Introduction
In 1824, Joseph Fourier calculated that Earth should be far colder than it is — and realised the atmosphere must be acting like an insulating blanket. By 1896, Svante Arrhenius had computed how much warmer Earth would get if CO₂ doubled. The science of climate began not in the 21st century, but 200 years ago.
Climate is the long-term statistical behaviour of the atmosphere and ocean system — the average weather, its variability, and the probability of extremes at a given location and time of year. Weather is the specific state of the atmosphere at a given moment (today's temperature, wind, and precipitation); climate is the distribution of all possible weather states over decades (the 30-year climatological normals used by meteorologists). Climate is not simply 'average weather' — it includes variance, extremes, and temporal patterns (seasonality, interannual variability, long-term trends) that cannot be captured by means alone.
Earth's climate system consists of five coupled components that exchange energy, water, and matter: (1) the atmosphere — the gaseous envelope, the most rapidly responding component, with variability on timescales of days to years; (2) the hydrosphere — liquid water in the ocean, lakes, and rivers; the ocean dominates, storing ~93% of Earth's surface heat and modulating climate on timescales of decades to millennia; (3) the cryosphere — all frozen water (sea ice, ice sheets, glaciers, permafrost, snow cover); changes in ice extent alter albedo, sea level, and ocean circulation on timescales of years to millions of years; (4) the biosphere — all living organisms, which modulate atmospheric CO₂, surface albedo (vegetation), and the water cycle through evapotranspiration; (5) the lithosphere — the solid Earth; relevant to climate through volcanic CO₂ degassing, weathering reactions that remove CO₂, and plate tectonics that repositions continents over millions of years. These five components interact through fluxes of energy and mass, forming a coupled system that exhibits internal variability (natural oscillations and chaotic dynamics) as well as responses to external forcing.
Energy from the Sun drives the climate system. The solar constant (the flux of solar radiation at Earth's mean orbital distance) is approximately 1,361 W m⁻², reduced to an effective average insolation of ~340 W m⁻² when averaged over Earth's spherical surface (dividing by 4 for geometry). Of this incoming solar radiation, approximately 30% is reflected back to space (the planetary albedo), leaving ~238 W m⁻² absorbed by the surface and atmosphere. The system is in approximate radiative equilibrium: the absorbed solar radiation is balanced by outgoing longwave (infrared) radiation to space. The greenhouse effect, surface albedo, and atmospheric circulation all modulate how this energy budget is distributed spatially and temporally.
Key Terms
The coupled system of five major components — atmosphere, hydrosphere, cryosphere, biosphere, and lithosphere — that together determine Earth's climate. Each component has different response timescales (atmosphere: days–years; ocean surface: weeks–years; deep ocean: centuries–millennia; ice sheets: millennia–millions of years; lithosphere: millions–hundreds of millions of years). The climate system exchanges energy, water, and matter between components through fluxes and feedbacks.
An external perturbation to Earth's energy budget that causes the climate system to depart from its previous equilibrium state. Positive forcings add energy to the system (warming); negative forcings remove energy (cooling). Examples: changes in solar output, volcanic aerosol injection (negative, temporary), changes in greenhouse gas concentrations (positive, persistent), changes in Earth's orbital parameters (Milankovitch cycles). Measured in W m⁻² globally averaged. Distinguished from internal variability, which arises from within the climate system without an external trigger.
A process in the climate system that amplifies (positive feedback) or dampens (negative feedback) the initial response to a forcing. The Planck (blackbody) feedback is the primary negative feedback: as the surface warms, it radiates more energy to space (Stefan-Boltzmann law), restoring equilibrium. Positive feedbacks include ice-albedo feedback (warming melts ice → lower albedo → more warming) and water vapour feedback (warming → more atmospheric H₂O → enhanced greenhouse → more warming). The net feedback determines climate sensitivity.
The equilibrium global mean surface temperature increase expected from a doubling of atmospheric CO₂ concentration (abbreviated ECS — Equilibrium Climate Sensitivity). The IPCC Sixth Assessment Report (2021) assessed ECS as "likely" (66% probability) to be in the range 2.5–4.0°C (4.5–7.2°F) and "very likely" (90%) in the range 2.0–5.0°C (3.6–9°F), with a best estimate of 3°C (5.4°F). Climate sensitivity integrates all climate feedbacks and is the key parameter linking greenhouse gas emissions to future warming.
Natural, unforced fluctuations in the climate system arising from the chaotic dynamics of the coupled atmosphere-ocean system, including: ENSO (El Niño-Southern Oscillation) — 3–7 year Pacific oscillation dominating interannual global temperature variability; PDO (Pacific Decadal Oscillation) — 20–30 year mode; AMO (Atlantic Multidecadal Oscillation) — 60–80 year mode. Internal variability can temporarily mask or amplify forced climate trends on timescales of years to decades, complicating the attribution of observed climate change to specific causes.