A carbon offset is a reduction in greenhouse gas emissions made to compensate for emissions produced elsewhere. Organizations and individuals purchase offsets to 'balance' emissions they cannot eliminate—for example, buying offsets to compensate for air travel emissions. Offset projects include tree planting (which absorbs carbon), renewable energy installations (which displace fossil fuel generation), and methane capture from landfills. Quality varies significantly: credible offsets must represent real, verifiable, additional emission reductions that would not have occurred anyway. Critics argue offsets allow continued emissions rather than driving genuine reductions. Standards and certification schemes attempt to ensure offset integrity.