Glaciers are ancient rivers of moving ice and snow that creep through the landscapes and shape the planet’s surface. Glaciers store water over relatively long timescales compared to rivers and lakes - hundreds to a few thousands of years. Many river systems depend on glacier melt, which maintain their water supply through the summer. Glacier melt water eventually makes its way to the ocean, where it can affect global sea level. So it is important to measure how and understand why glaciers change over time. Glaciers are the earth’s largest freshwater reservoir with a total area of ~0.6 million sq km (Table 1). Glaciers and ice caps provide the most visible indications of the effects of climate change. Glaciers have been retreating worldwide since the end of the Little Ice Age (around 1850), but in recent decades glaciers have begun melting at rates that cannot be explained by historical trends. From 1940 onwards, glaciers around the world retreated as the climate warmed. Glacial retreat slowed and even reversed, in many cases, between 1950 and 1980 as a slight global cooling occurred. However, since 1980, a significant global warming has led to glacier retreat becoming increasingly rapid and ubiquitous, so much so that some glaciers have disappeared altogether, and the existence of a great number of the remaining glaciers of the world is threatened. In locations such as the Andes of South America and the Himalayas in Asia, the demise of glaciers in these regions will have potential impact on water supplies. The retreat of mountain glaciers, notably in western North America, Asia, the Alps, Indonesia and Africa and tropical and subtropical regions of South America, has been used to provide qualitative evidence for the rise in global temperatures since the late 19th century (IPCC 2007, NSIDC). The recent substantial retreat and an acceleration of the rate of retreat since 1995 of a number of key outlet glaciers of the Greenland and West Antarctic ice sheets, may foreshadow a rise in sea level, having a potentially dramatic effect on coastal regions worldwide.

The projected climate change over the next century will further affect the rate at which glaciers melt. Average global temperatures are expected to rise 1.4-5.8ºC by the end of the 21st century. Simulations project that a 4ºC rise in temperature would eliminate nearly all of the world’s glaciers (the meltdown of the Greenland ice sheets could be triggered at a temperature increase of 2 to 3ºC). Even in the least damaging scenario – a 1ºC rise along with an increase in rain and snow – glaciers will continue to lose volume over the coming century.