Global Climate

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.

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, the 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.

The 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 1920's, the Serbian astronomer, Milutin Milankovitch, 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 socalled 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, the 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 the 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 the 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 the 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 the 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 gasses 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

The 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 the 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-wave-length 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 the 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 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 natural environment. The complexity of the inter-related 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.