The Intergovernmental Panel on Climate Change (IPCC) concluded in 2007 that: “ ... discernible human influences extend beyond average temperature to other aspects of climate.” Human influences have:
very likely contributed to sea level rise during the latter half of the 20th century
likely contributed to changes in wind patterns, affecting extra-tropical storm tracks and temperature patterns
likely increased temperatures of extreme hot nights, cold nights and cold days
more likely than not increased risk of heat waves, area affected by drought since the 1970s and frequency of heavy precipitation events.
Anthropogenic warming over the last three decades has likely had a discernible influence at the global scale on observed changes in many physical and biological systems.
Extent of Arctic Sea Ice Near Lowest Levels
Sea ice at the poles is a vital component of the Earth’s current climate system. Sea ice controls key aspects of Arctic atmospheric circulation, polar warming and other critical components of the Earth’s climate system. Polar sea ice is of cultural and iconic value to some people. It also affects a number of human activities, such as shipping, fishing, resource accessibility, and tourism. Sea ice is important to current Arctic ecology, such as habitat for polar bear, seals, whales and others.
Arctic sea ice shrunk to its smallest extent in 2007, and nearly to the same extent in 2008, since satellite measurements began in 1979. Sea ice cover has reached perhaps 50% below the sea ice extent of the 1950s. According to the National Snow and Ice Data Center, Arctic sea ice extent for August 2009 was the third lowest August since 1978, continuing the downward trend observed over the last three decades. Only 2007 and 2008 had lower ice extent during August. The long-term trend indicates a decline of 8.7% per decade in August ice extent since 1979. Average sea ice extent in September 2008 was about 4.67 million square kilometers, compared to the record low of September 2007 of 4.28 million square kilometers (1.65 million square miles). Compared to the average extent of sea ice between 1979 and 2000, 2008 was 34% below and 2007 was 39% below. The rate of sea ice decline since 1979 has reached approximately 11% per decade, or 78,000 square kilometers (28,000 square miles) per year. While rapid Arctic ice loss appears in climate model runs, the loss of Arctic sea ice extent has been more rapid than produced by climate models.
While the record melting of Arctic sea ice is associated with Greenhouse Gas (GHG)-induced warming, the winds in 2007 pushed sea ice from the Arctic toward the Atlantic Ocean. Simultaneously, Arctic currents seemed to be reversing, returning to the pre-1990s direction. In addition, low cloudiness led to more solar warming than usual. According to NASA, “The results suggest not all the large changes seen in Arctic climate in recent years are a result of long-term trends associated with global warming.” In 2008, there were near-record lows despite return of more “normal” wind and atmospheric conditions.
Earlier seasonal melting of sea ice triggers a positive feedback that increases ocean warming, further increasing sea ice melting, and so on. Updated estimates now project that the Arctic Ocean could be ice-free in summer as early as 2040, 2030 (or sooner) if recent accelerations in sea ice loss continue. Some scientists have expressed concern that recently observed sea ice loss may have passed a “threshold” or a spiral of warming feedbacks. Melting ice in the Arctic would contribute very little to global sea level rise because it already floats, with its volume already displacing sea water; however, it contributes to concern because of the impact loss of Arctic sea ice would have in warming Arctic waters and atmosphere, and consequent effects on warming and melting of the Greenland Ice Sheet.
Melting of the Greenland Ice Sheet
Between 1979 and 2005, the area of Greenland that melted on at least one day per year grew by 42%, while the mean temperature rose by 2.4oC. However, recent changes in rates of melting of the Greenland Ice Sheet point to variability in the climate system and the difficulties in discerning trends among the changes. Beginning in the late 1990s, ice flows from two of Greenland’s biggest glaciers that flow into the ocean accelerated rapidly, surprising scientists at the speed of change. The lack of prediction of the phenomenon contributed to the IPCC’s decision not to include the contribution of ice sheet melting in its projections of sea level rise due to global warming in the 21st Century. The high melting rates in 2005 startled many scientists and raised major concerns about the potential impacts on sea level rise. Some scientists have argued that policy goals to address climate change should avoid passing certain “tipping points” of the climate system that could have potentially catastrophic impacts, naming melting of the Greenland Ice Sheet on one of these thresholds. (See section “Projections of Future Climate” for further discussion of possible tipping points.) However, as melting rates in 2006 returned closer to the average, they have exposed greater variability and complexity in ice dynamics than previously understood. Now some scientists believe that warming waters near the glacier outlets accelerate ice flows and results in retreat of their floating leading margins. However, after a point, the glacier regains stability on its grounding rock and the retreat slows, though it will continue to melt with warm air temperatures.
Melting and Thickening of Ice in Antarctica
Over the past few decades, the atmosphere over Antarctica has warmed. Satellite observations analyzed in 2007 indicate that the Antarctic ice sheet is losing mass overall; the losses are mainly from the western Antarctic ice sheet. NASA satellites revealed that snow is melting farther inland, at higher altitudes than before and, increasingly, on the Ross Ice Shelf, which buffers land-based glaciers from the warmer ocean air. Some high elevation regions of the Antarctic ice sheet do not show a significant rate of change or show less melting. Researchers identified a link between changes in temperatures and the duration and area of melting in Antarctica, suggesting a connection to global climate change. In another 2007 study, the British Antarctic Survey found that 300 glaciers studied increased their average flow rate by 12% from 1993 to 2003. This was attributed to thinning of the lower glaciers at the edge of the sea, allowing the glaciers above them to flow faster, similar to phenomena observed in Greenland. Unlike Greenland, the Western Antarctic Ice Sheet is not well grounded like the outlet glaciers of Greenland, so that disintegration of the lead glacial margins could lead to persistently accelerated flows of ice to the sea. The researchers tied local warming on the Antarctic Peninsula—some of the fastest recent warming on Earth (nearly 3oC, or 4.4oF, over 50 years)—to retreat of 87% of its glaciers and the observed increase in their flow rates.
In 2008, parts of the Antarctic Pennisula’s Wilkins Ice Sheet disintegrated in three stages, which is especially significant because two of the stages occurred during the cold season. The pattern of breakup was smaller but similar to that of the Larsen A and B ice shelves, in1995 and 2002 respectively. According to the NSIDC, preliminary studies of the sea floor below the Larsen B ice shelf suggest that the 2002 disintegration was the first such in 12,000 years. While the general warming of ocean waters in both the Arctic and Antarctica contributes to loss of ice sheets, two studies in 2008 indicate that other factors (winds and current changes) may circulate warm water in the vicinity of ice shelves.
No Melting of Some Permanent Icefields
Not all glaciers and ice fields are experiencing increased melting. One study published in 2008 indicated that snow accumulation has doubled in the south-western Antarctica Peninsula since 1850, with rates accelerating in the past few decades. In Europe, while glaciers between 2,000 and 4,000 meters in altitude have lost an average of 1-1.5 kilometers of length through the 20th Century, others at high altitude—above 4,200 meters—have changed very little in the same period. Some melting did occur, however, during the 2003 extreme heat wave.
Contributions of Melting Ice and Warming Oceans to Sea Level Rise
A 2008 assessment of satellite-based data suggests that most of the sea level rise observed in recent years can be explained by an increased mass of the oceans (i.e., more water). Of the global melting of ice contributing to observed sea level rise, about half has come from relatively small land-based glaciers, with the other half contributed by melting of the Greenland and Ice sheetsAntarctic ice sheets. One report published in 2007 concluded that the net amount of melting ice from glaciers and ice caps flowing to the oceans each year is about 100 cubic kilometers—or about the volume of Lake Erie.
With further warming, the acceleration of dynamic ice melt could raise the estimates of sea-level rise by an additional 4 to 10 inches by 2100. Recent articles have proposed a range of new estimates for sea level rise in the 21st Century that would include contributions of sea ice melt, particularly from Greenland. Pfeffer et al. conclude that physical constraints would preclude more than 2 meters (6.6 feet) of sea level rise over the coming century (with a range of 2.6 to 6.6 feet), and put forward a best guess, with low confidence, of about 0.8 meters rise by 2100. Grinsted et al. suggest a range of 0.9 to 1.3 meters (3 to 4.3 feet) of sea level rise in 2090 to 2100, using a moderate climate change scenario.
Hydrological Changes in the Western United States
A modeling study published in 2008 concluded that human factors may have induced as much as 60% of the changes observed between 1950 and 1999 in the hydrological cycle in the western United States. Climate changes were found to have influenced river flows, winter air temperatures and snow pack. The authors concluded that these changes, and their human influences, suggest an impending water supply crisis in the West.
Observed Ecological Impacts of Climate Change
A growing number of studies are published each year investigating possible linkages between climate change and ecological changes. Results from a few released in 2008 are highlighted here. One study concluded that warming of the Southern Ocean around Antarctica is threatening King penguin populations on the continent.
A number of new studies continue to underscore threats to coral reefs globally by a variety of stressors that include heat stress from warm ocean events, and “ocean acidification” caused by absorption of CO2 from the atmosphere by the oceans. One of these studies concluded that almost one-third of 704 reef-building coral species that could be assessed with data show enhanced risk of species extinction. It further concluded that the share of coral species at risk has risen in recent decades. The Caribbean was the region with the highest share of corals at high risk of extinction, while the Coral Triangle in the western Pacific had the greatest share of coral species in all categories at risk. Another 2008 study concluded that throughout Australia’s Great Barrier Reef, coral calcification (a measure of growth) has decreased by 14% since 1990. The authors further concluded that the severity and abruptness of the observed decline was unprecedented in at least the past 400 years.
While risks to coral reefs from global warming and ocean acidification are increasingly studied, the associated risks to reef fish communities have been acknowledged but not documented. A 2008 study assessed the impacts of the major 2008 coral bleaching events across seven countries, 66 sites and 26 degrees of latitude in the Indian Ocean. The study concluded that, while impacts across sites were variable, ocean scale integrity of fish communities was lost, reflected in size structure, diversity, and food-chain composition of the reef fish. The authors also found that management regimes did not appear to affect the ecosystem responses to the bleaching event, suggesting a need to develop strategies for system-wide resilience to climate variability and change. At least study found evidence that some corals may be able to adjust to bleaching events by shifting the types of algae (zooxanthellae) with which they co-depend. In many ecological systems, climate is a primary—but not the sole—factor influencing the survival and behaviors of species. With the climate change experienced in recent decades, landuse, climate change and other factors have been associated with substantial range contractions, extinction of at least one species, and numerous changes in the timing of animal and plant behavior.
Polar bears are among the species that depend on sea ice for hunting and must fast during ice-free periods. The Western Hudson Bay of Canada has had ice-free summer periods for many years and, although the local polar bear population had previously appeared healthy, more recent observations have revealed lower survival rates among cubs and young bears. Similar patterns have now emerged in Southern Hudson Bay and the Southern Beaufort Sea.
Observations of several forest systems suggest that they are adapting to changes in climate more effectively than some scientists had expected. More specifically, NASA satellite imaging indicates that U.S. forests are adapting to the climate change experienced to date, and that the overall productivity response to weather and seasonal conditions has been closely linked to the number of different tree species in a forest area. In Brazil the productivity of Amazon forests has been resilient in spite of short but severe drought conditions in 2005, contrary to predictions of some ecosystem models, although whether the resistance will be sustained under longer drought—expected with climate change—is unknown. Studies have shown increases in primary productivity in the Amazon as well as in above-ground biomass. They also show, however, changes in the composition of plant species, with fast-growing species faring better than slowgrowing ones. The authors attributed these changes to global environmental changes, including elevated levels of CO2 in the atmosphere. Another study examined the influences of high temperatures on tropical forest uptake of CO2 from the atmosphere. It found that elevated temperatures initially raised CO2 uptake, then CO2 uptake declined. The authors concluded that, in the particular tract of forest studied, temperatures were approaching a threshold above which CO2 uptake would drop sharply.
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- ^Aslak Grinsted, J. Moore, and S. Jevrejeva, “Reconstructing sea level from paleo and projected temperatures 200 to 2100 A.D.,” Climate Dynamics, doi:10.1007/s00382-008-0507-2, http://dx.doi.org/10.1007/s00382-008-0507-2., using the IPCC “A1B” scenario of GHG emissions.
- ^Tim P. Barnett et al., “Human-Induced Changes in the Hydrology of the Western United States,” Science 319, no. 5866 (February 22, 2008): 1080-1083, doi:10.1126/science.1152538.
- ^Céline Le Bohec et al., “King Penguin Population Threatened by Southern Ocean Warming,” Proceedings of the National Academy of Sciences of the United States of America 105, no. 7 (February 19, 2008): 2493–2497, doi:10.1073/pnas.0712031105.
- ^Kent E. Carpenter et al., “One-Third of Reef-Building Corals Face Elevated Extinction Risk from Climate Change and Local Impacts,” Science (July 10, 2008): 1159196, doi:10.1126/science.1159196.
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- ^USGS, USGS Science to Inform U.S. Fish & Wildlife Service Decision Making on Polar Bears: Executive Summary (Reston, VA, 2007), http://www.usgs.gov/newsroom/special/polar%5Fbears/.
- ^NASA, “NASA Satellites Can See How Climate Change Affects Forests,” http://www.nasa.gov/centers/goddard/ news/topstory/2006/forest_changes.html, (accessed November 28, 2007).
- ^Scott R. Saleska et al., “Amazon Forests Green-Up During 2005 Drought,” Science (September 20, 2007); Yadvinder Malhi et al., “Climate Change, Deforestation, and the Fate of the Amazon,” Science (November 29, 2007).
- ^Jérôme Chave et al., “Assessing Evidence for a Pervasive Alteration in Tropical Tree Communities,” PLoS Biology 6, no. 3 (March 2008): e45, doi:10.1371/journal.pbio.0060045.
- ^Catia M. Domingues et al., “Improved estimates of upper-ocean warming and multi-decadal sea-level rise,” Nature 453, no. 7198 (June 19, 2008): 1090-1093, doi:10.1038/nature07080.
Note: The first version of this article was drawn from Climate Change: Science Highlights by Jane Leggett, Congressional Research Service, February 23, 2009.
Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Congressional Research Service. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Congressional Research Service should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.