4 Climate Through Time

Climate and weather

Climate is the pattern of weather in a place over long periods, typically assessed using averages over 30 years or more. It describes both typical conditions and their range of variation, whereas weather can change from day to day. Climate is shaped by how energy from the Sun is distributed and by the atmosphere, oceans, land, ice, and living things.

Controls on regional climate

Several interacting factors shape regional climate. Latitude affects how directly the Sun’s rays strike Earth: the tropics receive more direct sunlight than high latitudes and, on average, receive more solar energy. Earth’s curved surface and axial tilt shape this uneven heating.

Elevation also matters because air temperature generally decreases with height, making high plateaus and mountain regions often cooler than nearby lowlands. Water heats and cools more slowly than land, so coastal locations often have smaller temperature swings than inland areas; water also supplies moisture for precipitation.

Ocean currents transport heat and can warm or cool nearby coasts while influencing regional rainfall and winds. Topography and prevailing winds affect how heat and moisture move: mountains can force moist air upward, causing rain on one side and drier conditions on the other, while large-scale atmospheric circulation carries heat and moisture between regions.

Together, these controls produce broad Climate zones, including tropical, dry, temperate, continental, and polar climates. These are useful categories rather than sharp boundaries: conditions usually change gradually across a landscape, and local geography can create exceptions.

Why seasons occur

Earth’s axis is tilted relative to its orbit around the Sun. As Earth travels around the Sun, each hemisphere alternately tilts toward or away from it. A hemisphere tilted toward the Sun receives more direct sunlight and has longer days, producing summer; when tilted away, it receives less direct sunlight and has shorter days, producing winter. The Northern and Southern Hemispheres experience opposite seasons.

Seasons are caused mainly by the tilt of Earth’s axis, not by changes in Earth’s distance from the Sun. Seasonal effects vary by location. Near the equator, daylight changes relatively little through the year, while high-latitude regions have large changes in daylight. In some tropical regions, seasonal rainfall changes are more pronounced than seasonal temperature changes.

Natural climate variability

Climate varies naturally from year to year and decade to decade. El Niño–Southern Oscillation (ENSO) is one important source of this variability: it involves recurring changes in tropical Pacific Ocean temperatures and atmospheric circulation. Its warm phase, El Niño, and cool phase, La Niña, can shift temperature and rainfall patterns across many regions. ENSO influences climate variability but does not, by itself, explain a persistent global warming trend.

Other natural influences include volcanic eruptions, which can temporarily cool the surface by adding sunlight-reflecting particles to the atmosphere; small changes in the Sun’s output; and internal exchanges of heat between the ocean and atmosphere. These influences operate on different timescales and can temporarily raise or lower temperatures in particular regions or across the globe.

Long-term patterns and past climates

Over thousands to hundreds of thousands of years, gradual changes in Earth’s orbit and axial orientation alter the distribution of sunlight reaching different latitudes and seasons. Milankovitch cycles involve changes in orbital shape, axial tilt, and the direction of Earth’s axis. They help explain the timing of past glacial and interglacial periods, particularly through changes in summer sunlight at high northern latitudes. Their effects unfold over very long timescales and do not explain today’s rapid warming.

Scientists reconstruct past climates using evidence such as ice cores, tree rings, corals, and ocean sediments. Ice cores are especially useful because tiny trapped air bubbles preserve samples of ancient atmospheres, while the surrounding ice provides clues about past temperatures. These records show that climate has changed naturally and allow scientists to compare the pace and causes of past changes with those occurring now.

Human-driven climate change

Human activities—especially burning coal, oil, and gas, as well as land-use changes—have increased atmospheric greenhouse gases. These gases reduce the rate at which heat escapes to space, creating a warming influence on the climate system. The IPCC concludes that human influence has unequivocally warmed the atmosphere, ocean, and land.

Natural variability continues alongside this long-term, human-driven trend. As a result, a particular year or region may temporarily warm or cool more or less than the global average. A cold winter is a weather event; a run of unusually cold or warm years may reflect variability; a sustained shift in long-term averages and patterns is climate change. Scientists identify the causes of climate change by comparing observations with evidence about natural and human influences, not by relying on a single season or weather event.