Armed with the data that relate CO2 increase to temperature rise, models allow prediction of the global distribution of changing temperatures if the total amount of carbon dioxide in the air were to double or quadruple from today's values (arbitrarily set at 275° F). The 4x case could be truly catastrophic, leading to worldwide deserts unless the added heat in the atmosphere also notably increase vegetation growth (to tropical forms). Thus:
One obvious consequence of the significant rise in CO2 in the northern polar latitudes would be melting of Arctic Ocean and Greenland Ice Cap ices, releasing huge quantities of stored water that would have an extremely serious impact on global sea levels. A rise to 550 ppm CO2 by the end of year 2100 would bring about rises from 8 to 37 inches, enough to greatly modify present beach and shoreline configurations and even submerge such cities and New Orleans and Venice (which is also presently sinking because of groundwater withdrawal).
Evidence has been building, through satellite and ground measurements, that the Arctic region of the northern hemisphere has been steadily warming. The main consequence has been the shrinking of the area capped by ice in the Arctic, as shown in this plot based on ground data:
The general change pattern of Arctic temperatures is summarized in this map of the upper northern hemisphere using data from the last 30 years. The Arctic ice shelf has responded both by thinning and loss of ice at the margins. Further south, the western U.S. and Siberia are both in a warming trend. Note that the only region that has cooled during this time is in eastern Canada-Greenland.
A somewhat more precise map of temperature changes in the Arctic covering the two decades between 1972 and 1992 has been constructed from meteorological satellite data as shown here:
The net effect has been an overall shrinkage of the outer extent of the main sea ice cover within the Arctic Circle:
Owing to greater precipitation, the sea ice around the North Pole increased somewhat in 2008. But surrounding waters were more open. This meant that the famed Northwest Passage shipping lanes were even better suited to navigation by cargo ships from Europe to the Orient for the summer months, obviating the need to transit through the Panama Canal.
This ice fluctuation can be displayed visually using data from the SSM/I (Special Sensor Microwave Imager) instrument (on the U.S. Air Force Defense Meteorological Satellite series), taken in 1979 and again in 2003. The Arctic Ocean appears to be opening up off eastern Siberia and northern Alaska.
More recent observations have increased the alarm. NASA's Quikscat satellite has produced a map of the Arctic's perennial sea ice (ice that remains through the summer) for the years 2004 and 2005:
In 2007, the reduction of the most stable ice - perennial, in white - amounted to about 14% in one year - the largest yet observed. Overall, in the last 30 years ice has thinned from an average of 3.5 meters to less than 2 m. The areal extent is likewise shrinking, as this graph shows:
The ice is breaking away from Arctic islands. Here is a one year change next to Ellesmere Island in northern Canada:
Early explorers had hoped for at least a short season when the Arctic ice cap had melted enough to provide open water through which ships could pass - the famed Northwest Passage. In recent years this is now happening on an annual basis, as seen in this image in northern Canada:
This progressive diminishing of ice cover has been increasing steadily in the last few years, with 2005 now showing the least amount of frozen ice. Data acquired by satellite and ground truth lead to this plot:
Evidence is growing that ice in the Arctic Ocean is accelerating its loss. Near Ellesmere Island in northern Canada, the Ayles Ice Shelf began to break up in 2006. Here are a series of space observations that show the breakaway of a 64 square kilometer (41 sq miles) section of the Shelf over a period of just weeks:
NASA's IceSat has been monitoring the changes in Arctic sea ice thickness and volume since 2004. Here are results through 2008:
The decrease in sea ice has a potentially profound effect on the climate in the northern hemisphere. Ice reflects solar irradiation, thus keeping temperatures colder. As its areal coverage decreases, the reflectance diminishes and the open waters produced absorb more solar heat, causing the regional temperatures to rise. This in turn accelerates the general melting. If the Arctic ocean becomes notably warming, the lands to its south will experience warmer temperatures and the climates of North America, Europe, and Asia will change significantly. Some of the effects of this could be beneficial but others would be deletorius. If this warming trend is just part of the general interglacial cycle, then little can be done by humans to moderate or even reverse it. But if greenhouse gases are the culprit, then it becomes urgent to place these under much tighter control.
Experts studying the ice shrinkage in the Northern Hemisphere have, using projected temperature rises, produced a plot of area decrease during the full 20th Century and a prediction for loss in the 21st Century. The loss trend started around 1970:
The subcontinental Greenland land mass, mostly covered with ice, is also serving as a proving grounds for detection of widespread melting. Despite the interior cooling of Greenland (suggesting there should be enough snow to increase the ice cap's thickness), the edge of this ice cap is currently shrinking as confirmed by several separate aerial/ground/satellite studies in which remote sensing played a part. Most recent is the map produced by the GRACE satellite that uses gravity data to calculate the mass of ice that has melted, or increased, on Greenland:
A NASA team made survey passes across Greenland by air using a laser altimeter to measure elevations. A second group used the Global Positioning System (GPS) (page 11-6) to track movements of ice. When both results were analyzed, the map shown below was constructed. It shows increase in ice in the central part of this huge island (possibly related to the recent winter cooling) but significant shrinking near the edges (increases in summer temperature). The net loss of ice is estimated to be 51 cubic kilometers a year, enough to fill a lake 30 miles long, 30 miles wide, and 70 feet deep (spread globally to the oceans this would cause a rise of 0.005 inches a year; this contributes 7% of the total rise now being measured).
This map shows similar results from a different perspective:
Scientists visiting Greenland in the last few years have remarked about significant increases in the onset of melting along the edge of the western fringes. This is visible in MODIS images taken from 2001 into 2003, with the zone of active melting showing up as a darker gray (presence of water):
Individual glaciers along the edges of the Greenland ice cap, especially on its western side, have been undergoing retreat and flow rate increases (caused by increased water within the glacier that lubricates its movement) as the ice melts. Three big glaciers on both sides of Greenland are shown in this map:
This Terra ASTER image illustrates the loss through melting at the Jakobshavn Glacier:
This aerial photo shows the positions of the Jakobshavn glacier's toe or frontal boundary in the 21st Century. This front is now rapidly moving at a rate of 15 m/yr (49 ft/yr), as it calves off ice that enters the ocean along Greenland's western shore.
This next pair of images, made by ALI on EO-1, show in detail changes within the Jacobshavn glacier that have occurred between 2001 and 2010. The degradation of the glacier is obvious.
The two glaciers flowing into the sea along the southeast coast (see above map), Kangerdugssquaq and Helheim, from the latest ground measurements are now flowing (even as their fronts retreat) at 40 m (131 ft) and 25 m (82 ft) per year respectively for the period 2004-05 - the fastest movement rates ever observed for this pair:
Recent reports indicate that the amount of ice released from the Greenland sheet to the sea has nearly doubled in the last 10 years - the highest rate yet observed. This may be another sign that overall Arctic ice is wasting away at an accelerated pace. In the 21st Century, if this increase continues, worldwide sealevel can rise - from arctic melting alone - by about a meter (3 feet). This space image shows the coast around Helheim with a large raft of icebergs beyond the shoreline:
The concern is one of "volume", not just "area". While the areal coverage in the Arctic has decreased significantly - and looks dramatic - the volume loss in Greenland exceeds that of the sea ice. This is the kind of loss, if it accelerates, that will have the most impact on sealevel rise.
Ice melting is worldwide. Thus, melting is being observed in the Southern Hemisphere, mainly in South America and the Antarctic. Here is a ground photo pair showing the Qon Kalis glacier in the Peruvian Andes:
Glaciers in Asia also show notable retreats, as shown here for the Ganglotri glacier:
For a time, it was believed that the south polar region (the Antarctic) was not undergoing the degree of warming being observed over much of the rest of the world. But, a study of the last 50 years of satellite and ground station data has disclosed a continent-wide temperature rise of about 0.5 degrees centigrade. This next map shows the distribution of temperature rise in the last decade, with the reds denoting an increase of about 0.25° C grading into the browns that mark an increase of 0.10° C.
Ice builds up each year in shelves around the Antarctic. This diminishes in the Antarctic summers (Dec.-Feb.). Here is a composite view of Antarctica showing swirls of ice in the seas during an Antarctic summer.
There are many named ice shelves around Antarctica. This map shows the principal ones:
On page 14-14 evidence of major changes - mostly as thinning of ice along the fringes - in Antarctica was discussed. This is in response to notable rises in temperature in sea water along its coastal shelves; in the Antarctic Peninsula, a cumulative rise between 2 to 3°C (3.6 to 5.4°F) has occurred in recent years. This map, made from NOAA AVHRR data, shows that the exterior parts of the Antarctic and surrounding waters has warmed in the last 22 years, yet the interior has cooled. Perhaps the increased evaporation in the seas, where ice calving has exposed more open ocean, has produced more inland precipitation, accounting for the cooling.
Ice along the Antarctic Peninsula (west-northwest of the South Pole) is largely melting and thus retreating, although some measurements show ice advance
In the last few years parts of the fringing Antarctic ice shelves have broken loose as massive ice bergs floating northward. Use of laser altimetry and interferometric measurements of data obtained by InSAR (Interferometric SAR) are showing up as annual velocities of ice movement both within the continent and along its edges, as seen in this composite image:
The Antarctic ice shelves appear in gray in this map. Letters indicate 33 named glaciers. As the ice sheets thin and break off, the glaciers respond by increased (accelerating) outward flow.
The Ross ice shelf was one of the first in the western Antarctic to show dramatic changes. It has greatly diminished by 2006. Here are a series of MISR (Terra) images of parts of the Ross shelf as these broke off and entered the sea as ice(berg) sheets:
In February of 2008, most of the Wilkins ice shelf disintegrated, as ice cover the size of Delaware broke loose. The changes were imaged by the Terra satellite.
Formosasat, operated by the Taiwanese, obtained this detailed view of the disrupted Wilkins ice shelf:
Here is an aerial photo showing broken ice and separated slabs within the now detached Wilkins ice shelf:
The Larsen B ice shelf, located in the Weddell Sea off the Antarctic Peninsula, experienced almost disintegration in a few years just prior to 2002, amounting to a loss of continuity over an area comparable to the state of Rhode Island, as shown in these two satellite images:
Some of the changes along the Antarctic coastline can be dramatically rapid. Owing primarily to a sequence of warmer than usual Antarctic summers, the Crane glacier, feeding into the Larsen B ice shelf, underwent a collapse that moved its front back more than 6 kilometers from April of 2002 to March of 2003. This was due both to recession of the glacier through wasting and rapid breakoff of the ice as icebergs.
The West Antarctic ice shelves (primarily the Ross Shelf) are losing ~65 cubic kilometers per year, enough to raise ocean levels by 0.16 mm/yr; worldwide sealevel is rising by 1.8 mm/yr (0.7 inches/yr), primarily from Arctic, Antarctic, and Greenland sources. Scientists have calculated that if all of the ice in Greenland and the Antarctic were to completely melt through significant warming the worldwide sealevel would rise 70 meters (230 feet), which would be catastrophic to the large fraction of global populations living near the coasts.
Like the other evidence, a shrinking ice cover in each hemisphere does not confirm a direct response to global warming. Critics of the global warming hypothesis have challenged the conclusion reached from the now verified evidence of the linkage between warming and ice melting. Their argument is simple: Studies of global ice distribution during the last few million years have revealed a cyclic trend of advances and retreats. Debate still continues as to the cause(s) of these cycles. The periodicity is revealed by measuring the ratio of O18/O16 in ice cores retrieved from deep drilling into ice caps. O18 will increase during times of warming. This diagram displays the trends of warming and cooling determined by this method (individual core segments are dated by C14 and other radioactive isotopes):
Thus, natural cycles of freezing/thawing as continental glaciation waxes and wanes may indeed explain the current lines of evidence disclosed by temperature and ice retreat indicators. The crux of the argument between global warming advocates and those conservative thinkers who still believe the recent observations are not alarming and to be expected is this: The Earth may just be undergoing its latest natural warming in the cyclic sequence. But taken with other signs of temperature rises, and their effects, one can certainly postulate that global warming is a credible factor in these changes. Some argue that both cyclic warming and human-induced global warming are occurring concurrently. The present situation then may be made worse by the added effects of CO2 and other gases that are driving the atmosphere into a warmer than "normal" temperature rise.