Glacial Systems - Variations between Accumulation and Ablation and Feedback Cycles

Variations can occur over long periods of time, such as during glacial and interglacial periods of time or over much shorter time frames such as seasonally, depending upon the type of glacier.

Over time, when the world enters a glacial period, accumulation exceeds ablation. Existing glaciers will increase in size and mass, due to precipitation of snow and the ensuing albedo effect that the glacial ice has. New ones may form in areas of high accumulation.

The albedo effect occurs due to the reflective surface of the glacial ice reflecting incoming solar radiation. When the global temperature drops because of the earth’s orbital position around the Sun, and the tilt of its axis, global climate cools, and the ice sheets grow in high latitude areas and mountainous regions. With more extensive ice coverage, this in turn causes more heat to be reflected back into space, the temperature to fall further and more precipitation to fall as snow.

This creates a positive feedback cycle where an initial change in inputs (reduced incoming solar radiation) is amplified by the changes it brings to other components within the system. The earth gets ‘locked’ into an ice age, which is why they can commence so rapidly and last so long.

When the world enters an interglacial phase the reverse occurs: the earth receives more solar radiation, ablation will increase at the same time as less precipitation falling as snow with the result that ice sheets and glaciers start to retreat. The retreat occurs around coastal areas for ice sheets, due to the warming effects of the waters and at the snout of alpine glaciers. With reducing ice-cover albedo drops to a lower reflectivity state, darker waters and exposed rock absorbs more solar radiation, and the earth’s temperature shows a persistent rise.

The most recent ice advance episode was during the Pleistocene glacial period. Ice coverage at this time was much greater than can be seen today. This glacial period began 2.5 million years ago and ended approximately 11,800 years ago. Several ice advances took place during this period and occupied more prolonged periods of time than the periodic retreat of ice during shorter, warm interglacials (the latest of which we find ourselves in at present). In the latest major ice advance – the Devensian – starting around 110 000 years ago in the northern hemisphere, the ice stretched from the Arctic southwards over Canada and just into the northern USA. In Europe, the ice extended over northern areas, including all of Scandinavia, much of northern Europe and large areas of the British Isles. The Pleistocene period ended around 12 000 years ago with the present interglacial – known as the Holocene.

In the last 50 years, glacial retreat has become ever more evident, with most scientists suggesting that human processes have led to the change in global climate, leading to their retreat. Increased CO2 emissions have a strong correlation with increases in global temperature. The majority of the world’s alpine glaciers have significantly retreated and ice sheets, including the Antarctic and Greenland ice sheets show evidence of retreat at a level previously unseen.

Seasonally, there can also be an impact on glaciers. During winter months, accumulation will increase causing an advance in glaciers and summer months will see the glaciers retreat once more. Winter advance, however, does not occur at the same rate as it once did due to the changes in global climate. Many alpine areas are receiving lower levels of precipitation over a winter period and this leads to the most rapid retreat of some alpine glaciers in the subsequent summer that have ever been seen.