evidence: What causes sea-level rise? Most of the observed sea-level rise (about 3 mm per year) is coming from the meltwater of land-based ice sheets and mountain glaciers, which adds to the ocean’s volume (about 2 mm per year combined), and from thermal expansion, or the ocean water’s expansion as it warms (roughly 1 mm per year). [...] You can read more about what is causing the rise and how we observe sea-level change.