Tara Moran. Encyclopedia of Global Warming and Climate Change. Editor: S George Philander. Volume 1, Sage Publications, 2008.
Seasonal changes in snow and ice cover on the Earth’s surface result largely from the Earth’s position relative to the sun. However, because ice systems also respond to climate on longer timescales, they are considered good indicators of the Earth’s climate as a whole. In addition to being good climatic indicators, glaciers and ice sheets influence the Earth’s climate in two important ways. The first is their potential impact on sea-level rise and global water resources under declining ice regimes. Glaciers and ice sheets account for two-thirds of the Earth’s freshwater and would contribute approximately 229 ft. (70 m.) to sea-level rise if they were to melt completely.
The majority of this ice is located in the Earth’s polar regions, largely in the Greenland and Antarctic ice sheets, which combined account for approximately 97.5 percent of potential sea-level rise. Alternatively, glaciers located outside of the polar regions, commonly referred to as alpine glaciers, play a relatively small role in their overall contribution to global ice volume and, thus, sea-level rise. However, they are perhaps the most critical ice volumes when considering their locations with respect to populated areas. Alpine glaciers are found at the headwaters of river systems throughout the globe and, thus, are an important water source for many regions. These glaciers are also more susceptible to melt owing to their smaller size; as a result these systems have experienced the most visible decreases in ice volume.
Second, decreases in global ice and snow cover are considered particularly important to the Earth’s climate system because such changes are considered to be part of a positive feedback loop, commonly referred to as the snow-ice albedo feedback. Under this scenario, as temperatures increase, the extent of snow and ice decreases, and the highly reflective snow or ice surface is replaced with the darker (more absorptive) ocean or land surface underlying it. The increased absorption of energy completes the feedback loop, as more solar radiation is absorbed by the Earth’s surface, leading to an overall increase in air and sea surface temperatures, and further decreases in ice and snow volume.
Global air temperatures have increased in the past century. Climate observations indicate that increases in temperature over this timeframe have accelerated in recent decades, and are unlikely to slow under current climatic conditions. One key example of the positive feedback associated with the loss of ice mass is the increased rate of warming observed in the Arctic, which has warmed at nearly twice the global average. Global temperature increases are strongly correlated with decreases in global ice mass, which have been in a relatively constant state of decline throughout the last century. As global temperature rates increase, global ice volumes have experienced the largest rates of decline in recent decades.
Observations of global ice masses are a critical component of climate change research. They give scientists insight into how ice masses are changing over time, how quickly changes are taking place, and allow climate scientists to make informed predictions of how these ice masses are likely to change in the future. Improvements in technology have changed the way in which climate scientists make observations of glaciers, ice sheets, and sea ice. New techniques now include the use of satellite and remote-sensing data as a means of monitoring these systems.
Prior to the 19th century, records of weather and climate were rare. As a result, climate scientists use proxy records (such as ice cores, lake and ocean sediments, and ocean coral) to make inferences about how the Earth’s climate and ice masses have changed in the past. The current understanding of changes to the Earth’s cryosphere (snow and ice) is comprised, therefore, of a combination of data derived from proxy records (indirect observations) and from direct ice observations. Combined, direct and indirect observations provide insight into how global ice masses are changing over time and provide a context in which to view current changes in global ice masses.
Indirect Ice Observations
Proxy records are commonly used to reconstruct the Earth’s climate beyond the period of direct scientific observation. Some commonly used proxy records include ice cores, tree rings, ocean sediments, and coral records. Consistent results in the historical climatic data derived from these proxy records indicate the reliability of these records as accurate means of reconstructing past climates.
These records indicate large-scale changes in the Earth’s climate in the past, including periods of complete global glaciation (Snowball Earth) and contrasting ice-free periods. During the past 900,000 years, the Earth has fallen into a pattern of glacial-interglacial cycles operating on 100,000-year timescales. Glaciations are long periods of cold temperatures, resulting in the growth of large land and sea ice masses extending outward from the poles. These glaciations tend to come to an abrupt end with rapid warming and consequent ice recession.
The last glaciation, the Wisconsin glaciation, ended approximately 10,000 to 11,000 years ago. During this glaciation, ice caps in the Northern Hemisphere extended to latitudes of approximately 45 degrees N, and were 1.8-2.5 mi. (3-4 km.) thick. Since the end of the Wisconsin glaciation, the Earth’s climate has been in a period of relative climatic stability, a period commonly referred to as the Holocene (the past 10,000 years).
Synthesis of proxy records for the past 1,000 years in the Northern Hemisphere shows a gradual cooling for 900 years, culminating in the Little Ice Age (1400-1850). During the Little Ice Age, sea ice throughout the Arctic increased, and alpine glaciers across Europe expanded down valleys, covering farmland and destroying agriculture. Since the end of the Little Ice Age, the world has been in a state of declining ice extent.
Proxy records from the Southern Hemisphere are not as unanimous in their observations of 20th century warming. Some regions in the Southern Hemisphere, including Australia, New Zealand, and South America indicate the 20th century as a period of unprecedented warmth in the last millennium. However, ice core records from Antarctica do not indicate this same 20th-century warming. The data derived from both proxy records, and direct observation, indicate that climate does not always operate on a global scale, rather, changes occur on a more regional scale.
Direct Ice Observations
While indigenous people have populated the Arctic for thousands of years, exploration of the Arctic by European explorers did not begin until the 16th century. While these voyages were initially motivated by the search for the Northwest Passage, they also provided the first real records of Arctic sea ice conditions. However, it was not until numerous expeditions to the Arctic and several attempts for the North Pole had been made that the distribution of sea ice in the Arctic became well understood.
While early theories speculated the existence of a large southern continent, Antarctica eluded explorers for several centuries, until it was first sighted by Captain James Cook in 1773, after which many voyages were made to the area for both sealing and exploration. Similar to the Arctic, by the mid-1800s, there were rough models of sea, ocean currents, and winds for the Antarctic region.
The first scientific literature discussing Arctic, sea ice properties and extent were published in the 1870s, but it was the organization of the First International Polar Year (IPY) in 1882-83 that spurred scientific exploration to both the globe’s polar regions. From 1900-06, a major scientific expedition to the Arctic Ocean was undertaken by Fridtjof Nansen, aboard the Fram, to determine ocean currents and sea-ice extent. In the Southern Hemisphere, expeditions by Robert Falcon Scott, Erich von Dryglaski, and Ernest Shackleton to Antarctica marked the transition from early exploration to modern scientific expeditions.
Modern Ice Observations in the Polar Regions
In the 1950s, international scientific campaigns were undertaken on both the Greenland and Antarctic ice sheets. The scientific advances made during these research campaigns were unprecedented, and resulted in deep ice cores retrieved from both the Northern and Southern Hemispheres, as well as the establishment of long-term scientific monitoring sites. The information gained during these campaigns now form the foundations for the current understanding of ice volumes and ice-sheet dynamics in these regions, as well as providing high-resolution proxies of past climatic conditions. Insight into past glacial dynamics provides a context in which to view the current ice-sheet behavior.
Continued technological advances in transportation, computing, and communications have increased accessibility to the polar regions, and increased the number of scientists investigating glacial dynamics in these regions. In the late 1960s, an improved understanding of the potential environmental impacts associated with decreasing global ice volumes brought the study of glaciers and ice sheets to the forefront of scientific research. The current understanding of global ice volumes to serve a bellwethers of global climate change is largely the result of research done in the 1960s; it was during this time that glaciologists and climate scientists began expressing concern over 20th-century climate change.
In 1972, the Landsat 1 satellite was launched and provided the next major scientific advance in ice monitoring. The development of remote-sensing techniques has opened a whole new world for scientists studying sea ice, ice sheets, and glaciers. These techniques allow scientists to make observations of ice extents and changing surface heights, without having to make expensive and time-consuming visits to these remote regions.
Ice Observations of Alpine Glaciers
While alpine glaciers compose a relatively small component of the Earth’s cryospheric system, a large proportion of the world’s population relies on the melted water from these systems for drinking water and hydroelectric power. Understanding how alpine glaciers are likely to respond to future climatic changes is, therefore, a crucial component of sustainable planning and development. Direct ice observations from alpine glaciers and ice fields have a longer history than those in the more remote polar regions, largely because they are situated closer to population centers.
The longest records of glacial dynamics come from Europe, where records extend over 150 years. These records include glacial mass balance measurements (measurements of snow accumulation and melt), ice extent, and thickness. The large number of alpine glaciers existing on the mountain ranges of Africa, North America, South America, Europe, and Asia mean that individual monitoring of all glaciers is not feasible. As a result, the same advances in remote-sensing technology that have improved monitoring of polar ice masses have also been an asset to the collection of information about alpine glaciers and ice fields.