Extreme weather

Glacial melt

The size and movement of a glacier is controlled by rates of ablation (melting) and accumulation (of snow and ice). During periods of global cooling (such as the onset of the Pleistocene ice-age) rates of ablation will fall and the result is a world-wide advance of glacial ice. However, during periods of warming the opposite happens. Ablation rates rise and the snout of a glacier recedes up-valley, exposing landforms such as moraine and fluvial deposits.

The production of vast quantities of melt-water and the growth of pro-glacial lakes was one of the most significant impacts of global warming at the end of the last glacial period - both in terms of landform development and for local climates. Notably, the sudden release of vast amounts of cold melt-water from melting glaciers and over-flowing melt-water lakes in North America caused sudden changes to ocean circulation in the north Atlantic. Interfering with the flow of warmer waters into the region, the cold melt-water served to cool Europe's air to such an extent that it delayed the return of warmer temperatures to the continent for a further 1,500 years. (see http://cgrg.geog.uvic.ca/abstracts/PerkinsOnceDuring.html )

The huge volume of ice-cold water had disrupted the flow of the Gulf Stream - the ocean current that brings warm waters to areas of the North Atlantic such as the UK. Also known to oceanographers as the Atlantic thermohaline circulation, the Gulf Stream carries one billion watts of heat annually from the tropics to the Arctic via Scotland.

Could another interruption of the Gulf Stream happen again as a result of climate change? Could the UK really get colder on account of climate change?

How might melting ice impact on ocean currents and surface air temperatures?

Will the Gulf Stream really get diverted as result of glacial melting caused by climate change? Scientists are certainly concerned about the possibility of the Gulf Stream being diverted, leading to the cooling of the UK. One recent conference on climate change reported that the chances of this occurring are now almost fifty-fifty. The current has recently been weakened by 10% and scientists warn that a 3C rise in global temperatures – which is well within current predictions – would bring a 45% chance of complete shutdown.

This is because patterns of ocean circulation will be modified by huge inputs of cold glacial melt-water from Greenland. The resulting climate in Europe could be even colder than that experienced in the 1600s, during the so-called Little Ice Age when the Thames regularly froze over in winter. Without the Gulf Stream, scientists warn that the UK's air temperatures could actually fall by 10C!

Melting of Antarctic ice is also a cause of great concern. Temperatures on the Antarctic Peninsula (an 800-mile long westward extension of the continent) have risen by 4C over the last 50 years and the winter temperature of the surrounding waters is thought to have risen by 10C. Physical changes are also evident. In March 2000, the world’s biggest iceberg, B15, broke away from the Ross Ice Shelf in Antarctica. 200 feet high and the size of Jamaica, it split in half in October 2003 and is now beginning to waste further.

Scientists believe this is a result of global warming and is linked to recent reports that glaciers in neighbouring Chile and Argentina are melting at twice the rate observed in the 1970s. Given that 70% of the earth’s fresh water is stored as ice in Antarctica, there is a clear threat of global eustatic sea level rises should Antarctica’s ice mass begin to decline further.

Student Practice Question:

Discuss the causes and consequences of increased rates of glacial ablation

As well as referencing important year-to-year ideas about seasonality, sun-spot activity and avalanching, a good answer will recognise that most glaciers are now experiencing a long-term retreat and negative mass balance as a result of ablation that most scientists believe is the result of climate change. Local consequences (creation of depositional landforms and flooding of low-lying areas) can be contrasted with possible long-term regional impacts (interference with Gulf Stream).