983): 245-251. -. "A Working Model of the Primitive Earth." American Journal of Science 272 (1972): 537-548.

clouds Clouds are visible masses of water droplets or ice crystals suspended in the lower atmosphere, generally confined to the troposphere. The water droplets and ice crystals condense from water vapor around small dust, pollen, salt, ice, or pollution particles that aggregate into cloud formations, classified according to their shape and height in the atmosphere. Luke Howard, an English naturalist, suggested the classification system still widely used today in 1803. He suggested Latin names based on 10 genera, then further divided into species. In 1887 the British naturalist Ralph Abercromby and H. Hildebrand Hildebrandsson of Sweden further divided the clouds into high, middle, and low-level types, as well as clouds that form over significant vertical distances. The basic types of clouds include the heaped cumulus, layered stratus, and wispy cirrus. If rain is falling from a cloud, the term nimbus is added, as in cumulonimbus, the common thunderhead cloud.

High clouds form above 19,685 feet (6,000 m) and are generally found at mid to low latitudes.

The air at this elevation is cold and dry, so the clouds consist of ice crystals, and appear white to the observer on the ground except at sunrise and sunset. The most common high clouds are the cirrus—thin, wispy clouds typically blown into thin, horsehairlike streamers by high winds. Prevailing high-level winds blow most cirrus clouds from west to east, a sign of generally good weather. Cirrocumulus clouds are small, white puffy clouds that sometimes line up in ripplelike rows and at other times form individually. Their appearance over large parts of the sky is often described as a Mackeral sky, because of the resemblance to fish scales. Cirrostratus are thin, sheetlike clouds that typically cover the entire sky. They are so thin that the Sun, Moon, and some stars can be seen through them. They are composed of ice crystals, and light that refracts through these clouds often forms a halo or sun dogs. These high clouds often form in front of an advancing storm and typically foretell of rain or snow in 12-24 hours.

Middle clouds form between 6,560 and 22,965 feet (2,000 and 7,000 m), generally in middle latitudes. They are composed mostly of water droplets, with ice crystals in some cases. Altocumulus clouds are gray, puffy masses that often roll out in waves,

Cirrus clouds over coast range at Purísima Creek Redwoods, Bay Area, California (NOAA/Department of Commerce)

with some parts appearing darker than others. Altocumulus are usually less than 0.62 miles (1 km) thick. They form with rising air currents at cloud level, and a morning appearance often predicts thunderstorms by late afternoon. Altostratus are thin, blue-gray clouds that often cover the entire sky, and the sun may shine dimly through, appearing as a faint, irregular disk. Altostratus clouds often form in front of storms that bring regional steady rain.

Low clouds have bases that may form below 6,650 feet (2,000 m) and are usually composed entirely of water droplets. In cold weather they may contain ice and snow. Nimbostratus are the dark gray, rather uniform-looking clouds associated with steady light to moderate rainfall. Rain from the nimbostratus clouds often causes the air to become saturated with water, and a group of thin, ragged clouds that move rapidly with the wind may form. These are known as stratus fractus, or scud clouds. stratocumulus clouds are low, lumpy-looking clouds that form rows or other patterns, with clear sky visible between the cloud rows. The sun may form brilliant streaming rays known as crepuscular rays through these clouds. stratus clouds have a uniform gray appearance and may cover the sky, resembling fog but not touching the ground. They commonly appear near the seashore, especially in summer months.

Some clouds form over a significant range of atmospheric levels. Cumulus are flat-bottomed, puffy clouds with irregular, domal, or towering tops. Their bases may be lower than 3,280 feet (1,000 m). On warm summer days small cumulus clouds may form in the morning and develop significant vertical growth by the afternoon, creating a towering cumulus or cumulus congestus cloud. These may continue to develop further into the giant cumulonimbus, giant thunderheads with bases that may be as low as a few hundred meters, and tops extending to more than 39,370 feet (12,000 m) in the tropopause. Cumulonimbus clouds release tremendous amounts of energy in the atmosphere and may be associated with high winds, vertical updrafts and downdrafts, lightning, and tornadoes. The lower parts of these giant clouds are made of water droplets, the middle parts may contain both water and ice, whereas the tops may consist entirely of ice crystals.

Many types of unusual clouds form in different situations. Plieus clouds may form over rising cumulus tops, looking like a halo or fog around the cloud peak. Banner clouds form over and downwind of high mountain tops, sometimes resembling steam coming out of a volcano. Lenticular clouds form wavelike figures from high winds moving over mountains, and may form elongate, pancakelike shapes. Unusual and even scary-looking mammatus clouds form bulging, baglike sacks underneath some

Mammatus Clouds Arizona
Cumulus cloud over Arizona desert (Aleksander Bochenek, Shutterstock, Inc.)

cumulonimbus clouds, forming when the sinking air is cooler than the surrounding air. Mammatus-like clouds may also form underneath clouds of volcanic ash. Finally, jet airplanes produce condensation trails when water vapor from the jet's exhaust mixes with the cold air, which becomes suddenly saturated with water and forms ice crystals. Pollution particles from the exhaust may provide the nuclei for the ice. In dry conditions condensation, or con trails, will evaporate quickly, but in more humid conditions the con trails may persist as cirruslike clouds. With the growing numbers of jet flights in the past few decades, con trails have rapidly become a significant source of cloudiness, contributing to the global weather and perhaps climate.

Clouds greatly influence the Earth's climate. They efficiently reflect short-wavelength radiation from the sun back into space, cooling the planet. But since they are composed of water, they also stop the longer-wavelength radiation from escaping, causing a greenhouse effect. Together these two apparently opposing effects of clouds strongly influence the climate of the Earth. In general the low- and middle-level clouds cool the Earth, whereas abundant high clouds tend to warm the Earth with the greenhouse effect.

See also atmosphere; climate; climate change; energy in the Earth system.


schaefer, Vincent J., and John A. Day. A Field Guide to the Atmosphere: A Peterson Field Guide. Boston: Houghton mifflin, 1981.

comet Comets are bodies of ice, dust, and rock that orbit the sun and exhibit a coma (or atmosphere) extending away from the sun as a tail when they are close to the sun. They have orbital periods that range from a few years to a few hundred or even thousands of years. short-period comets have orbital periods of fewer than 200 years, and most of these orbit in the plane of the ecliptic in the same direction as the planets. Their orbits take them past the orbit of Jupiter at aphelion, and near the sun at perihelion. Long-period comets have highly elongated or eccentric orbits, with periods longer than 200 years and extending to thousands or perhaps even millions of years. These comets range far beyond the orbits of the outer planets, although they remain gravitation-ally bound to the sun. Another class of comets, called single-apparition comets, have a hyperbolic trajectory that sends them past the inner solar system only once, then they are ejected from the solar system.

Before late 20th-century space probes collected data on comets, comets were thought to be com posed primarily of ices and to be lone wanderers of the solar system. Now, with detailed observations, it is clear that comets and asteroids are transitional in nature, both in composition and in orbital character. Comets are now known to consist of rocky cores with ices around them or in pockets, and many have an organic-rich dark surface. Many asteroids are also made of similar mixtures of rocky material with pockets of ice. There are so many rocky/icy bodies in the outer solar system in the Kuiper belt and Oort Cloud that comets are now regarded as the most abundant type of bodies in the universe. There may be one trillion comets in the solar system, of which only about 3,350 have been cataloged. Most are long-period comets, but several hundred short-period comets are known as well.

The heads of comets can be divided into several parts, including the nucleus; the coma, or gaseous rim from which the tail extends; and a diffuse cloud of hydrogen. The heads of comets can be quite large, some larger than moons or other objects including Pluto. Most cometary nuclei range between 0.3 and 30 miles (0.5-50 km) in diameter and consist of a mixture of silicate rock, dust, water ice, and other frozen gases such as carbon monoxide, carbon dioxide, ammonia, and methane. Some comets contain a variety of organic compounds including methanol, hydrogen cyanide, formaldehyde, etha-nol, and ethane, as well as complex hydrocarbons and amino acids. Although some comets have many organic molecules, no life is known to exist on or be derived from comets. These organic molecules make cometary nuclei some of the darkest objects in the universe, reflecting only 2-4 percent of the light that falls on their surfaces. This dark color may actually help comets absorb heat, promoting the release of gases to form the tail. Cometary tails can change in length, and can be 80 times larger than the head when the comet passes near the sun.

As a comet approaches the sun, it begins to emit jets of ices consisting of methane, water, and ammonia, and other ices. Modeling of the comet surface by astronomers suggests that the tails form when the radiation from the sun cracks the crust of the comet and begins to vaporize the volatiles like carbon, nitrogen, oxygen, and hydrogen, carrying away dust from the comet in the process. The mixture of dust and gases emitted by the comet then forms a large but weak atmosphere around the comet, called the coma. The radiation and solar wind from the sun causes this coma to extend outward away from the sun, forming a huge tail. The tail is complex and consists of two parts. The first part contains the gases released from the comet forming an ion tail that gets elongated in a direction pointing directly away from the Sun and may extend along magnetic field lines for more than 1 astronomical unit (9,321,000 miles; 150,000,000 km). The second part is the coma, or thin atmosphere from which the tail extends, which may become larger than the Sun. Dust released by the comet forms a tail with a slightly different orientation, forming a curved trail that follows the orbital path of the comet around the Sun.

Short-period comets originate in the Kuiper belt, whereas long-period comets originate in the oort Cloud. Many comets are pulled out of their orbits by gravitational interactions with the Sun and planets or by collisions with other bodies. When these events place comets in orbital paths that cross the inner solar system, these comets may make close orbits to the Sun, and may also collide with planets, including the Earth.

Several space missions have recently investigated the properties of comets. These include Deep Space 1, which flew by Comet Borrelly in 2001. Comet Bor-relly is a relative small comet, about 5 miles (8 km) at its longest point, and the mission showed that the comet consists of asteroid-like rocky material, along with icy plains from which the dust jets that form the coma were being emitted. In 1999 the National Aeronautics and Space Administration (NASA) launched the Stardust Comet Sample Return Mission, which flew through the tail of comet Wild 2 and collected samples of the tail in a silica gel and returned them to Earth in 2006. Scientists were expecting to find many particles of interstellar dust, or the extrasolar material that composes the solar nebula, but instead found little of this material; instead they found predominantly silicate mineral grains of Earthlike solar system composition. The samples collected revealed that comet Wild 2 is made of a bizarre mixture of material that includes some particles that formed at the highest temperatures in the early solar system, and some particles that formed at the coldest temperatures. To explain this, scientists have suggested that the rocky material that makes up the comet formed in the inner solar system during its early history, then was ejected to the outer bounds of the solar system beyond the orbit of Neptune, where the icy material was accreted to the comet. Calcium-aluminum inclusions, which represent some of the oldest, highest temperature parts of the early solar system, were also collected from the comet. one of the biggest surprises was the capture of a new class of organic material from the comet tail. These organic molecules are more primitive than any on Earth and than those found in any meteorites; they are known as polycyclic aromatic hydrocarbons. Some samples even contain alcohol. These types of hydrocarbons, thought to exist in interstellar space, may yield clues about the origin of water, oxygen, carbon, and even life on Earth.

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