Global climate is the term used to describe the sum total of all weather patterns formed over all parts of the planet. In contrast to the term weather, which applies to relatively short-term phenomena such as storms, global climate is used to describe patterns that extend over much longer periods of time, at least a few decades.
Ten distinct climatic zones exist on the planet: tropical, subtropical, arid, semi-arid, mediterranean, temperate, northern temperate, mountain, polar, and coastal. The specific designation of each climate zone depends on two major factors: the average temperature and the amount of precipitation received in the zone. For example, temperate zones have fairly uniform rainfall patterns and four different seasons, while tropical zones have high rainfall and temperature and a short dry season. Each of the climatic zones on Earth is inhabited by particular kinds of plants and animals that have adapted to the conditions that exist there.
A number of factors account for Earth's climatic pattern. Among these factors are the amount of solar energy that reaches Earth's surface, the shape and orientation of Earth's orbit around the Sun, and the composition of Earth's atmosphere.
Solar energy. The driving force behind almost all climatic changes on Earth is the Sun. Sunlight that reaches Earth's atmosphere experiences different fates. About 30 percent of the solar radiation reaching the atmosphere is reflected back into space, another 20 percent is absorbed by the atmosphere, and the rest (about 50 percent) is absorbed by Earth's surface.
Global warming: The rise in Earth's temperature that is attributed to the buildup of carbon dioxide and other pollutants in the atmosphere.
Greenhouse effect: The warming of Earth's atmosphere due to water vapor, carbon dioxide, and other gases in the atmosphere that trap heat radiated from Earth's surface.
Radiation: Energy emitted in the form of waves or particles.
Wavelength: The distance between one peak of a wave of light, heat, or energy and the next corresponding peak.
Earth's orbit. The fact that weather conditions vary on different parts of the planet and that they change throughout the year is a consequence of two features of Earth's orbit around the Sun. First, Earth follows an oval-shaped path, called an ellipse, around the Sun. Because of this, Earth comes closer to the Sun at certain times of the year, absorbing more solar energy. Second, Earth's axis is tilted at an angle of 23.5 degrees in relation to the plane of the Sun. Because of this tilt, different parts of the planet are tilted toward the Sun at different times of the year. Summer occurs in the northern hemisphere in the middle of the year not because the planet is closer to the Sun (it is not), but because the hemisphere is tilted closer to the Sun.
The atmosphere. Earth's climate is strongly affected by the way heat is absorbed in the atmosphere. Sunlight consists largely of radiation with relatively short wavelengths, radiation that is not absorbed by most atmospheric gases. Once that radiation has been absorbed by Earth's surface, however, it is reradiated to the atmosphere in a form that consists of longer wavelengths. These forms of radiation are more readily absorbed by atmospheric gases, especially water vapor and carbon dioxide. As these gases absorb reradiated energy, the temperature of the atmosphere increases, a phenomenon known as the greenhouse effect.
Changes in global climate have occurred over the course of Earth's history. Volcanic eruptions, more prevalent in the past than today, spewed huge quantities of dust and ash into the atmosphere, reducing the amount of solar radiation reaching Earth and lowering global temperatures. These conditions may have lasted for months or even years.
The movement of landmasses across the planet's face may have had significant climatic effects. The breaking apart of ancient continents probably had a measurable effect on heat exchange between Earth's land surface, ocean waters, and the atmosphere. The creation of major landforms such as mountains had a large effect on the formation and pattern of winds, clouds, and precipitation.
Glacial periods—ice ages—are incidents of large-scale climatic changes on Earth over very long periods of time. In the 1930s, Serbian mathematician Milutin Milankovitch proposed a theory to explain such changes. The Milankovitch theory states that three periodic changes in Earth's orbit around the Sun affect the amount of sunlight reaching Earth at different latitudes, leading to ice ages. First, Earth's axis wobbles like a gyroscope, tracing a complete circle every 23,000 years or so. Second, at the same time while wobbling, the axis tilts between 22 and 24.5 degrees every 41,000 years. Third, Earth's elliptical orbit pulses, moving outward or inward every 100,000 and 433,000 years.
Oscillations (back-and-forth changes) in ocean current temperatures seem to lead to decades-long changes in global climate (the oceans and the atmosphere are linked by their mutual transfer of heat and moisture). Scientists have long known of the existence of a huge, powerful current in the Atlantic Ocean that carries water from the area around Florida northeast to the coast of Ireland, then heads westward where it cools and sinks near the Labrador Peninsula of Canada (there is a similar one in the Pacific). This pipeline of water alternates between warm and cool currents every 20 years or so. When the currents are warm, Northern Europe and Asia experience mild winters. When the currents are cooler, Northern Europe is much colder and drier, while the Mediterranean region, Africa, and the Middle East are warm and wet.
Human effects. During the twentieth century, humans have burned very large quantities of fossil fuels (coal, oil, and natural gas) to operate factories, heat homes and offices, run automobiles, and perform similar tasks. Since carbon dioxide is always produced during the combustion (burning) of a fossil fuel, these activities have contributed to the concentration of that gas in the atmosphere.
Strong evidence now supports the contention that these activities have significantly raised the level of carbon dioxide in Earth's atmosphere. Many scientists now believe that this change means that higher concentrations of carbon dioxide will inevitably lead to a greater retention of heat in the atmosphere and, hence, a higher annual average global temperature. This phenomenon has been called global warming.
If global warming should occur, a number of terrestrial (land) changes may follow. For example, portions of the polar ice cap may melt, increasing the volume of water in the oceans and flooding coastal cities. In addition, regional weather patterns may undergo significant changes.
[ See also Atmospheric circulation ; Greenhouse effect ]