Deglaciation

Deglaciation describes the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice sheet or frozen surface layer, and the resulting exposure of the Earth's surface. The decline of the cryosphere due to ablation can occur on any scale from global to localized to a particular glacier. After the Last Glacial Maximum (ca. 21,000 years ago), the last deglaciation begun, which lasted until the early Holocene. Around much of Earth, deglaciation during the last 100 years has been accelerating as a result of climate change, partly brought on by anthropogenic changes to greenhouse gases. Deglaciation describes the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice sheet or frozen surface layer, and the resulting exposure of the Earth's surface. The decline of the cryosphere due to ablation can occur on any scale from global to localized to a particular glacier. After the Last Glacial Maximum (ca. 21,000 years ago), the last deglaciation begun, which lasted until the early Holocene. Around much of Earth, deglaciation during the last 100 years has been accelerating as a result of climate change, partly brought on by anthropogenic changes to greenhouse gases. The previous deglaciation took place between approximately 22ka until 11.5ka. This occurred when there was an annual mean atmospheric temperature on the earth that increased by roughly 5 °C, which was also accompanied by regional high-latitude warming that exceeded 10 °C. This was also followed by noteworthy deep-sea and tropical-sea warming, between about 1-2 °C (deep-sea) and 2-4 °C (tropical sea). Not only did this warming occur, but the global hydrological budget also experienced noticeable changes and regional precipitation patterns changed. As a result of all of this, the world's main ice sheets, including the ones located in Eurasia, North America and parts of the Antarctic melted. As a consequence, sea levels rose roughly 120 metres. These processes did not occur steadily, and they also did not occur at the same time. The process of deglaciation reflects a lack of balance between existing glacial extent and climatic conditions. As a result of net negative mass balance over time, glaciers and ice sheets retreat. The repeated periods of increased and decreased extent of the global cryposhere (as deduced from observations of ice and rock cores, surface landforms, sub-surface geologic structures, the fossil record, and other methods of dating) reflect the cyclical nature of global and regional glaciology measured by ice ages and smaller periods known as glacials and interglacials. Since the end of the Last glacial period about 12,000 years ago, ice sheets have retreated on a global scale, and Earth has been experiencing a relatively warm interglacial period marked by only high-altitude alpine glaciers at most latitudes with larger ice sheet and sea ice at the poles. However, since the onset of the Industrial Revolution, human activity has contributed to a rapid increase in the speed and scope of deglaciation globally. Research published in 2014 suggests that below Greenland's Russell Glacier's ice sheet, methanotrophs could serve as a biological methane sink for the subglacial ecosystem, and the region was at least during the sample time, a source of atmospheric methane. Based on dissolved methane in water samples, Greenland may represent a significant global methane source, and may contribute significantly more due to ongoing deglaciation. A study in 2016 concluded based on past evidence, that below Greenland's and Antarctica's ice sheet may exist methane clathrates. At every scale, climate influences the condition of snow and ice on Earth's surface. In colder periods massive ice sheets may extend toward the Equator, while in periods warmer than today, the Earth may be completely free of ice. A significant, empirically demonstrated, positive relationship exists between the surface temperature and concentration of Greenhouse gases such as CO2 in the atmosphere. The higher concentration, in turn, has a drastic negative impact on the global extent and stability of the cryosphere. On the millennial time scales of Pleistocene glacial and interglacial cycles, the pacemaker of glaciation onset and melting are changes in orbital parameters termed the Milankovitch cycles. Specifically, low summer insolation in the northern hemisphere permits growth of ice sheets, while high summer insolation causes more ablation than winter snow accumulation. Human activities promoting climate change, notably the extensive use of fossil fuels over the last 150 years and the resulting increase in atmospheric CO2 concentrations, are the principal cause of the more rapid retreat of alpine glaciers and continental ice sheets all across the world. For example, the West Antarctic Ice Sheet has receded significantly, and is now contributing to a positive feedback loop that threatens further deglaciation or collapse. Newly exposed areas of the Southern Ocean contain long-sequestered stores of CO2 which are now being emitted into the atmosphere and are continuing to impact glacial dynamics. The principle of isostasy applies directly to the process of deglaciation, especially post-glacial rebound, which is one of main mechanisms through which isostasy is observed and studied. Post-glacial rebound refers to the increase in tectonic uplift activity immediately following glacial retreat. Increased rates and abundance of volcanic activity have been found in regions experiencing post-glacial rebound. If on a large enough scale, an increase in volcanic activity provides a positive feedback to the process of deglaciation as a result CO2 and methane released from volcanos. Periods of deglaciation are also caused in part by oceanic processes. For example, interruptions of the usual deep cold water circulation and penetration depths in the North Atlantic have feedbacks that promote further glacial retreat. Deglaciation influences sea level because water previously held on land in solid form turns into liquid water and eventually drains into the ocean. The recent period of intense deglaciation has resulted in an average global sea level rise of 1.7 mm/year for the entire 20th century, and 3.2 mm/year over the past two decades, a very rapid increase.