The same atoms of life cycle repeatedly through the biosphere. The carbon cycle moves carbon among the atmosphere, oceans, soil, and living organisms through photosynthesis, respiration, decomposition, and exchanges with seawater. The nitrogen cycle moves nitrogen through the atmosphere, soil, and organisms, with bacteria playing key roles in converting atmospheric nitrogen into biologically usable forms. The water cycle continuously moves water through evaporation, condensation, and precipitation, distributing heat and fresh water around the planet. The phosphorus cycle, unlike the others, has no major atmospheric phase; phosphorus moves mainly through rocks, soil, water, and organisms, which makes its supply more limited and its losses through runoff a long-term concern.
Human activity has altered these cycles, especially carbon. The greenhouse effect is the warming of Earth's surface by gases that trap outgoing infrared radiation in the atmosphere. The main greenhouse gases are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), water vapor, and fluorinated gases. Their relative impact is measured by global warming potential, or GWP: methane has a GWP roughly 28 to 36 times that of CO₂ over 100 years, meaning a ton of methane warms the planet far more than a ton of CO₂ over that timescale. Burning fossil fuels, deforestation, and industrial agriculture release greenhouse gases faster than natural sinks can absorb them, driving anthropogenic climate change.
Climate is distinct from weather: weather refers to short-term atmospheric conditions, while climate describes long-term averages and patterns. The clearest empirical evidence of changing climate is the Keeling curve, a continuous record of atmospheric CO₂ measured since 1958 at Mauna Loa, Hawaii. As of recent years, CO₂ concentrations have risen above 420 parts per million, compared with a pre-industrial level of about 280 ppm. The global average temperature anomaly, the difference between today's average surface temperature and the pre-industrial baseline, stands at roughly 1.1 to 1.4°C and continues to rise. Scientists working through the Intergovernmental Panel on Climate Change (IPCC) synthesize these findings for policymakers, and the annual Conference of the Parties (COP) is where nations negotiate responses, including the 2015 Paris Agreement, which aims to limit warming to well below 2°C, ideally 1.5°C, above pre-industrial levels.