# Sea Level Rise Sea level rise is the slow, grinding impact of warming with the longest lead time and the deepest commitment. Even if emissions stopped today, the oceans would keep rising for centuries, because the heat already added is still working its way into the deep sea and the great ice sheets respond slowly. It is the impact that most clearly exposes the gap between political time horizons and physical ones. ## The Two Causes Sea level rises from two processes. The first is thermal expansion: water expands as it warms, and the ocean has absorbed more than 90% of the excess heat trapped by greenhouse gases, so the whole water column is swelling. The second is the addition of water from melting land ice — mountain glaciers (the subject of [[Glaciology]]) and, far more consequentially, the Greenland and Antarctic ice sheets. Melting sea ice does not raise sea level, because floating ice already displaces its weight; only ice resting on land adds new water to the ocean. Global average sea level rose about 20 centimeters over the twentieth century, and the rate has accelerated to over 4 millimeters per year, measured precisely by satellite altimetry since 1993. The acceleration matters more than the current rate, because it signals that the ice sheets — not just thermal expansion — are beginning to contribute meaningfully. ## The Ice-Sheet Wild Card The deepest uncertainty, and the gravest risk, lies in the ice sheets. Greenland holds enough ice to raise sea level about 7 meters; West Antarctica about 3–4 meters; East Antarctica far more. These are classic Climate Feedbacks and Tipping Points elements: marine-based ice sheets like West Antarctica can become unstable once warm water undercuts their grounding lines, potentially committing the world to multi-meter rise that unfolds over centuries but cannot be stopped once begun. Because the IPCC's consensus process tends toward caution, its sea-level projections have historically excluded the hardest-to-model ice dynamics and have repeatedly been revised upward. ## The Politics Sea level rise has a peculiar political profile. It is gradual enough that it rarely commands urgency, yet relentless enough to be among the most certain long-term outcomes. It also lands unevenly — on low-lying island nations, on coastal cities, on communities least able to fund defenses — linking it to [[Climate Migration]] and to questions of who pays for seawalls versus who is told to retreat. The phenomenon has produced some of the most direct collisions between science and political denial, as when officials in coastal U.S. states moved to bar state agencies from using accelerating sea-level projections in their planning, legislating against the forecast rather than against the sea. ## Connections Sea level rise is driven by ocean heat and by ice loss covered in [[Glaciology]], governed by the ice-sheet Climate Feedbacks and Tipping Points, and is a leading driver of [[Climate Migration]]. For the field overview, see [[Climate Science]].