The long-term distribution of heat and precipitation on Earth’s surface is called global climate. Heat from the sun keeps the Earth’s average temperature at about 60°F (16°C), within a range that allows for biological life and maintains the planet’s life-sustaining reservoirs of liquid water. Astronomical variations and atmospheric shielding cause incoming solar radiation to fall unevenly on the Earth’s surface. Ocean currents and winds further redistribute heat and moisture around the globe, creating climate zones. Climate zones have characteristic annual precipitation, temperature, wind, and ocean current patterns that together determine local, short-term weather, and affect development of ecologically adapted suites of plants and animals. Changes in the astronomical, oceanographic, atmospheric, and geological factors that determine global climate can lead to global climate change over time. The term climate is reserved for regional patterns of temperature and precipitation that persist for decades and centuries. Local atmospheric, oceanic, and temperature phenomena like storms and droughts that occur over hours, days, or seasons, is generally referred to as weather.
Earth’s climate zones are classified according to their average temperature and rainfall accumulation, and, in general, form latitudinal, east-west oriented bands on the Earth’s surface. Average temperatures increase with latitude and decrease with altitude; temperatures are highest near the equator and near sea level. This pattern of uneven heating drives convection, or heat-driven circulation, of the oceans and atmosphere. Warm, moisture-laden air at the equator rises and flows toward the poles, cooling, releasing precipitation, and sinking as it flows. The tropical zone, which extends about 15° north and south of the equator, is extremely warm and wet. Earth’s hot semi-arid and arid zones lie beneath dry, sinking air between about 15° and 30° North and South. This vertical convection cycle of rising warm, wet air and sinking cool, dry air is called a Hadley cell. The Earth’s has six Hadley cells that are responsible for the Earth’s alternating wet and dry climate bands. The temperate zones, between 30° and 60° North and South, lie beneath Hadley cells with rising limbs at 60° and a sinking limbs at 30°. The polar climates form beneath sinking, dry, very cold air at the north and south poles.
Earth’s rotation, the global distribution of ocean basins and continents, and the location of high mountain ranges add complexity to the pattern of latitudinal climate bands. The Coriolis effect, a phenomenon that deflects air and water currents to the right in the northern hemisphere, and to the left in southern hemisphere, is a consequence of the Earth’s eastward spin. For example, surface air flowing south in the northern equatorial Hadley cell creates the southwesterly Trade Winds instead of a direct, southerly wind. (Winds are named for the direction from which they originate; a nor’easter, for instance, blows from the northeast toward the southwest.) Belts of alternating easterly and westerly winds drive corresponding west-and east-flowing ocean currents, and distribute heat and moisture east and west within the climate bands. Air flowing across a continent loses moisture the farther it travels from the ocean. Consequently, the windward side of a continent is often wetter than its leeward side, and the interior a large continent is dryer than its coasts. When flowing air reaches a mountain front, it rises, cools, and releases its moisture as precipitation. Large mountain ranges thus receive heavy rain and snowfall on their upwind flank, and arid deserts and semi-arid grasslands form on their leesides.
The German climatologist, Wladimir Köppen (1846-1940), developed the most common classification nomenclature for climatic zones in the early 1900s. The Koppen system recognizes five general types of regional climate based on average temperature and precipitation: humid tropical, dry, humid mid- latitude with mild winters, humid mid-latitude with cold winters, and polar. The system further divides the general categories into sub-types. Dry regions, for example, can be arid deserts or semi-arid steppes, and polar regions contain frozen tundra as well as ice sheets. The Koppen system has been modified over the years to include finer sub-divisions, and a sixth category for alpine environments was added, but the system remains a valuable and widely used tool for general climatic mapping.
Ecosystems of specifically adapted plants and animals inhabit each climatic zone. The climatic zones delineated by the Koppen system generally correspond to characteristic networks of species that have evolved to survive the region’s seasonal temperature changes, precipitation fluctuations, and weather events. Desert plants, for example, have waxy leaves and stems that reduce the amount of water lost by transpiration, and many desert animals are nocturnal, an adaptation that has allowed them to survive in some of the hottest regions on the planet. Biologically productive rainforests and corral reefs flourish in the warmth and humidity of tropical zones. Arctic plants and animals are adapted to take advantage of the short polar summer season by reproducing and storing nutrients quickly before the long, dark, cold polar winter.
A complex group of astronomical, atmospheric, geological, and oceanographic factors account for Earth’s global climate. Many of these factors vary naturally over decades, centuries, and millennia. Furthermore, astronomical and geological variations begin a cascade of compensatory adjustments in the coupled, or linked, ocean-atmosphere system, which, in turn, require major adjustments to biological systems. These variations force changes in the global pattern of long-term precipitation and temperature, or global climate change. Global climate change causes permanent redistribution of climatic zones, alteration of major weather patterns, and establishment of new ecosystems. Global climate change has occurred throughout Earth’s history, and has been a major driving force in biological evolution; species unable to adapt to new climate regimes have become extinct, while others have flourished. Scientists predict that human activities, notably combustion of carbon-based fossil fuels like oil and coal, will affect the climate-regulating properties of the atmosphere, which may cause anthropogenic (human-induced) global climate change.
Astronomical factors affecting global climate change
Energy from the sun drives the Earth’s climate. Changes that affect the amount of solar radiation reaching the planet, called insolation, and that alter the distribution of sunlight on its surface, can cause global climate change. Each minute, Earth’s outer atmosphere receives about two calories of energy per square centimeter of area, a value known as the solar constant. In spite of its name, the solar constant varies over time. Astronomers have, for example, observed a correlation between the solar constant and changes in the pattern of sunspots, or solar storms, on the sun’s surface.
Earth’s position with respect to the sun over time affects its climate. During its annual circuit around the sun, the Earth’s present elliptical orbit brings it closest to the sun in January (perihelion), and carries it farthest away in July (aphelion). The planet receives about 6% more solar energy in January than in July. The Earth’s axis, a line through the poles, is tilted 23.4° with respect to the sun. Consequently, the sun’s rays strike the northern hemisphere most directly on June 21st, the summer solstice, and the southern hemisphere most directly in December 21st, the winter solstice. The equinoxes, on April 21st and September 21st, mark the dates when the sun shines directly on the equator, and day and night are the same length around the globe. Orbital geometry and axial tilt together determine the Earth’s pattern of seasons. Variations in this astronomical geometry would cause climatic variations.
In the 1920s, the Serbian astronomer, Milutin Milankovitch (1879-1958), proposed an astronomical explanation for long-term, cyclical global climate changes that caused the Pleistocene “ice ages.” By observing variations in the Earth’s orbital geometry and axial tilt, and calculating the time for a complete cycle of change to occur, Milankovitch predicted a pattern of varying insolation and global climate change. According to his theory, three so-called Milankovitch cycles—precession, obliquity, and eccentricity—repeat approximately every 21, 41, and 100 thousand years, respectively. The 21,000-year precession cycle occurs because the direction of the Earth’s spin axis changes over time, much in the way a spinning top wobbles. This phenomenon, called the precession of the equinoxes, causes a particular season, northern hemisphere summer for example, to occur at different places along the Earth’s orbital path, and hence, at a different time of year. During the 41,000-year obliquity cycle, the tilt angle of the Earth’s axis changes, altering the intensity of the seasons.
Changes in the shape, or eccentricity, of the Earth’s orbit cause the 100,000-year Milankovitch cycle. The Earth’s present orbit is almost circular, so the difference in insolation between aphelion and perihelion is fairly minor. When the orbit becomes more elliptical, Earth receives more radiation at the perihelion, and less at the aphelion. The eccentricity cycle also modulates the precession and obliquity cycles. The most intense northern hemisphere summer, for example, would occur when the June solstice coincided with the perihelion of an eccentric elliptical orbit, and the axial tilt was at its highest.
Geological data from the most recent portion of Earth’s history seem to support Milankovitch theory. The pattern of insolation variations that Milankovitch predicted generally matches the pattern of polar ice sheet advance and retreat since about two million years ago. Observations of northern hemisphere glacial features, deep sea cores that record the amount of water stored in glacial ice, and sea-level records all corroborate the timing of global cooling and warming predicted by Milankovitch theory. The correlations are more difficult to prove farther back in geologic history.
Geological factors affecting global climate
Geological changes on Earth’s surface can also affect global climate. The distribution of continental landmasses and ocean basins affects the pattern of global atmospheric and oceanographic circulation, and the shape, or topography, of Earth’s surface directs winds and ocean currents. According to the widely accepted, and well-supported theory of plate tectonics, the continents move, ocean basins open and close, and mountain ranges form over time. The continents have assumed new configurations on Earth’s surface throughout geologic history, and geologists know, from examination of fossil environments and organisms, that the movement of landmasses had significant climatic effects. For example, during the Cretaceous period, about 100 million years ago, continents covered the poles, and a warm ocean called Teethes circled the equator. An intense period of volcanic activity added insulating gases to the atmosphere. The Cretaceous was the warmest and wettest period in Earth history. There is no evidence of Cretaceous polar ice caps, shallow seas covered many continental interiors, and tropical plants and animals lived on all the continents. The collision of the Indian subcontinent with Asia, and formation of the Himalayan mountain range about 40 million years ago is another example of a plate tectonic event that caused significant climate change. The Himalayas obstruct equatorial winds and ocean currents, and contribute to major climatic phenomena, namely the monsoon seasons of southern Asia and the Indian Ocean, and the El Niño Southern Oscillation in the Pacific Ocean.
Changes in atmospheric composition and anthropogenic global warming
Earth’s climate is strongly affected by the way solar radiation is reflected, absorbed, and transmitted by the atmosphere. Presently, about 30% of the incoming solar energy reflects back into space, the atmosphere absorbs about 20%, and the remaining 50% reaches the Earth’s surface. The major gaseous components of Earth’s atmosphere are nitrogen, oxygen, argon, and carbon dioxide. Other components include relatively small amounts of neon, helium, methane, krypton, hydrogen, xenon and ozone gases, water vapor, and particulate matter. Except for relatively uncommon natural events, such as volcanic eruptions, the composition of the atmosphere stays constant over long periods of time.
The structure and composition of the atmosphere function to maintain Earth’s surface temperature within the phase boundaries of liquid water, and to protect organisms from damaging ultraviolet radiation. Gases, like ozone, in the outer atmosphere reflect or absorb much of the incoming short-wavelength solar radiation. Much of the sunlight that reaches the Earth’s surface is re-radiated into the atmosphere as longer-wavelength infrared energy, or heat. Gases in the middle and lower atmosphere, namely carbon dioxide and water vapor, absorb this infrared radiation, and the temperature of the atmosphere increases, a phenomenon known as the greenhouse effect. This heat, trapped in the atmosphere, drives atmospheric and oceanographic circulation, keeps the oceans liquid, and maintains global climate zones. The greenhouse effect makes Earth livable for biological organisms, including humans.
In the last century, humans have burned large quantities of fossil fuels like coal, oil, and natural gas to operate factories, generate electricity, and run automobile engines. Because carbon dioxide is always produced during the combustion of a carbon-based fuel, these activities have significantly (by about 36%) increased the concentration of that greenhouse gas in the atmosphere. Many scientists now believe that higher concentrations of carbon dioxide will enhance the greenhouse effect, and lead to global warming. If global warming should occur, a number of terrestrial changes could follow. Some simulations predict melting of the polar ice caps, increasing volume of water in the oceans, and inundation of coastal cities. Models also show changes in ocean currents and wind patterns and redistribution of the Earth’s major climate zones. Such events would have severe consequences for human agriculture, fishing, and civil planning, as well as for the
Anthropogenic effect —Any effect on the environment caused by human activities.
Aphelion —The point in Earth’s orbit at which it is at its greatest distance from the sun.
Axis of inclination —The angle at which Earth’s axis is tipped in relation to the plane of Earth’s orbit around the sun.
Climate —The sum total of the weather conditions for a particular area over an extended period of time, at least a few decades.
Greenhouse effect —The warming of Earth’s atmosphere as a result of the capture of heat re-radiated from the Earth by certain gases present in the atmosphere.
Ice age —An extended period of time in Earth’s history when average annual temperatures were significantly lower than at other times, and polar ice sheets extended to lower latitudes.
Perihelion —The point in Earth’s orbit when it makes its closest approach to the sun.
Solar constant —The rate at which solar energy strikes the outermost layer of Earth’s atmosphere.
natural environment. The complexity of the interrelated systems that create global climate, however, makes predicting the climatic effect of increased atmospheric carbon dioxide extremely difficult. The issue of anthropogenic global climate change remains a subject of heated debate among scientists and policy makers.
See also Atmospheric circulation.
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