Climate is the long-term pattern of weather in a particular place, averaged over a standard period of 30 years. While weather describes what’s happening outside your window right now, climate describes what you’d typically expect for any given day or season based on decades of data. The World Meteorological Organization has used 30-year averaging periods as the global standard for nearly a century, partly because 30 data points is a reliable statistical threshold for producing stable averages.
Those averages cover more than just temperature. Climate encompasses patterns in precipitation, wind speed and direction, humidity, and other atmospheric conditions recorded day after day, year after year. When all of that data is compiled for a location, it produces a profile: the climate of that place.
How Climate Differs From Weather
Weather is what you check before leaving the house. Climate is why you own a winter coat in Minnesota but not in Miami. Weather can shift in minutes. Climate shifts over decades. A single unusually cold summer doesn’t change a region’s climate, just as one warm winter doesn’t make the Arctic tropical. Climate is the statistical personality of a place’s atmosphere, built from averaging daily maximum and minimum temperatures, rainfall totals, wind patterns, and other measurements across those 30-year windows.
Countries have been computing these averages in coordinated global updates since the early 1900s, starting with the 1901 to 1930 period and refreshing every 30 years. The current reference period used by most national weather services is 1991 to 2020.
What Powers Earth’s Climate
Everything starts with the sun. Of all the solar energy that reaches the top of the atmosphere, about 47% is absorbed by Earth’s surface, 19% is absorbed by gases in the atmosphere, and the rest is reflected back to space by clouds (23%) and the surface itself (7%). The energy Earth absorbs is eventually radiated back out as heat, and this incoming-versus-outgoing energy balance is what sets the planet’s baseline temperature.
Greenhouse gases in the atmosphere, primarily water vapor and carbon dioxide, trap some of that outgoing heat and redirect it back toward the surface. This natural greenhouse effect is what keeps Earth warm enough to support life. Without it, the planet would be far too cold for liquid water.
Five Parts of the Climate System
Climate isn’t controlled by the atmosphere alone. It emerges from interactions between five interconnected parts of the Earth system.
- Atmosphere: The layer of gases surrounding the planet, where greenhouse gases trap heat and weather systems form.
- Oceans: Water absorbs and stores enormous amounts of heat and carbon dioxide, acting as a temperature buffer that dampens extreme swings in climate.
- Ice (the cryosphere): Ice sheets, glaciers, and sea ice reflect a large share of incoming sunlight back to space. They also influence deep ocean circulation patterns that redistribute heat around the globe.
- Land surface: Vegetation and soils determine how solar energy is absorbed or returned to the atmosphere. Soil moisture evaporation, for instance, has a strong influence on local surface temperatures.
- Living things (the biosphere): Plants absorb carbon dioxide through photosynthesis and store significant amounts of carbon. Ecosystems on land and in the ocean influence how greenhouse gases cycle through the system.
Changes in any one of these components ripple through the others. When ice melts, less sunlight gets reflected, so the ocean absorbs more heat, which melts more ice. These feedback loops are a central feature of how climate behaves.
What Determines a Region’s Climate
Two cities at the same latitude can have very different climates depending on local geography. Several physical factors shape what a region’s climate looks like in practice.
Latitude is the most fundamental. As you move away from the equator, average yearly temperatures drop because sunlight hits the surface at a lower angle and spreads over a larger area. Latitude also drives precipitation patterns: belts of rising air near the equator and around 60° north and south produce heavy rainfall, while belts of sinking air near 30° north and south create dry conditions and deserts.
Elevation works like a second latitude. Higher altitudes are cooler, which is why mountaintops can be snow-capped even in the tropics. Proximity to large bodies of water matters too. Oceans and large lakes moderate temperature swings, keeping coastal areas milder in winter and cooler in summer compared to inland locations at the same latitude. Coastal regions also tend to receive more precipitation, especially on the side of a landmass where prevailing winds blow in from the water.
The Köppen Climate Classification
Scientists organize the world’s climates into five broad categories using temperature and precipitation thresholds. This system, called the Köppen classification, gives every region on Earth a climate type.
- Tropical (A): Every month averages 18°C (64°F) or warmer. These are the hot, often humid climates near the equator.
- Arid (B): Too dry to support most vegetation. Subdivided into hot deserts and cold deserts based on whether the annual average temperature is above or below 18°C.
- Temperate (C): The coldest month falls between −3°C and 18°C (27°F to 64°F). This covers a wide range of mild climates, from Mediterranean coastlines to humid subtropical regions.
- Continental (D): At least one month averages below −3°C. These climates have harsh winters and are found mainly in the interiors of large continents.
- Polar (E): The warmest month stays below 10°C (50°F). Polar tundra regions see temperatures rise above freezing in summer, while polar frost regions never do.
How Scientists Measure and Reconstruct Climate
Modern climate monitoring relies on networks of ground stations equipped with precision instruments: platinum thermometers for air temperature, weighing rain gauges for precipitation, anemometers for wind speed, pyranometers for solar radiation, and soil sensors buried at multiple depths. These stations transmit data via satellite to central databases where it undergoes quality checks and becomes part of the long-term climate record.
But the instrument record only goes back about 150 years. To understand climate further into the past, scientists use proxy data: natural archives that preserve clues about ancient conditions. Ice cores drilled from glaciers and ice sheets contain trapped air bubbles and dust layers that reveal temperature, precipitation, atmospheric composition, and volcanic activity stretching back hundreds of thousands of years. Tree rings record growing conditions in their width, density, and chemical composition. Ocean and lake sediment cores contain tiny fossils and pollen grains that show what organisms and plants thrived at different times, offering a window into past temperatures and rainfall.
How Climate Changes
Climate has never been perfectly static. Natural forces have pushed it warmer and cooler throughout Earth’s history. Slow shifts in the planet’s orbit and axial tilt alter how much solar energy different parts of Earth receive over cycles spanning tens of thousands of years. Volcanic eruptions can temporarily cool the climate by injecting particles into the upper atmosphere that reflect sunlight. Variations in the sun’s energy output also play a role, though a relatively small one on human timescales.
What makes the current period different is the speed and cause of change. Since about 1750, human activities have added greenhouse gases to the atmosphere at rates with no precedent in at least 10,000 years. Carbon dioxide has risen from a pre-industrial concentration of about 280 parts per million to 422.8 ppm in 2024. For context, ice core records show that over the previous 650,000 years, CO2 levels naturally fluctuated between 180 and 300 ppm and never came close to today’s levels. Methane and nitrous oxide concentrations have followed similar trajectories, driven primarily by agriculture and fossil fuel use.
The combined warming effect of these added greenhouse gases is significant. The net impact of human activities since 1750 has been a warming influence on the climate system, with fossil fuel combustion and land use change as the primary drivers. This is layered on top of natural variability, but it now far exceeds what natural factors alone would produce.