Abrupt climatic change related to unstable ice-sheet dynamics: toward a new paradigm

Abstract

Ice streams are fast currents of ice that drain some 90% of present-day ice sheets, and probably drained a similar percent of ice from former ice sheets. Ice streams may have life cycles akin to, but much longer than, surge cycles of certain mountain glaciers. The floating tongues of marine ice streams coalesce to form floating ice shelves that buttress marine ice sheets, and the grounded lobes of terrestrial ice streams stabilize terrestrial ice sheets. Ice shelves and ice lobes form during the life cycles of ice streams, and disintegrate rapidly to initiate new life cycles. Successive life cycles continue until the ice sheet collapses. A new paradigm is postulated in which abrupt climatic change is driven by iceberg outbursts associated with life cycles of marine ice streams and dust storms associated with life cycles of terrestrial ice streams. For Pleistocene Northern Hemisphere ice sheets, these mechanisms regulated North Atlantic surface temperatures and salinity, and perhaps planetary albedo and atmospheric CO 2, and thereby drove climatic change, which tended to be abrupt because ice-stream life cycles began and ended abruptly. Ice streams proliferated along the margins of Northern Hemisphere ice sheets after they advanced beyond the crystalline shields of North America and Eurasia, and spread over softer sedimentary rocks overlain by thick ice-cemented sediments that progressively thawed, especially along river valleys and marine troughs. An increasing fraction of these ice sheets was then subjected to the unstable life cycles of ice streams, as seen in the pronounced spikiness of the late glacial oxygen isotope stratigraphy in the Greenland ice core at Dye 3. Life cycles of Greenland ice streams during the Holocene seem to be linked to episodes of regional climatic cooling; most recently, the Little Ice Age. When superimposed on the present Milankovitch hemicycle of reduced insolation over the North Atlantic, this could abruptly initiate a new worldwide glaciation cycle in the aftermath of present-day CO 2 “greenhouse” warming. Proliferation of Antarctic ice-stream life cycles during the period of “greenhouse” warming may also trigger global climatic change by forcing a reorganization of circumpolar thermohaline circulation in the Southern Ocean.