# What factors affect climate? Climate is the long-run average of weather, usually measured over about 30 years. It is shaped by five interacting components: atmosphere, living things, ice, water and the Earth's crust, plus external forcings such as the Sun, volcanoes, orbital changes and human activity. Before listing factors, it helps to be precise about what is being affected. Weather is what happens over hours or days; climate is the statistical picture of those conditions over decades. The Met Office usually defines a region's climate over a 30-year period, while Fairfax County's environmental office describes climate as typical meteorological conditions based on statistical averages across 20 years or longer. Either way, a single cold week or a single record-setting day says nothing on its own about climate, because reports of extreme events like these also existed decades ago. What matters is the long-term trend in averages and in how often extremes occur. References: [https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate](https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate), [https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate](https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate). ## The five components of the climate system The Met Office breaks the climate system into five parts that interact in complex cycles, producing knock-on effects and feedback loops. When any of these components varies, the overall climate can change. References: [https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate](https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate). - Atmosphere: the air and the clouds within it; atmospheric changes affect how heat is retained and can modify cloud patterns. - Biosphere: living things, including vegetation on the land surface. - Cryosphere: ice in the form of sea ice, glaciers, ice sheets and permafrost. - Hydrosphere: water on, under or above the Earth's surface, such as oceans, lakes, rivers and clouds. - Lithosphere: the Earth's crust. References: [https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate](https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate). ## External forcings: what pushes the system from outside Separate from the five components are external forcings. The Met Office lists variations in the Earth's orbit and the Sun's output, volcanic eruptions that alter the atmosphere, and human activities such as the release of greenhouse gases and changes in land use. Most of the energy in the climate system ultimately comes from the Sun, so changes in solar output affect how much heat enters the system. Land cover changes, such as an ice sheet melting or a forest being cut down, can also produce climate changes. References: [https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate](https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate). ## Natural cycles within the system The Met Office distinguishes external forcings from internal variability: natural variation inside the climate components, some of it predictable cycles and some of it random. Named examples include ocean circulation changes such as the Atlantic Meridional Overturning Circulation, sea surface temperature cycles such as El Niño-Southern Oscillation and the Pacific Decadal Oscillation, atmospheric cycles such as the North Atlantic Oscillation, the Madden-Julian Oscillation and the Quasi-biennial Oscillation, plus chemical cycling such as the carbon and nitrogen cycles. These cycles are part of why climate varies even without any outside push. References: [https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate](https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate). ## A worked example: one region, two timescales Take Fairfax County, Virginia. Its environmental office describes a humid subtropical climate: historically moist, warm summers, cool winters and precipitation spread fairly evenly through the year. Suppose one July brings a week of unusually cool, rainy days. That is weather. If instead the long-term averages show heat waves becoming more frequent and heavy rainfall events becoming stronger over decades, that is a climate shift, and the county reports exactly this kind of measurable change in recent decades. The Met Office wardrobe analogy captures the same idea: climate determines what clothes belong in your wardrobe, while weather tells you what to wear on a given day. References: [https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate](https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate), [https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate](https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate). ## A checklist for interpreting any climate claim - Check the timescale: is the claim about days (weather) or decades (climate)? - Check the period used for the average, since climate is typically defined over roughly 20 to 30 years of data. - Match the indicator, geography, units and observation period before comparing places or years. - Separate country-level averages from local conditions; a national figure is not a local forecast. - Remember that a single record event is not evidence of climate change on its own; trends across many years of data are the indicator. References: [https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate](https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate), [https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate](https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate), [https://climate99.com/](https://climate99.com/). One limitation worth stating plainly: the Met Office and Fairfax County explain the components and forcings of climate, but neither ranks the factors by size or attributes any specific amount of change to any specific cause. For that kind of quantified attribution, you would need to consult current scientific assessments matched to the exact indicator, region and time period you care about. References: [https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate](https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate), [https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate](https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate). A practical next step: pick one country and compare its dated indicators, such as CO₂ per person or forest area, against a neighbour's, keeping the source year attached to every number so you are comparing evidence rather than impressions. References: [https://climate99.com/](https://climate99.com/). ## Sources - [https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate](https://weather.metoffice.gov.uk/climate/climate-explained/what-is-climate) — accessed 2026-10-18 - [https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate](https://www.fairfaxcounty.gov/environment-energy-coordination/climate-matters/weather-vs-climate) — accessed 2026-10-18 - [https://climate99.com/](https://climate99.com/) — accessed 2026-10-18 Published 2026-10-18. Climate99 editorial; AI-assisted preparation. ## Next step - [Open climate99.com](https://climate99.com/) - [Compare](https://climate99.com/compare) - [Map](https://climate99.com/map) - [Co2 emissions per capita](https://climate99.com/metric/co2-emissions-per-capita)