Thermal expansion accounts for ~40% of 20th-century rise — as oceans warm, water expands. Each 1°C of average ocean warming raises global sea level by ~0.5 m over centuries. Greenland's ice sheet holds ~7 m sea-level equivalent (SLE); Antarctica holds ~58 m SLE. Current melt contributions: Greenland ~0.8 mm/yr, Antarctica ~0.5 mm/yr. The IPCC AR6 (2021) projects 0.3–1.0 m by 2100 under SSP scenarios, with "low likelihood but high impact" scenarios of 1.5–5 m from marine ice sheet instabilities (MISI). The West Antarctic Ice Sheet is especially vulnerable: its bedrock slopes inward (retrograde), creating a potential runaway feedback (Marine Ice Sheet Instability). Each centimeter of rise threatens ~3–4 million additional people from annual flooding.