Global climate is the long-term pattern of temperature and precipitation on Earth’s surface. Heat and water are unevenly distributed around the globe, and Earth has many climate zones (areas with a characteristic climate) and subclimates (areas with unique climate features within a climate zone) with unique patterns of temperature, rainfall, winds, and ocean currents (the circulation of ocean waters that produce a steady flow of water in a prevailing direction).
Climate zones support communities of plants and animals (ecosystems) that have adapted to thrive there. The term climate refers to temperature and moisture patterns that characterize a large region over tens, hundreds, or even thousands of years. Local changes that last days, weeks, or seasons, like storms and droughts, are called weather.
Regulating sunlight: the ozone and greenhouse layers
Energy from the Sun drives Earth’s climate and biology. Sunlight heats the surface and nourishes plants that, in turn, feed animals. Heat drives ocean currents, winds, and the hydrologic cycle (the circulation of water between the land, oceans, and the layer of air surrounding Earth, called the atmosphere).
Layers of gas in the atmosphere regulate incoming solar energy and maintain the planet’s average temperature at about 60°F (16°C). The gaseous layers keep Earth within the temperature range where life-sustaining oceans are liquid and life flourishes.
Unfiltered sunlight is too strong for organisms; it damages plants and burns animal tissues. A layer of ozone gas in the outer atmosphere acts as a shield that protects Earth from the Sun’s most dangerous rays—ultraviolet radiation. Ozone absorbs most of the Sun’s ultraviolet rays.
The filtered sunlight that reaches the surface has the correct intensity to set off a process in green plants in which they produce their own food. When sunlight strikes objects on Earth’s surface, they warm up and radiate heat back toward outer space.
Gases like carbon dioxide and water vapor keep Earth warm by trapping heat in the lower atmosphere. They are called greenhouse gases because they warm Earth’s surface the same way a greenhouse stays warm in the winter.
How climate works
Sunlight falls unevenly on Earth’s surface. The Sun’s rays are more direct at the equator and less direct at the North and South Poles, causing surface temperature to decrease the farther away the area is from the equator.
Temperature also decreases with altitude (elevation), making it very cold on high mountain peaks. This uneven heating creates heat-driven flows in the oceans (currents) and atmosphere (winds).
Circular patterns of rising warm air and sinking cool air in the atmosphere (called Hadley cells) control the distribution of rainfall. Six Hadley cells, three in each hemisphere (half of the Earth), create wind belts (consistent winds in a prevailing direction) and climate zones.
To illustrate, follow a volume of air as it completes a trip around the Hadley cell north of the equator: Intense sunlight heats ocean water in the tropical zone around the equator. The air warms, and gains moisture from the warm water below it. It rises and flows north of the equator, cooling as it moves.
Because cool air holds less moisture than warm air, the water vapor condenses (changes to liquid from a gas) into clouds and falls as heavy rain in the tropics. Once dry, the air flows north and sinks over one of the hot, dry deserts north of the tropics like the Sahara.
The dry air then flows back along the surface toward the equator. Earth’s eastward rotation causes the returning winds to bend toward the west; they are the strong, steady Trade Winds that blow on either side of the equator.
Earth’s climate zones are defined by their average yearly rainfall (or snowfall) and temperature. In general, they are alternating, east-west oriented, wet and dry zones under the rising and falling Hadley cells.
If Earth were a simple, water-covered ball, without complicating factors like continents, high mountain ranges, and ocean currents, there would be five climate zones in each hemisphere: tropical, arid, temperate, cold, and polar.
- Tropical (hot, wet): Lush, biologically-diverse rainforests thrive in the tropical zone at the equator. The jungles of central Africa, the Amazon basin in South America, and south Pacific Islands like Borneo lie in the tropical zone.
- Subtropical arid and semi-arid (hot, dry): Earth’s great deserts lie in arid zones north (Saharan, Arabian) and south (Kalahari, Australian Outback) of the equator. Dry, semi-arid (mostly arid) grasslands form the bordering lands around the subtropical deserts. The African savannah, Asian steppe, and Great Plains of North America are semi-arid grasslands that support large mammals like elephants, horses, and buffalo.
- Temperate (mild temperatures, moderate rainfall): A large percentage of Earth’s population lives in mild and temperate regions of North and South America, Europe, and Asia. These climates often have warm, dry summers and cool, wet winters. Coastal regions are usually wetter and have less extreme temperature variations than inland temperate regions.
- Cold (cold, moderate rainfall): The cold, snowy, northern forests of North America, Scandinavia, and Asia are called the boreal zone. The treeless plain of the sub-arctic between the boreal forest and the polar ice cap (the thick covering of permanent ice and snow at the North and South Poles) is called tundra.
- Polar (very cold, very dry): The North and South Poles are cold, dry deserts. The polar ice caps have formed from the accumulation of light snows over thousands of years.
The positions of continental land masses, ocean currents, and high mountain ranges also affect the pattern of climate zones. Land heats up and cools down faster than water.
Many coastal areas have climates affected by wet onshore winds that bring rain, dry offshore winds that create coastal deserts, or reversing winds (monsoons) that cause alternating wet and dry seasons. Warm ocean currents keep some regions that are far from the equator warm, and cold water upwelling (rising up) from the deep ocean cools some tropical coastlines and islands.
Winds that flow from the oceans onto land generally lose their moisture as they travel inland or uphill. The interiors of large continents like Asia, Australia, and North America are generally dry.
When moist air reaches a tall mountain range, it drops rain as it rises and cools. The slopes of mountain ranges exposed to the wind are typically wetter than the sides away from the wind.
Arid deserts and semiarid grasslands form in the rainshadows (an area of decreased precipitation on the downwind side of a mountain), behind tall mountains. In the United States, high winds called the jet stream carry moisture-rich air from west to east.
It is rainy in the Pacific Northwest, and snowy on the western slopes of the Cascade, Sierra Nevada, and Rocky Mountains. The air is bone dry when it reaches the Mojave, Sonoran, and Chihuahuan deserts of the American Southwest, and northern Mexico.
Changes in the factors that determine climate can lead to global climate change over time. Cooling leads to global sea level fall and glacial advance (increased ice formation and spread of ice at the polar ice caps); warming melts the polar ice caps and water rises to cover the edges of the continents.
In either case, Earth’s climate zones and regional subclimates must adjust to the new temperature and rainfall patterns. Geologic data confirms that Earth has warmed and cooled throughout its history as its position has changed relative to the Sun.
Plate tectonic forces (the bumping together and moving apart of large plates of Earth’s crust) have rearranged the continents, changing the paths of ocean currents and the pattern of dry land, bodies of water, and ice. The amounts of greenhouse gases and ozone in the atmosphere have changed naturally and because of human activities.
Global warming is the increase in the average temperature of the Earth’s surface. Many scientists and environmentalists are concerned that increased carbon dioxide in the atmosphere from the use of fossil fuels like oil and coal could lead to global warming and climate change.