The geologic record shows that glaciers have existed on Earth for billions of years. Geologic evidence unique to glacier movement, such as glacial till and erratic boulders, exist in many parts of the world where glaciers are not present today. This suggests that glaciers have moved across the Earth shrinking and growing in response to a changing climate on Earth. Currently, glaciers are undergoing a period of retreat as Earth experiences a shift in climate towards warmer global atmospheric and ocean temperatures.
Glaciers and Changing Sea-levels
The extent to which glaciers have covered Earth has changed many times over the course of the geologic record. Periods in the geologic area that show evidence of maximum glacial coverage of the Earth are referred to as glacial maximums (GM) and were dominated by a colder climate. Another characteristic of glacial maximums is that sea-levels were lower. This is because colder climates cause more evaporation of ocean water and precipitation of that water in the form of snow and accumulation of glaciers.
The last glacial maximum (LGM ) recorded in the geologic record occurred about 21,000 years ago. During this time, the sea-level was about 410 feet lower than it is today. During the last warmest period on Earth, about 125,000 years ago, sea-level was about 19.7 feet higher than it is today. About 2.2 million years ago, during an even warmer period on Earth, the sea-level was about 82 to 164 feet higher than it is today.
How Much Have Glaciers Retreated?
During the LGM, about 21,000 years ago, glaciers covered about 8 percent of the Earth's surface. Of this, about 25 percent of Earth's land area and about 30 percent of Alaska was covered by glaciers. About 15,000, the Earth's climate began to warm and glaciers were retreating. Glaciers continued to retreat until about 6,000 years ago, when the sea-level reached its current height. Since then, glacial coverage, and subsequently sea-level, has fluctuated. But very recently, glacial retreat has been more persistent than in the last few thousand years.
Glaciers started to retreat more vigorously around the 1800s, about 40 years after the Industrial Revolution began. Scientists attribute this high rate of retreat to a warming global atmosphere caused by the emission of green house gases by human activity. Currently, glaciers cover about 3.1 percent of the Earth's surface. Of this, about 10.7 percent of Earth's land area and about 5 percent of Alaska is covered by glaciers.
Hazards Associated with Shrinking Glaciers
Shrinking glaciers can cause a variety of hazards that threaten humans. Of these, two of the more dangerous are rising sea-levels and glacial lake outburst flooding. If all of the glaciers on the Antarctica continent melted, the result would be a 240 foot increase in sea-level. That's significantly higher than the sea-level of the last warmest period 2.2 million years ago. Glacial lake outburst flooding is caused by the sudden release of a lake that was dammed by a glacier or a terminal moraine - an accumulation of unconsolidated glacial material such as rocks and ice. Weakening of Tulsequah glacier in Alaska caused glacial lake outburst flooding of Tulsequah Lake in late May of 2005. What remains now is an empty lake bed behind the glacier.
Source:
USGS - Glacier and Landscape Change in Response to Climate Change
USGS - Glaciers and Glacier Hazards
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Alexandra Matiella Novak - With a PhD in Geology and expertise in science education, Alexandra is passionate about increasing the public's Earth science literacy.
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