Key Terms

📖 Key Terms

greenhouse gases — Gases that absorb and re-emit outgoing infrared (heat) radiation from Earth's surface, trapping heat in the lower atmosphere. The main greenhouse gases are CO₂, methane (CH₄), nitrous oxide (N₂O) and water vapour. Human activities have increased CO₂ from ~280 ppm pre-industrially to 421 ppm in 2023 — the highest level in at least 3 million years.

enhanced greenhouse effect — The intensification of the natural greenhouse effect caused by human emissions of greenhouse gases. Higher concentrations of CO₂ and other GHGs trap more outgoing heat, creating a planetary energy imbalance that drives global warming. The IPCC Sixth Assessment Report (2021) states it is "unequivocal" that human influence has caused this warming.

mitigation — Actions that reduce the causes of climate change by cutting greenhouse gas emissions or enhancing carbon sinks. Examples: renewable energy, energy efficiency, reforestation, carbon capture, international agreements (Paris Agreement). Mitigation addresses the root cause but requires sustained global cooperation.

adaptation — Adjusting human activities to cope with the effects of climate change that are already unavoidable. Examples: sea walls (Maldives), flood-tolerant crops (Bangladesh), Room for the River (Netherlands). Adaptation manages consequences rather than causes.

Milankovitch cycles — Natural variations in Earth's orbital shape, axial tilt, and wobble that alter the distribution of solar radiation over tens of thousands to hundreds of thousands of years, driving ice ages and warm interglacials. They are far too slow to explain modern warming.

sea level rise — The rise in global mean sea level caused by thermal expansion of seawater and melting of land-based ice (glaciers, ice sheets). Sea levels have risen 21–24 cm since 1880 and are currently rising at 3.7 mm/year, threatening low-lying countries and coastal cities.

carbon sink — A natural system that absorbs more CO₂ from the atmosphere than it releases. Forests, oceans and soils act as carbon sinks. Deforestation destroys carbon sinks and releases stored carbon, contributing to rising atmospheric CO₂ concentrations.

carbon capture and storage (CCS) — Technology that captures CO₂ from industrial processes or power generation and stores it permanently underground, preventing it from entering the atmosphere. Example: the Sleipner Project, Norway, which has injected 1 million tonnes of CO₂ per year into North Sea sandstone since 1996.

phenology — The study of seasonal biological events such as first flowering, migratory bird arrival and hibernation. Phenological records provide biological evidence of climate change — UK spring is arriving 26 days earlier than in the 1980s.

positive feedback — A process in which an initial change amplifies further change in the same direction. Example: melting Arctic sea ice exposes darker ocean water, which absorbs more solar radiation than white ice, causing further warming and further melting. Positive feedbacks are a major concern in climate science because they can accelerate warming beyond what direct emissions would cause.