The earth's polar regions--the Arctic and Antarctic--are essential to maintaining the planet's climate as we know it. The vast expanses of ice and snow found at high latitudes help to cool the earth by reflecting incoming solar radiation, and the temperature gradient between the equator and the poles is a major driver of ocean and atmospheric currents, the pathways by which heat is distributed around the globe.
With global warming, these dynamic processes have caused temperatures in many polar locations to rise about twice as fast as the global average during the past few decades. As a result, polar ecosystems have already undergone significant physical changes, often to the detriment of these regions' inhabitants. For example, recent media coverage has focused on climate change's negative effects on Arctic polar bear populations.
It may be that the potential loss of an iconic mammal provides much-needed impetus for more rigorous global action on climate change, but the physical changes to polar systems resulting from increasing temperatures have far greater implications worldwide for human well-being.
Extent of September Sea Ice (in million square km.)
Source: The National Snow and Ice Data Center
Melting Sea Ice
Arctic summer sea ice is currently receding by an average of approximately 8 percent per decade (NASA), a pace that has been steadily quickening since 2001. Recent observations show that winter sea ice is also undergoing unprecedented declines in aerial extent.
The thinning and loss of sea ice limits access to food sources for the polar bear, as well as indigenous hunters, and removes important breeding and feeding grounds for other marine mammals and seabirds. Also affected by the loss of sea ice are microscopic phytoplankton, which constitute the base of the ocean food web. Changes to phytoplankton distribution or abundance are likely to have "trickle-up" effects on higher trophic-levels; in other words, on any predator higher on the food chain.
Source: NASA
Global warming's impact on sea ice is compounded by the fact that ice's albedo is much greater than that of water, meaning that ice is a more reflective surface than the ocean water beneath it. This property sets up a positive feedback: a physical change that is exacerbated by reinforcing processes within the system. In this instance, as the planet warms and polar ice melts, water, which was previously covered by ice, is left exposed. Incoming radiation from the sun is more readily absorbed by water and water temperatures rise, thus melting more ice and amplifying overall warming (see NASA animation).
Although some negative feedbacks--processes that would slow melting--are also possible, scientists predict that the Arctic may experience summers that are completely ice-free by the end of this century, and perhaps as early as 2040. An ice-free Arctic would open up shipping lanes and provide new economic opportunities, but could also result in dramatic changes to Earth's ocean and atmospheric circulation patterns.
Melting Land Ice
Polar ice sheets contain 68 percent of Earth's fresh water (Millennium Ecosystem Assessment). Although the thermal expansion of sea water is the most important driver of future sea level rise, the melting of land-based ice in Greenland and Antarctica is also likely to significantly contribute to increasing sea level.
The latest IPCC report estimates that by 2100, sea level will be 28-58cm (11-23in) higher. However, studies of past climate reveal that sea level rise can occur much more rapidly than current rates of about 3mm/year. During the previous interglacial period (approximately 125,000 years ago) polar temperatures were 3-5°C warmer than present and subsequent reductions in ice volume led to a 4-6m (13-20ft) rise in sea level relative to the twentieth century (IPCC, 2007). If melting rates unexpectedly increase (an abrupt climate change), projections of sea level rise may need to be revised.
Future sea level rise will likely have the greatest consequences for low-lying, developing countries (for example, Bangladesh; see figure). Maldives, Kiribati, Tuvalu, and other island nations may be largely inundated or dissapear completely by the end of the century as sea level rise persists.
Source: UNEP/GRID-Arendal
Designer: Philippe Rekacewicz, UNEP/GRID-Arendal.
Countries such as the United States, China, and the Netherlands, with low-elevation areas and densely populated coastal regions, could also have a large number of residents forced to become "climate refugees." Implementing comprehensive adaptation strategies in these susceptible areas is especially critical.
Areas Susceptible to Sea Level Rise: Northeast United States
Susceptible areas are in red; high population areas are in white.
Source: Weiss and Overpeck, 2003.
Department of Geosciences Environmental Studies Laboratory (DGESL), University of Arizona
Sea level rise susceptibility maps for other regions, including Florida and Louisiana in the U.S., Europe & Middle East, Asia, South America, Africa, and Australia and technical notes can be found on the DGESL website.
The longevity of carbon dioxide (CO2) and other greenhouse gases (GHGs) in the atmosphere means that regardless of when global emissions are reduced, melting will continue (perhaps for centuries), but the extent to which GHGs continue to be emitted will ultimately determine how much melting occurs.
The Global Ocean "Conveyor Belt"
Another, more uncertain, consequence of melting ice in the Arctic is the freshening of North Atlantic waters. Typically, the submergence of cold, salty water in the North Atlantic propels the movement of water along a global network of surface and deep-water currents. Known as thermohaline circulation or the ocean "conveyor belt," this system is driven by differences in water density. Increasing the influx of meltwater run-off into the North Atlantic could reduce the salinity (and therefore density) of surface waters, disrupting downwelling and, ultimately, slowing the "conveyor" system, with impacts worldwide. For example, reducing the strength of the Gulf Stream, which currently transports warm water north, would cool western Europe.
Source: UNEP/GRID-Arendal
Designer: Philippe Rekacewicz, UNEP/GRID-Arendal.
Terrestrial Impacts
Global warming could also strongly affect terrestrial environments. High-latitude wetlands, or tundras, are some of the world's largest sources of methane gas. As temperatures rise, the soil that stays frozen year-round (permafrost) thaws, releasing methane into the atmosphere. Although uncertainties remain, it is believed that this release of methane--a GHG with 23 times more global warming potential than CO2--will cause additional warming at a global scale (a positive feedback).
Locally, the melting of permafrost reduces stability for infrastructure (e.g., buildings, roads, pipelines) and creates difficulties for ground transportation. It also enables the migration of other vegetation types northward, reducing the extent of Arctic tundra.
International Polar Year
Despite advancements in data collection and modeling technologies in recent years, more information is needed regarding the timing, extent, and global implications of anticipated changes to polar regions. The physical systems of polar regions are complex, involving many variables, and accurate forecasts are predicated on how well the relationships between these variables are understood.
To that end, March 1 marked the launch of International Polar Year (IPY), an internationally-coordinated, cross-disciplinary research initiative that focuses on the geologic and physical components of polar ecosystems. For the first time, IPY will also focus on relevant human dimensions (some 3.8 million people reside in the Arctic) and assess how the physical and socio-economic aspects of polar regions are linked.
IPY participants will gather new data and information and convey their findings to the public through education and outreach initiatives that will occur throughout the event's duration. Work resulting from IPY will hopefully provide new measurements, more robust insights into past climate variability, a broader analysis of global climate system interactions, and, ultimately, offer better information to assess future scenarios in a warming world.
Insight into the Future
What happens in the polar regions as a result of global warming is an important harbinger for future climate change. The magnitude and rapidity with which these physical changes occur could inform expectations for the consequences of future climate change in non-polar regions.
Today, observable, quantifiable changes in climate are already harming polar regions, as well as human societies and ecosystems worldwide. Although many of the effects noted above are gradual processes that may not become catastrophic for several decades, assessing societal vulnerabilities, as well as opportunities, and instigating both mitigation and adaptation strategies now, will undoubtedly prove advantageous for future prosperity.
RELATED LINKS:
International Polar Year
Main site: www.ipy.org
U.S. sites: www.us-ipy.org and www.us-ipy.gov
Eurasian Arctic Sub-office: www.ipyeaso.aari.ru
National Academies Polar Research Board
National Science Foundation Office of Polar Programs
Other Polar Resources
Arctic Climate Impact Assessment
World Environment Day 2007: Melting Ice - A Hot Topic?
United Nations Environment Programme Key Polar Centre
Millennium Ecosystem Assessment: Polar Systems (pdf)
International Polar Foundation
Vital Arctic Graphics - People and global heritage on our last wild shores (UNEP/GRID-Arendal)
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