January 25, 2005
Changes in the Arctic: Consequences for the World
Goddard -- Observations and computer models have long proven that the Arctic plays an important role in maintaining a stable climate on Earth. However, significant changes in the Arctic environment, especially those over the past decade, could lead to dramatic swings in weather and climate patterns across the rest of the globe, with potentially far-reaching consequences for ecosystems and human populations. Societies that have adapted to their current climates may be faced with highly disruptive changes over relatively short time periods.
Ground-based surface temperature data shows that the rate of warming in the Arctic from 1981 to 2001 is eight times larger than the rate of Arctic warming over the last 100 years. There have also been some remarkable seasonal changes. Arctic spring, summer, and autumn have each warmed, lengthening the seasons when sea ice melts from 10 to 17 days per decade.
Recently, a research study led by atmospheric scientist Jiping Liu of the Georgia Institute of Technology discovered that the total Arctic sea ice extent and area decreased, respectively, by 30,848 km2/year (11,910 square miles per year) and 35,372 km2/yr (13,660 square miles per year) using ice data between 1978 and 2002, derived from NASA's Nimbus 7 satellite and several defense meteorological satellites. And, "if the current trends continue, Arctic sea ice will become much thinner in winter and almost non-existent in the summer, in keeping with increased greenhouse loading in the atmosphere," said Liu. The paper, "Recent Arctic Sea Ice Variability: Connections to the Arctic Oscillation and the ENSO," was published in the May 2004 issue of Geophysical Research Letters.
The Arctic is so important to the world's climate because it acts as the "collection bed" for the world's excess energy. In an attempt to balance energy across the Earth's surface, heat is constantly being transported through atmospheric circulations and ocean currents from the equator to the poles, where it is ultimately released out to space.
But if the climate continues to warm faster in the Arctic than at lower latitudes, this transfer of heat will slow down, weakening overall atmospheric circulation. The weakening circulation would alter storm tracks, and their intensity, but the most profound impact would be on temperature. Oceans are capable of holding a tremendous amount of heat and moisture, which, when transferred through its surface to the atmosphere, can significantly alter temperature and pressure patterns.
Some scientists speculate that as low-latitude surface waters warm, forces like the El NiÃ±o-Southern Oscillation (ENSO) will strengthen and become even bigger players in the world's climate.
El Nino (EN) is signaled by a warming of the ocean surface off the western coast of South America that occurs every 4 to 12 years when cold, nutrient-rich water does not come up from the ocean bottom. It causes die-offs of plankton and fish and affects Pacific jet stream winds, altering storm tracks and creating unusual weather patterns in various parts of the world. Southern Oscillation (SO) refers to a see-saw of high and low pressure that varies between Tahiti and Darwin, Australia.
Other researchers believe another cyclical atmospheric pressure system, called the Arctic Oscillation (AO) may also be responsible for declining Arctic sea ice. This oscillation refers to a pattern of low- and high-pressure systems between the Arctic and the mid-latitudes. When the oscillation is in its positive phase, as it has generally been over the last 20 years, air pressure tends to be low over the Arctic Ocean. Some scientists theorize that a general warming of the Earth could be pushing the oscillation toward a phase that warms the Arctic. The oscillation helps explain why summer sea ice is thinner than in years past. Since the 1980s, wind changes associated with the oscillation have pushed ice apart and shoved more ice from the Arctic into the Atlantic Ocean between Greenland and Norway.
Although Liu's study showed that AO and ENSO trends cannot explain the recent regional sea ice trends, his research found they do influence the Arctic sea ice to some degree on time scales from year to year. "For example, with a positive phase of the AO, we usually observe more ice in the western Arctic and decreased ice coverage in the eastern Arctic," said Liu. With strong El Nino events, however, there is more ice in both the eastern and western Arctic.
Liu also says that more study is needed to better understand how regional ice trends might respond to a warmer climate, including less understood large-scale processes such as the Pacific Decadal Oscillation (a long-lived, El Nino-like pattern of Pacific climate variability) and other influences, like river discharge into the Arctic Basin from Russia and Canada and glacier discharge from Greenland.
While melting Arctic sea ice will influence the atmospheric circulations in the high- and mid-latitudes, therefore altering the world's weather patterns and storm tracks, it could also threaten the biodiversity of the Arctic Ocean.
A study led by Kevin Arrigo of Stanford University, "Annual Cycles of Sea Ice and Phytoplankton in Cape Bathurst Polynya, Southeastern Beaufort Sea, Canadian Arctic," published in the April 2004 issue of Geophysical Research Letters, surveyed the impact of declining sea ice on marine ecosystems in the Canadian Arctic. Specifically, the research examined the association between annual sea ice cycles and biological productivity in the Cape Bathurst polynya. Polynyas are areas of open water or reduced ice cover, usually created by strong winds that blow ice away from the coast.
Although relatively small in area, coastal polynyas play a disproportionate role in many physical and biological processes in polar regions. In eastern Antarctica, for example, more than 90 percent of all Adelie penguin colonies live next to coastal polynyas.
Arrigo found that the Cape Bathurst polynya contained considerable variability, in terms of initial polynya formation and in the extent and persistence of open water, over a five year period (1998-2002). Phytoplankton blooms also varied considerably in intensity and timing. Phytoplankton are plantlike organisms that contain green chlorophyll and are a primary food source for many marine mammals and birds, are tiny organisms that are responsible for most of the photosynthetic activity in the oceans.
Polynyas, combined with shallow coastal waters, provide the top layers of the ocean with added sunlight, creating ideal conditions for phytoplankton to flourish. "The open waters retain more heat, further thinning ice cover and leading to early, intense, and short-lived plankton blooms," said Arrigo.
"Understanding the dynamics of polynya formation and phytoplankton bloom development is important because of their ramifications for other components of the marine ecosystem," added Arrigo. Several fish species use polynyas as feeding and nursery grounds and since seasonal temperatures influence polynya formation, it is clear that climate changes can have a major impact on the marine food web, in both the short and long term.
To determine the amount of phytoplankton produced in the Arctic, Arrigo collected data from SeaWiFS -- NASA's Sea-viewing Wide Field-of-view Sensor satellite. SeaWiFS measures the amount of light coming out of the ocean at different wavelengths and can measure the intensity of the greenness coming from the chlorophyll in the phytoplankton.
Global warming might reduce the amount of sea ice cover in the Arctic, which could result in an increase in the amount of phytoplankton produced. But, global climate change will do more than just melt ice; it will alter precipitation and wind patterns. Increasing winds could reduce biological productivity by mixing the surface waters where phytoplankton grows too deeply, as happens now in much of the Antarctic. The effects of global warming are complex, and scientists do not yet know how Arctic ecosystems are likely to change in response.
"Many Arctic organisms have adapted to a life on, in, or near sea ice and further reductions in ice cover will almost certainly have an impact on these biological communities," said Arrigo. "Whether the Arctic will become more or less biologically productive as a result in declining ice cover is uncertain. What is virtually certain is that we will see shifts in the types of species that we see today [because of changes in the food chain]," Arrigo said.
Changes in the food chain will mean that some species will be able to adapt to the sea ice changes, while others will won't and will die off. Over time, the biology of the species may evolve to ensure survival in their new environment and climate.
Scientists do know that adding greenhouse gases like carbon dioxide to the atmosphere increases the greenhouse effect, warming the planet. Because a warm atmosphere holds more water vapor, precipitation across the Arctic has already increased more than at any other latitude on Earth, by about 15 percent over the past 40 years. This water flows off the land and into rivers. Records show that the fresh water in Siberia's three largest rivers has swelled by roughly a quarter of the annual flow of the Mississippi River, and that water is now being poured directly into the Arctic Ocean.
The global ocean circulation is regulated by cold, dense water that sinks in the Arctic. This water moves south toward the equator and well below the surface in the Atlantic. Upon its circular return northward, it pulls warm tropical water north along the surface, where, like a hot-water heater, it releases heat back into the atmosphere. An influx of fresh water to the Arctic Ocean could prevent the water there from sinking and essentially halt this conveyor-belt-like flow. Changes in ocean currents can greatly complicate overall climate change and, among other things, leave some regions, like England and eastern Canada, much cooler than they otherwise would be.
An important consequence of global warming is the possible reduction in albedo, a measure of the reflection of the Sun's rays back into space. Because of its white color, snow-covered sea ice reflects most of the incoming solar radiation, which is in part why it is so cold throughout the Arctic region. Melt the snow and ice, replace it with the darker surface of water, and much of the energy will be absorbed leading to warming. Heated oceans in turn will lead to further melting and removal of snow and ice, increasing warming, a "positive feedback" to global warming. It is especially this feedback process that leads to predictions that warming in the Arctic will be more pronounced, fueling climate changes for other areas of the world.
But, it's not quite that simple. Melting sea ice will also leave more of the ocean exposed, increasing evaporation and cloud cover, which can block sunlight and diminish warming - a "negative feedback." Scientists are still trying to understand the Arctic's feedbacks and how they might play out in the future.
Perhaps the best tool scientists have today to answer these questions are computer models, that simulate present oceanic-atmospheric behavior as well as future and past climates. But, due to the limitations of today's computers, it is not possible to explicitly represent all the important physical processes that govern the climate.
Most computer models predict continued precipitation increases in high latitudes and some warming over the Arctic waters within the next 70 years, assuming a doubling of carbon dioxide. But, these models only offer a "best guess" as to how scientists believe different climate processes interact.
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