Antarctic Peninsula Climate Change

Cenozoic Climate Change

The Cenozoic Era has been a time of gradually decreasing temperatures and increasing ice volumes in Antarctica. With the opening of the Drake Passage (and the isolation of the continent by the Circumpolar Current), the gradual movement of the Antarctic Ice Sheet southwards to polar latitudes, and gradually decreasing atmospheric CO₂ concentrations, small ice caps and mountain glaciers began to develop on the Antarctic Peninsula and outlying islands around 35 million years ago.

Glaciation on the Antarctic Peninsula (Figure 3) began to develop during the latest Palaeogene and early Neogene, with initially small-scale ice caps and mountain-style glaciers eventually developing into large ice sheets during the Quaternary. The Quaternary period has been characterised by oscillations of climate, from 40,000 year cycles until 400,000 years ago, and then 100,000 year cycles after this period. During the last 400,000 years, large continental-shelf wide glaciations dominated the area, with ice retreating to the higher mountains during interglacials (such as in the present period). The Holocene is the present interglacial. During the Last Glacial Maximum (LGM; 18,000 years ago), a large ice sheet dominated the area, with grounded ice reaching the continental shelf. Sea levels were 125 m lower then than they are now.

ERA	PERIOD		EPOCH (and Ma)			GLACIAL EVENTS
CENOZOIC	QUATERNARY		Holocene	0.01		Retreat from shelf edge; ice shelf formation; strong climate variability and repeated small re-advances.
			Pleistocene		Late	18 ka BP. LGM glaciation to the continental shelf edge; ice streams emanating from the central mountain spine.
					Middle	Development of ice streams during glacial periods; excavation of glacial troughs. Glaciations become longer in duration.
				2.54	Early	Colder conditions; small ice sheet; short-lived glacial periods. Onset of Northern Hemisphere glaciation.
	TERTIARY	Neogene	Pliocene		Late	Latest Pliocene; Seymour Island glaciation.
				5.33	Early	5.0-4.3 Ma: Local ice-cap glaciation of James Ross Island. Antarctic Peninsula Ice Sheet expansion in James Ross Basin, with glaciations throughout Late Miocene, Pliocene and into Pleistocene.
			Miocene		Late	5.9-5.4 Ma: Small APIS fluctuations. 6.2 Ma: Small APIS fluctuations on Pacific Margin.
					Middle	11-12 Ma: James Ross Basin: WAIS expansion.
				23.03	Early	21-22 Ma: APIS and WAIS expansion.
		Palaeogene	Oligocene		Late	26-29 Ma: APIS and WAIS expansion. 24-28 Ma: James Ross Basin ice-rafting.
				33.09	Early	Early Oligocene APIS expansion, Seymour Island.
			Eocene		Late
					Middle	45-41 Ma: Alpine Glaciation on King George Island.
				55.8	Early
			Palaeocene		Late	Preglacial

Table 1. Timescale in the Antarctic Peninsula, showing glacial events from the Cenozoic to the present day. APIS - Antarctic Peninsula Ice Sheet. JRI - James Ross Island. WAIS - West Antarctic Ice Sheet. Small ice caps began to develop in the area about 45 million years ago. Larger ice sheets began to develop about 5 million years ago. Large continental wide ice sheets began to develop during the Quaternary, with oscillations at 100,000 year periodicities after about 400,000 years ago.

Twentieth Century climate change

The Antarctic Peninsula region has suffered rapid warming over the past half-century, with around 2.5°C temperature increase since 1950. This temperature rise is significantly higher than the global average. The peninsula is particularly susceptible to climate change due to its small size and relatively northern location. This warming has been exhibited by strong warming in the Southern Ocean, increased melt seasons, and the recession of Antarctic Peninsula glaciers. The IPCC predict continued warming in this region, which questions the viability of ice cover in the region. The melting and retreat of glaciers and ice sheets releases large volumes of fresh water into the oceans. This not only raises sea level, but also influences deep sea circulation and regional climate. The loss of glacier mass to the oceans has clear implications for sea level rise. The extent to which Antarctic Peninsula glaciers will retreat is important, as it will influence future predictions of sea level change.

In the Antarctic Peninsula, many marine-terminating glaciers are buffered by large ice shelves. Ice shelves form when glaciers reach sea level and spread out to form a floating shelf. These ice shelves provide back stress and stabilise the glaciers. Recent media coverage of twentieth century ice shelf collapse has focussed attention on this topic. Many ice shelves around the Antarctic Peninsula are now becoming unstable as a result of climatic warming. As the ice shelves disintegrate, the velocity of Antarctic Peninsula glaciers has increased, glaciers have thinned, and grounding lines have retreated. The recession of Antarctic Peninsula ice shelves has been mapped by the US Geological Survey (http://pubs.usgs.gov/imap/2600/A/). Although the removal of floating ice shelves does not directly raise sea levels, ice-shelf collapse destabilises changes the boundary conditions of their tributary glaciers, which leads to glacier recession and sea level rise.

The collapse of Antarctic ice shelves (Figure  3) is widely attributed to atmospheric warming, but it appears that oceanographic change and ice-shelf structure provide an important control in 'pre-conditioning' ice shelves to collapse. It is still uncertain whether future ice-shelf change will be dominated by oceanographic or atmospheric drivers.  Against this background, it is important that we understand the behaviour of Antarctic glaciers and ice shelves in the past, present and future.