Warm Oceans, Melting Ice: How Ocean Heat Is Accelerating Glacier Loss From Below

The world's oceans have absorbed approximately 90% of the excess heat trapped by greenhouse gases since the Industrial Revolution. This has buffered atmospheric warming considerably — but at a profound cost to marine systems and, increasingly, to the ice that borders and underlies the world's great ice sheets. Warm ocean water intruding beneath ice shelves and at glacier grounding lines is now one of the primary drivers of accelerating ice loss in both Greenland and Antarctica — a mechanism that operates largely out of sight but with consequences that are visible in rising sea levels worldwide.

The Submarine Melting Mechanism

When warm ocean water reaches the base of a glacier or ice shelf — the grounding line where ice meets the seafloor or ocean floor — it melts ice with extraordinary efficiency. Saltwater at -1°C can melt ice at rates of metres per year; water at +1°C above freezing melts at rates of tens of metres per year. In West Antarctica, warm Circumpolar Deep Water — a water mass that typically circulates at depth but is increasingly intruding onto the Antarctic continental shelf — reaches temperatures of up to +1.5°C. At Thwaites Glacier, submarine melt rates at the grounding line have been measured at up to 25 metres per year.

Fjord Circulation and Greenland

In Greenland, the mechanism is similar but the geography is different. Greenland's outlet glaciers — the rivers of ice that drain the interior ice sheet to the ocean — flow through deep fjords. Atlantic Water, a warm, salty ocean mass, has been intruding further into these fjords and penetrating to greater depths. At Jakobshavn Glacier, one of Greenland's fastest-moving glaciers, the arrival of warm Atlantic Water in the fjord in the 1990s is closely correlated with a dramatic acceleration in ice flow and a tripling of mass loss rates. The glacier has retreated approximately 40 kilometres since 1990.