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Ice
Ice K M Stewart, State University of New York, Buffalo, NY, USA J J Magnuson, University of Wisconsin, Madison, WI, USA ã 2009 Elsevier Inc. All rights reserved.
Introduction For most people, ice is simply frozen water. It is, but, in North Temperate and Polar regions of the earth, ice has a far greater spatial and temporal influence on soils, rivers, ponds, lakes, and oceans, and the distribution of biota, than generally realized. Ice can be a benefit (cooling our drinks, providing a base for indoor and outdoor skating, and a surface for ice fishing) as well as a detriment (driving hazards, coastal damage when ice is pushed into shore, and shipping dangers, e.g., the sinking of the Titanic). As described in more detail elsewhere, a large amount of heat exchange (80 cal g1) is associated with the freezing (heat released) or melting (heat required) of a winter ice cover on a lake. Thus, a lake that develops an ice cover can serve as a limited source and sink for heat at various times of the year. In addition to a general reduction in physiological activity in aquatic biota as water temperatures drop, if cells do freeze, ice crystals may form internally and rupture cell walls and membranes. An important associated damage is that as the crystals form they may take up the cell water, effectively desiccate the cell, and create concentrated solutes that may induce further damage or death to the cell. Interestingly, cold tolerance of various biota to freezing or near-freezing temperatures, may be aided by both adequate acclimation and the development of internal solutes that act as antifreeze agents. An ice cover on a lake restricts gas exchange across the frozen ice boundary, prevents wind-induced mixing of water, and helps preserve the inverse thermal structure and stability of the water column below the ice. Additionally, compared with the relative ease of light penetration through clear ice, an ice sheet covered with much snow may markedly restrict or prevent the transmission of solar radiation through ice such that there may be periods of time during which ‘darkness’ extends from the ice–water interface downwards and photosynthesis is not possible. A cover of snow over the ice may also act as an insulator and reduce heat transmission in either direction. There are many big freshwater lakes in the world but the biggest known volumetric quantities of fresh water on Earth are in the form of ice (i.e., the Greenland and the Antarctic ice caps), not liquid water. Indeed, as human populations expand further, atmospheric greenhouse gases (such as carbon
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dioxide and methane) continue to rise, and governments fail to take timely corrective measures, there is an increased probability that global warming will cause further melting of those ice caps. The result is anticipated to raise water levels of the oceans enough to inundate numerous coastal cities around the world. Tens of millions of people may be displaced and resources diminished further. Other than the limited activities of a few earlier polar explorers, the extent of exploration of the Earth, particularly in the Northern Hemisphere, has been mainly limited by ice. For example, earlier shipping and commerce between countries bordering the Baltic Sea in Europe were seasonally limited by the extent of winter ice on the Baltic. Within North America, many lives and much effort were expended in efforts to find a ‘Northwest Passage’ for boats through the northern archipelago region of Canada. Here also, the earlier resupply of far-northerly coastal towns and villages of North America, by water, was very much a seasonal affair, and success was a function of reduced quantities of sea ice. In more recent years, the main transport of supplies and fuel to some northerly mines in Canada is done by heavy trucks during winter over an ‘ice road.’ The ‘ice road’ starts near the capital City of Yellowknife (province of Northwest Territories) and leads about 400 km north to some mines up in the Canadian Province of Nunavut. The ice road is so named because well over three-fourth of the distance consists of roads prepared over many frozen lakes. After the lakes are well frozen (more than a meter of ice is recommended), large numbers of truck trips may be undertaken each winter to transport fuel and supplies. Trips by truck are far less expensive than transport by airplane.
Variability and Degree Days
A bit of background information may help understand some of the variation seen in the freeze-up (complete ice-on) or break-up (complete ice-off) dates of lakes that normally freeze each winter. For example, at the start of some winters, the sudden arrival of exceptionally cold and persistent quantities of Arctic air may cause both shallow and nearby moderately deep lakes to develop an ice cover on nearly the same dates. Similarly, in the spring, the
Light and Heat in Aquatic Ecosystems _ Ice
arrival and persistence of exceptionally warm air temperatures may cause the same lakes to lose their ice on nearly the same dates. However, other things being equal and during an average winter, shallow freshwater lakes tend to both freeze earlier (in late fall or early winter) and lose their ice earlier (in late winter or early spring) than is the general case for deeper lakes. Shallow lakes have less total heat (as calories) to exchange with the atmosphere than do deep lakes with the same surface area. Consequently, shallow lakes respond more rapidly to episodes of cold/warm atmospheric fronts and decreases/ increases in solar radiation. Of course, unusually strong and persistent winds may play a significant role in retarding or advancing the dates when a lake may freeze over or lose its ice. One method to help quantify the amount of cold or heat, leading up to a full ice cover in winter, or no ice cover in the spring, is by keeping tract of the number of freezing degree days (FDDs) and thawing degree days (TDDs) leading up to those respective events. A day when the average daily air temperature is 0 C does not count as a FDD or a TDD. (Note, an average daily temperature is more accurate if determined from multiple equally spaced measurements, e.g., hourly recordings, and not just the average of the maximum and minimum for that day.) A day when the air temperature averages 5 C (i.e., five degrees below 0 C) counts as 5 FDDs, and a day when the air temperature averages þ5 C (i.e., five degrees above 0 C) counts as 5 TDDs. During the normal course of events leading up to an ice-on or ice-off event, some days may average below 0 C, and some days may average above 0 C. As late fall and winter progress, there will be an increasing number of FDDs. As late winter and spring progress, there will be an increasing number of TDDs. Therefore, it is the running sum of the FDDs or of the TDDs, leading up to an ice-on or ice-off event, which may serve as a rough predictor of when a lake may freeze over or thaw. Interestingly, one investigation applied the FDD and TDD concept to the freeze and thaw dates of a limited set of 22 lakes (ranging from 5.5 to 83.5 m max depth, 1.9 to 38.8 m mean depth) in the State of New York. The results there showed that the correlation between the number of FDDs and the maximum (or the mean) depth of the lake was statistically significant whereas the correlation between TDDs and lake depths (for that particular set of 22 lakes) was not. Knowledge of a lake’s depth is helpful in trying to predict freeze dates. Generally, the depth of a lake is a more important variable than surface area in influencing when a lake will freeze. For example, consider that there are three hypothetical lakes at the same elevation and located not far from each other. Assume
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the first lake has a mean depth of 5 m and has a surface area of 1 km2, the second lake also has a mean depth of 5 m but has a surface area of 100 km2, and the third lake has a mean depth of 50 m deep and has a surface area of 1 km2. The first and second lakes (with mean depths of 5 m) are both likely to freeze before the third lake with its mean depth of 50 m. Some smaller shallow ponds and lakes may freeze over with 50 FDDs whereas several hundred FDDs may be required before an ice cover develops on some deeper lakes. Strong winds may play an increasingly important role as the surface area of a lake increases. Consequently, if there was a 5 m deep lake with a surface area of 1000 km2, the chances of it freezing over on the same date as a nearby 5 m deep lake with a surface area of only 1 km2 are substantially reduced. Ice Types
Many terms are used to describe and differentiate various types of ice. The terms provide some commonality of usage for fisherman, scientists, engineers, and others. Shipping companies also benefit because careful descriptions of ice types on large lakes influence decisions as to whether transport of material to and through ice by boats is feasible. What follows now are some simple descriptions and example figure/photos of only a few of the many ‘ice types’ known. Interested readers may wish to look over some more extensive earlier glossaries, e.g., the U.S. Dept of Commerce (1971) of NOAA, among others. Black ice This ice is clear but appears black because an observer on the ice surface can see through the ice into the dark depth of the lake (Figure 1(a)). This is usually the first ice to form and thicken over a lake surface. There may be some selective loss at various wavelengths, but generally, light is transmitted easily through such ice. This clear ice is where the word crystal came from in Greek. If a block of ice were cut out of a frozen lake surface during mid-winter, the black or clear ice would be on the bottom of the block and may or may not (depending on the snow fall and winds) extend to the upper surface of the block of ice. White ice or snow ice This is the whitish or grayish appearing ice that is generally above the black ice. The thickness of the ‘white ice’ varies with snow fall and also spatially across a lake (Figure 1(b)). As the wind energies are modified near boundaries, nearshore regions of a lake may have slightly thicker amounts of ‘white ice’ than is the case at mid-lake. The reason for the whitish appearance is that the majority of white ice originates from accumulated
666 Light and Heat in Aquatic Ecosystems _ Ice
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
(l)
Figure 1 Various types of ice (a)–(l) seen on Lake Erie from mid February through early March in 1972 (Most photos by K.M.S). (a) ‘Black ice’ with patches of wind-swept snow on top. (b) Some irregular patches of ‘white ice’ (formerly melted snow but now frozen) and a few streaks of white ice where earlier cracks in black ice allowed wetting of snow from beneath. (c) Just beyond the flotationequipped helicopter and man, both of which are on a piece of floating ice just large enough for support, are numerous pieces of small ‘brash ice’ and other ice fragments. (d) Rounded ‘pancake ice’ that has been refrozen into an ice plate thick enough ( 25 cm) to support the ice researchers and the flotation-equipped helicopter. (e) Hundreds of pieces of floating ‘cake ice’ (roughly 5–10 m across). (f, g) Two different aerial photos of ‘pressure ridges.’ Pressure ridges may be many times thicker than the surrounding ice cover and constitute a real barrier for all boats except ice-breakers. (h) This photo, made while standing on the ice, shows some ridging and rafting of ice in a ‘pressure ridge’ just beyond the helicopter. (i) ‘Ice foot’ or ‘foot ice’ in a narrow ridge around the shores of Long Point (a peninsula extending down into the easterly part of Lake Erie from the Province of Ontario, Canada). Much of the surrounding portion of Lake Erie froze over after the ‘ice foot’ formed. (j) An aerial view of ‘ice volcanoes’ on the Canadian shore near the easterly end of Lake Erie. These ‘ice volcanoes’ formed during the development of the ‘foot ice,’ i.e., before the lake froze over. Note that the size of the ‘ice volcanoes’ compares to that of some nearby cottages. (k) ‘Rectilinear thrusts.’ This rectangular ice fracture was noted in some fairly thin ice (5–6 cm) that had probably formed only 2–3 days prior to when this aerial photo was taken. This type of fracture, seen only rarely, was noted a few km off the Canadian shore in the central basin of Lake Erie. (l) ‘Barchans.’ These beautiful features are crescent-shaped structures of snow that have their two tails pointed downwind. Similar shaped structures of sand occur in some of the world’s deserts.
snow falls of varying depths. Following some irregular periods of above freezing air temperatures, snow may melt onto the top of the black ice. White ice may sometimes be seen where water has seeped up, through cracks in the black ice, and melted a streak of overlying snow from beneath. The latter case (most easily apparent from the air) can be seen as irregular white lines just above the cracks in the ice. Both a wet and more widespread melted snow, such as one may ‘slosh through’ on top of black ice, as well as snow that is wetted above cracks, may refreeze with many bubbles. The many bubbles that reduce the amount of light transmitted through such ice, increase the
amount of radiant energy reflected back into space, and also help give such ice its whitish appearance. Frazil ice This type of ice is associated with the initial stages of freezing in both rivers and lakes. Frazil ice is composed of tiny crystals, needle-like spicules, or thin plates of ice. When large quantities of such tiny crystals are present, the surface of the water may appear as dull, opaque, or oily. Brash ice This refers to the variable-sized smaller pieces of ice (generally from a few centimeters to <2 m across) that result from randomly broken
Light and Heat in Aquatic Ecosystems _ Ice
pieces, or wreckage, of ice of many different types (Figure 1(c)). Brash ice is commonly seen along the track of a ship breaking through ice. Brash ice may also be seen near the edge of an ice pack where open water waves and ice are interacting. Pancake ice This type of ice may result when large numbers of formerly broken pieces of floating ice (size may vary as a function of ice thickness and strength, but roughly 0.5–3 m in diameter) are abrading each other’s sides (Figure 1(d)). The result of such abrading action is to cause the ice to lose some of its sharper edges and begin to look like a rounded pancake. Of course, the effects of pure physical abrasion may be noted in nature elsewhere, e.g., rounded stones left in fields after glacial action thousands of years earlier, or simply some beach/shore areas where pebbles and stones have been rolled or pushed around in areas of wave-energy dissipation. In the case of the pebbles and rocks, all sides generally get abraded whereas, in the case of ice, the abrasion is primarily limited to the edges of the floating piece of ice. Lake and river pancake ice may exist as discrete rounded pieces or become frozen into an extended ice sheet. Cake ice Cakes may be variable-sized fragments of ice that are roughly 5–10 m across (Figure 1(e)). Their presence suggests the partial break-up of larger subdivisions of ice such as ice floes (10–2000 m) or ice fields (2–20 km). Pressure ridge This type of ice formation is a generic term for ice that has been squeezed or forced together (Figure 1(f), 1(g), and 1(h)). It is a feature reflective of energy release such that fractured pieces of ice plates may be forced into, upward, or beneath each other in a jumbled mass of ice. Geophysicists can note a surprising similarity of the action of pressure ridges on frozen lakes to the mountain-forming and subduction seen in the plate tectonics on Earth. Being able to break through the thickness of the ice (from one to possibly many meters) at pressure ridges is one of the reasons ice-breaking ships are needed in some water bodies. Foot-ice or ice-foot This term refers to the narrow strip of relatively thick ice observed where the shore meets the lake during freezing conditions (Figure 1(i)). ‘Foot-ice’ is a type of ice seen most commonly along the shores of the larger lakes of the world in the North Temperate Zones, but is generally of little to no consequence in very small lakes. When Arctic or subfreezing air moves across large lakes, which are still ice-free or only partially covered with ice, breaking waves on shore may toss spray into the air. The below freezing air temperatures may cause the spray to freeze in the
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air and/or upon contact with the cold shore. A narrow ‘ice foot’ gradually builds there and thickens with time and freezing temperatures. As the ice builds up, the base of the ice may extend to below the water surface and the heavy strip of foot ice may gradually move lake-ward. The ‘foot ice’ may provide a temporary advantage in that it becomes a seasonal barrier against some types of coastal erosion. A disadvantage may accrue in spring if strong on-shore winds drive heavy pieces of ice into shore and accelerate erosion. Ice volcanoes Ice volcanoes are structures that are sometimes seen to develop at the edge of the foot-ice during freezing conditions (Figure 1(j)). Some sections of a shoreline may have many ice volcanoes side-by-side and other sections may have few to none. Ice volcanoes are an interesting and, depending on the winter and winds, fairly striking feature along the downwind shores of some North American Great Lakes. The eastern end of Lake Erie has stretches of shore line where ice volcanoes are fairly common in average to harsh winters. In some of the areas along the lake-side edge of foot-ice, arriving waves (of a certain energy or wavelength?) are sprayed up into the air through a developing tube-like structure in the foot-ice. The tube may be partially open, on the side facing the open water, or mostly closed except for an opening at the bottom. Some of the wave energy is funneled into the open side of the tube, or into an opening at the bottom of the developing vertical cone. Water and slush may be propelled up through the tube, sprayed out the top, and, when falling back in freezing air or on ice, form a conical top. More spray continues this process and the growing conical ice structure (some many meters high) resemble a small volcano made of ice . . . thus the name of ‘ice volcano.’ Rectilinear thrusts This is a striking feature where lateral stresses on ice may cause portions of a cracked ice sheet to override or slide under one another in a fractured rectangular manner (Figure 1(k)). These beautiful rectangular fractures (not normally seen in small lakes, and observed infrequently in the developing ice of some Laurentian Great Lakes) are probably a reflection of the crystalline structure, strength, and limited thickness of the ice. Barchans Barchans are crescent-shaped structures composed of snow (Figure 1(l)). They lie on top of the ice, where winds can easily transport particles of snow, but they are not necessarily a type of ice by themselves. The crescents are oriented with their widest central portion toward the direction from which the wind is coming, and their two narrower tails are pointed downwind or toward the direction the wind is heading
668 Light and Heat in Aquatic Ecosystems _ Ice
River Ice
It is easy to forget but the departure of river ice, during the spring runoff and increased stream levels, indirectly supported thousands of people as North Temperate Zones were being settled. In areas of original forest in northerly states of the United States and provinces of Canada, huge numbers of people were employed by various lumber industries to harvest the trees. Many people were employed to chop and saw trees in the winter. Other people helped move the felled trees to a river bank by the aid of horses, sleds, tractors, and skidding while the snow and icy logging roads through forests made the task much easier than in summer. Some people were involved in the dangerous job of ‘riding the logs’ down the river, or clearing log jams. Still others were involved in more sustained work at the lumber mills. The rising stream waters of spring not only cleared the ice from the streams, but also served as a vital conveyer belt for transporting cut timber to the lumber mills. The historical and more recent interest in river ice is a reflection of societal concerns about ice jams and flooding as well as interest in navigation and hydroelectric power. Ice jams tend to recur in the same sections of a stream because, in combination with variable flow, some stream and bank morphologies are more favorable for the development of ice jams. Research has shown that ice jams develop most commonly where the slope of the river changes from steep to mild, not necessarily within narrowed reaches of the river. As the flow of the river is lessened floating ice may accumulate and form jams. Ice jams in streams may cause surprisingly rapid ‘over-the-bank’ flooding and subsequent damage to development within flood zones. During the onset of winter, the development of an ice cover on streams may be preceded by water that is supercooled, i.e., cooled a bit below 0 C. If the stream flow is rapid and turbulent, tiny crystals or spicules of ice may form within the vertical flow field and become part of an ice cover, attach to rocks on the bottom, or both. If the stream water moves slowly, or hardly at all, the supercooling at the surface film may cause a thin film of ice to form there and thicken if subfreezing air temperatures persist. Supercooled conditions and variable flows tend to generate a chaotic mix of small crystals and variable-sized chunks of ice such that a jumbled mix at an ice jam may be called ice rubble.
Just as is the case at some lakes, some communities or businesses on the shores of a few rivers hold contests to see who can predict the time when the ice will be gone. The definitions of when the river ice is gone varies among stake holders but, after establishing some local definitions, prediction of the ‘break-up’ date is a social event enjoyed by the happy winner(s) as well as the many losers. Nature retains surprises! Ice as a Climate Indicator
Ice is a powerful indicator of climate change and variability. The ice on lakes and streams and in glaciers, permafrost, and the winter snow cover serve as indicators of climate. On the water and land surfaces, the change in state from water to ice and rain to snow, and visa versa, is obvious. This visible threshold between the hydrosphere and the cryosphere enhances the usefulness because a human observer, whether directly or from photographs or satellites, can personally perceive the transition dates and the geographic extent of frozen water. Ice is perhaps an even more powerful indicator in a less observable way because when glaciers form and thicken over the centuries and millennia, a paleorecord can be obtained from ice cores, e.g., from the Greenland and Antarctic glaciers, that reveal long-term changes in air temperature and atmospheric chemistry for hundreds of thousands of years into the past. Ice and snow cover is very sensitive to warming at the temperatures typical at the Earth’s surface. The shorter snow and ice cover seasons and the melting of glaciers and loss of Arctic sea ice have become graphic images of the current warming and provide the public with strong evidence that global warming is occurring. In the State of Alaska, the end of the snow melt season at Point Barrow has advanced from about
Ice duration on lake mendota 6
19 days less per 100 years
5 4 Months
Barchans are also seen in some of the world’s deserts where blowing sand is transported by wind and may create some crescent-shaped sand dunes.
3 2 1
10 longest ice covers 10 shortest ice covers
0 1850
1900
1950
2000
Figure 2 A long-term record showing the decreasing duration of winter ice cover, with time, for Lake Mendota at Madison, Wisconsin. More evidence of global warming.
Light and Heat in Aquatic Ecosystems _ Ice
16 June to 7 June over the last 40 years and some of the Alaskan glaciers have receded by 30% of their length between 1959 and 1990. The extent of Arctic sea ice has decreased by 15–20% over the last 30 years and, in the Antarctic, sea ice is being rapidly lost around the Palmer Peninsula. Since the 1960s and 1970s, the thickness of the Arctic sea ice cover has thinned from an average of 3.1–1.6 m, or by about 45% across six areas of the floating ice. For the same time period, the thickness of ice at the North Pole has decreased from 3.8 to 2.8 m, or by 25%.
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In Southern Wisconsin, the duration of ice cover on Lake Mendota has declined by an overall average of 19 days per 100 years (see Figure 2), but by about 9 days per decade just since the mid-1970s. The recent general warming since the mid-1800s resulted in later freeze-up and earlier break-up dates on many lakes and rivers. Around the Northern Hemisphere over the last 150 years, the rates of change for freeze-up have been 5.7 days later per 100 years and the break-up 6.3 days earlier per 100 years. These rates of loss of ice cover have been even more rapid since the
Table 1 Evidence of change over time in freeze-up dates, break-up dates, and ice thickness of lakes and rivers at many locations around the world Water body
Measure
Change per C warmer air temperature
Location
Dates
Lake
Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Fall freeze over date Fall freeze over date Fall freeze over date Fall freeze over date Fall freeze over date Fall freeze over date Fall freeze over date Maximum ice thickness Spring break-up date Spring break-up date Ice thickness
3.8–5.0 days earlier
Finland, n ¼ 3
April air temp. 1836–2002
4.4 days earlier 5 days earlier
N. America, n ¼ 2076 data points from 143 lakes N. Sweden, n ¼ 14
Feb–June air temp. Various and not specified 1961–2002
15 days earlier
S. Sweden n ¼ 14
1961–2002
2 days earlier
Minnesota, n ¼ 6
March air temp. 1960–2004
3 days earlier
Laurentian Great Lake region
Fall to spring 1975–2004
6.4 days earlier
Lake Mendota Wisconsin
Fall and early winter 1855–1990
2.8 days earlier
Grand Traverse Bay, Michigan
8.3 days earlier
Wisconsin, n ¼ 45
Jan–March 1990–1991
5.5 days earlier
N. Hemisphere 24 time series
1846–1995
5.6 days earlier
Lake Mendota Wisconsin, n ¼ 65
2.6–5.1 days later
Finland, n ¼ 3
Model sensitivity to Temp., 1961–1990 Nov. air temp 1836–2002
3.3 days later 3.9 days later
N. America, n ¼ 733 data pts from 143 lakes Lake Mendota Wisconsin
4.9 earlier
Lake Mendota Wisconsin
2.0 days later
Grand Traverse Bay, Michigan
4.8 days later
N. Hemisphere 15 time series
1846–1995
4.7 days later
Fall to spring 1975–2004
3.6 days earlier
Laurentian Great Lake region, n ¼ 65 N. America, n ¼ 831 data points from 143 lakes Finland, n ¼ 1
April air temp. 1802–2002
4.4 days earlier
Maine n ¼ 1
Dec–Feb air temp. 1931–2002
16 cm thinner on 28 Feb
Maine, n ¼ 1
Dec–Feb air temp. 1900–1999
Lake Lake Lake Lake Lake Lake Lake Lake Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river River River River
Data compiled from many sources.
6.8 cm thinner
Sept–Dec air Temp. Various and not specified Model sensitivity to Temp., 1961–1990 Fall and early winter
Sept–June Mean temp.
670 Light and Heat in Aquatic Ecosystems _ Ice
mid-1970s. For example, across the Laurentian Great Lakes region of North America, freeze-up and breakup dates for lakes and rivers have been 3.3 days later and 2.1 days earlier, respectively, per 100 years, or about 3–6 times more rapid than for the longer 150 year records. Some specific examples of climateinduced change are given in Table 1. Some of these cryosphere records have been related to temperature and other factors to form a quantitative indicator of temperature change and variability (note changes indicated in Table 1). For lakes and rivers, the midwinter thickness of ice decreases 7–16 cm per 1 C increase in temperature. Dates of freeze-up and break-up are observed and recorded more commonly than is ice thickness. These dates have been changing around the Northern Hemisphere at rates near 4–6 days per 1 C change in temperature, but a greater range from 2 to 15 days per 1 C is observed for some locations. Ice dates are more sensitive to temperature increases in warmer (more southerly) areas of the regions where inland waters become ice covered than in cooler (more northerly) regions. Ice cover on rivers appears to be more sensitive to increases in temperature than is ice cover on lakes. Other explanatory variables such as altitude, latitude, and snow cover relate to ice dates on lakes and rivers as well. Some inland waters that typically have been ice covered are experiencing occasional winters that are warm enough to prevent complete ice formation. When this becomes common, ice cover changes from a continuous variable (day or thickness) to a categorical variable (present or not present). When this happens, ice cover becomes a less precise indicator of further climate change except on some large lakes like the Laurentian Great Lakes where the extent of ice cover can be measured from satellites. The recession of valley glaciers and loss of glacial mass is a visible indication of recent climatic warming. Examples of decreases in the mass of ice characterize the Arctic glaciers for the last 40 years with a loss in the Northern Hemisphere of about 90 km3 year1. The east lobe of Twin Glacier, near the Alexandria Fiord in Greenland, receded about 4 m year1 from 1960 to 1985 and about 8 m year1 from 1985 to 1990. There are many such cases. A few glaciers are growing because glacial growth and loss are complicated by warmth that melts them and increased snow deposition, associated with some of
the warmth that thickens them. Greenland is rapidly losing ice around the edges and thickening centrally for this reason. So glaciers can be a complex indicator of warming with snowfall as a part of the equation. Differences in ice cover from year to year are strong indicators of climate variability as well as of climatic change. For example, lake ice cover dates are related to large-scale climate drivers such as the El Nino Southern Oscillation (ENSO) index, the North Atlantic Oscillation (NAO), and North Pacific Index (NPI). The interactions between ice dates and these drivers vary across the continents with strong signals in some areas and weak signals in others. Their influence also differs with the years being considered, e.g., the influence of El Nino is apparent on break-up dates after 1960 but not before 1950. See also: Physical Properties of Water.
Further Reading ACIA (2004) Impacts of a Warming Arctic; Arctic Climate Assessment. Cambridge University Press. http://www.acia.uaf.edu. Ashton G (1979) River ice. American Scientist 67: 38–45. Huntington TG, Hodgkins GA, and Dudley RW (2003) Historical trend in river ice thickness and coherence in hydroclimatological trends in Maine. Climatic Change 61: 217–236. Korhonen J (2006) Long-term changes in lake ice cover in Finland. Nordic Hydrology 37: 347–363. Magnuson JJ, Robinson DM, Benson BJ, et al. (2000) Historical trends in lake and river ice cover in the northern hemisphere. Science 289: 1743–1746. Erratra 1291–1254. Marshall EW (1966) Air photo interpretation of Great Lakes ice features. Special Report 25, Great Lakes Res. Div., University of Michigan, Ann Arbor, MI. 92 pp. Rondy DR (1969) Great lakes ice atlas. U.S. Lake Survey Research Report 5–6. Dept. of the Army, Corps of Engineers Detroit, MI. (8 pages, but 6 Figures and 31 Plates appended). Shen HT (2003) Research on river ice processes: Progress and missing links. Journal of Cold Regions Engineering 37: 135–142. Stewart KM and Haugen RK (1990) Influence of lake morphometry on ice dates. Verhandlungan International Verein Limnology 24: 122–127. U.S. Department of Commerce (1971) Ice Glossary (No. 75–602). National Oceanographic and Atmospheric Administration. National Ocean Survey, p. 9, Lake Survey Center, Detroit, MI. Weyhenmeyer GA, Meili M, and Livingston DM (2004) Nonlinear temperature response of lake ice breakup. Geophysical. Research Letters 31: L07203, 4. Williams G, Layman KL, and Stefan HG (2004) Dependence of lake ice covers on climatic, geographical and bathymetric variables. Cold Regions Science and Technology 40: 145–164.
Introduction For most people, ice is simply frozen water. It is, but, in North Temperate and Polar regions of the earth, ice has a far greater spatial and temporal influence on soils, rivers, ponds, lakes, and oceans, and the distribution of biota, than generally realized. Ice can be a benefit (cooling our drinks, providing a base for indoor and outdoor skating, and a surface for ice fishing) as well as a detriment (driving hazards, coastal damage when ice is pushed into shore, and shipping dangers, e.g., the sinking of the Titanic). As described in more detail elsewhere, a large amount of heat exchange (80 cal g1) is associated with the freezing (heat released) or melting (heat required) of a winter ice cover on a lake. Thus, a lake that develops an ice cover can serve as a limited source and sink for heat at various times of the year. In addition to a general reduction in physiological activity in aquatic biota as water temperatures drop, if cells do freeze, ice crystals may form internally and rupture cell walls and membranes. An important associated damage is that as the crystals form they may take up the cell water, effectively desiccate the cell, and create concentrated solutes that may induce further damage or death to the cell. Interestingly, cold tolerance of various biota to freezing or near-freezing temperatures, may be aided by both adequate acclimation and the development of internal solutes that act as antifreeze agents. An ice cover on a lake restricts gas exchange across the frozen ice boundary, prevents wind-induced mixing of water, and helps preserve the inverse thermal structure and stability of the water column below the ice. Additionally, compared with the relative ease of light penetration through clear ice, an ice sheet covered with much snow may markedly restrict or prevent the transmission of solar radiation through ice such that there may be periods of time during which ‘darkness’ extends from the ice–water interface downwards and photosynthesis is not possible. A cover of snow over the ice may also act as an insulator and reduce heat transmission in either direction. There are many big freshwater lakes in the world but the biggest known volumetric quantities of fresh water on Earth are in the form of ice (i.e., the Greenland and the Antarctic ice caps), not liquid water. Indeed, as human populations expand further, atmospheric greenhouse gases (such as carbon
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dioxide and methane) continue to rise, and governments fail to take timely corrective measures, there is an increased probability that global warming will cause further melting of those ice caps. The result is anticipated to raise water levels of the oceans enough to inundate numerous coastal cities around the world. Tens of millions of people may be displaced and resources diminished further. Other than the limited activities of a few earlier polar explorers, the extent of exploration of the Earth, particularly in the Northern Hemisphere, has been mainly limited by ice. For example, earlier shipping and commerce between countries bordering the Baltic Sea in Europe were seasonally limited by the extent of winter ice on the Baltic. Within North America, many lives and much effort were expended in efforts to find a ‘Northwest Passage’ for boats through the northern archipelago region of Canada. Here also, the earlier resupply of far-northerly coastal towns and villages of North America, by water, was very much a seasonal affair, and success was a function of reduced quantities of sea ice. In more recent years, the main transport of supplies and fuel to some northerly mines in Canada is done by heavy trucks during winter over an ‘ice road.’ The ‘ice road’ starts near the capital City of Yellowknife (province of Northwest Territories) and leads about 400 km north to some mines up in the Canadian Province of Nunavut. The ice road is so named because well over three-fourth of the distance consists of roads prepared over many frozen lakes. After the lakes are well frozen (more than a meter of ice is recommended), large numbers of truck trips may be undertaken each winter to transport fuel and supplies. Trips by truck are far less expensive than transport by airplane.
Variability and Degree Days
A bit of background information may help understand some of the variation seen in the freeze-up (complete ice-on) or break-up (complete ice-off) dates of lakes that normally freeze each winter. For example, at the start of some winters, the sudden arrival of exceptionally cold and persistent quantities of Arctic air may cause both shallow and nearby moderately deep lakes to develop an ice cover on nearly the same dates. Similarly, in the spring, the
Light and Heat in Aquatic Ecosystems _ Ice
arrival and persistence of exceptionally warm air temperatures may cause the same lakes to lose their ice on nearly the same dates. However, other things being equal and during an average winter, shallow freshwater lakes tend to both freeze earlier (in late fall or early winter) and lose their ice earlier (in late winter or early spring) than is the general case for deeper lakes. Shallow lakes have less total heat (as calories) to exchange with the atmosphere than do deep lakes with the same surface area. Consequently, shallow lakes respond more rapidly to episodes of cold/warm atmospheric fronts and decreases/ increases in solar radiation. Of course, unusually strong and persistent winds may play a significant role in retarding or advancing the dates when a lake may freeze over or lose its ice. One method to help quantify the amount of cold or heat, leading up to a full ice cover in winter, or no ice cover in the spring, is by keeping tract of the number of freezing degree days (FDDs) and thawing degree days (TDDs) leading up to those respective events. A day when the average daily air temperature is 0 C does not count as a FDD or a TDD. (Note, an average daily temperature is more accurate if determined from multiple equally spaced measurements, e.g., hourly recordings, and not just the average of the maximum and minimum for that day.) A day when the air temperature averages 5 C (i.e., five degrees below 0 C) counts as 5 FDDs, and a day when the air temperature averages þ5 C (i.e., five degrees above 0 C) counts as 5 TDDs. During the normal course of events leading up to an ice-on or ice-off event, some days may average below 0 C, and some days may average above 0 C. As late fall and winter progress, there will be an increasing number of FDDs. As late winter and spring progress, there will be an increasing number of TDDs. Therefore, it is the running sum of the FDDs or of the TDDs, leading up to an ice-on or ice-off event, which may serve as a rough predictor of when a lake may freeze over or thaw. Interestingly, one investigation applied the FDD and TDD concept to the freeze and thaw dates of a limited set of 22 lakes (ranging from 5.5 to 83.5 m max depth, 1.9 to 38.8 m mean depth) in the State of New York. The results there showed that the correlation between the number of FDDs and the maximum (or the mean) depth of the lake was statistically significant whereas the correlation between TDDs and lake depths (for that particular set of 22 lakes) was not. Knowledge of a lake’s depth is helpful in trying to predict freeze dates. Generally, the depth of a lake is a more important variable than surface area in influencing when a lake will freeze. For example, consider that there are three hypothetical lakes at the same elevation and located not far from each other. Assume
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the first lake has a mean depth of 5 m and has a surface area of 1 km2, the second lake also has a mean depth of 5 m but has a surface area of 100 km2, and the third lake has a mean depth of 50 m deep and has a surface area of 1 km2. The first and second lakes (with mean depths of 5 m) are both likely to freeze before the third lake with its mean depth of 50 m. Some smaller shallow ponds and lakes may freeze over with 50 FDDs whereas several hundred FDDs may be required before an ice cover develops on some deeper lakes. Strong winds may play an increasingly important role as the surface area of a lake increases. Consequently, if there was a 5 m deep lake with a surface area of 1000 km2, the chances of it freezing over on the same date as a nearby 5 m deep lake with a surface area of only 1 km2 are substantially reduced. Ice Types
Many terms are used to describe and differentiate various types of ice. The terms provide some commonality of usage for fisherman, scientists, engineers, and others. Shipping companies also benefit because careful descriptions of ice types on large lakes influence decisions as to whether transport of material to and through ice by boats is feasible. What follows now are some simple descriptions and example figure/photos of only a few of the many ‘ice types’ known. Interested readers may wish to look over some more extensive earlier glossaries, e.g., the U.S. Dept of Commerce (1971) of NOAA, among others. Black ice This ice is clear but appears black because an observer on the ice surface can see through the ice into the dark depth of the lake (Figure 1(a)). This is usually the first ice to form and thicken over a lake surface. There may be some selective loss at various wavelengths, but generally, light is transmitted easily through such ice. This clear ice is where the word crystal came from in Greek. If a block of ice were cut out of a frozen lake surface during mid-winter, the black or clear ice would be on the bottom of the block and may or may not (depending on the snow fall and winds) extend to the upper surface of the block of ice. White ice or snow ice This is the whitish or grayish appearing ice that is generally above the black ice. The thickness of the ‘white ice’ varies with snow fall and also spatially across a lake (Figure 1(b)). As the wind energies are modified near boundaries, nearshore regions of a lake may have slightly thicker amounts of ‘white ice’ than is the case at mid-lake. The reason for the whitish appearance is that the majority of white ice originates from accumulated
666 Light and Heat in Aquatic Ecosystems _ Ice
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
(l)
Figure 1 Various types of ice (a)–(l) seen on Lake Erie from mid February through early March in 1972 (Most photos by K.M.S). (a) ‘Black ice’ with patches of wind-swept snow on top. (b) Some irregular patches of ‘white ice’ (formerly melted snow but now frozen) and a few streaks of white ice where earlier cracks in black ice allowed wetting of snow from beneath. (c) Just beyond the flotationequipped helicopter and man, both of which are on a piece of floating ice just large enough for support, are numerous pieces of small ‘brash ice’ and other ice fragments. (d) Rounded ‘pancake ice’ that has been refrozen into an ice plate thick enough ( 25 cm) to support the ice researchers and the flotation-equipped helicopter. (e) Hundreds of pieces of floating ‘cake ice’ (roughly 5–10 m across). (f, g) Two different aerial photos of ‘pressure ridges.’ Pressure ridges may be many times thicker than the surrounding ice cover and constitute a real barrier for all boats except ice-breakers. (h) This photo, made while standing on the ice, shows some ridging and rafting of ice in a ‘pressure ridge’ just beyond the helicopter. (i) ‘Ice foot’ or ‘foot ice’ in a narrow ridge around the shores of Long Point (a peninsula extending down into the easterly part of Lake Erie from the Province of Ontario, Canada). Much of the surrounding portion of Lake Erie froze over after the ‘ice foot’ formed. (j) An aerial view of ‘ice volcanoes’ on the Canadian shore near the easterly end of Lake Erie. These ‘ice volcanoes’ formed during the development of the ‘foot ice,’ i.e., before the lake froze over. Note that the size of the ‘ice volcanoes’ compares to that of some nearby cottages. (k) ‘Rectilinear thrusts.’ This rectangular ice fracture was noted in some fairly thin ice (5–6 cm) that had probably formed only 2–3 days prior to when this aerial photo was taken. This type of fracture, seen only rarely, was noted a few km off the Canadian shore in the central basin of Lake Erie. (l) ‘Barchans.’ These beautiful features are crescent-shaped structures of snow that have their two tails pointed downwind. Similar shaped structures of sand occur in some of the world’s deserts.
snow falls of varying depths. Following some irregular periods of above freezing air temperatures, snow may melt onto the top of the black ice. White ice may sometimes be seen where water has seeped up, through cracks in the black ice, and melted a streak of overlying snow from beneath. The latter case (most easily apparent from the air) can be seen as irregular white lines just above the cracks in the ice. Both a wet and more widespread melted snow, such as one may ‘slosh through’ on top of black ice, as well as snow that is wetted above cracks, may refreeze with many bubbles. The many bubbles that reduce the amount of light transmitted through such ice, increase the
amount of radiant energy reflected back into space, and also help give such ice its whitish appearance. Frazil ice This type of ice is associated with the initial stages of freezing in both rivers and lakes. Frazil ice is composed of tiny crystals, needle-like spicules, or thin plates of ice. When large quantities of such tiny crystals are present, the surface of the water may appear as dull, opaque, or oily. Brash ice This refers to the variable-sized smaller pieces of ice (generally from a few centimeters to <2 m across) that result from randomly broken
Light and Heat in Aquatic Ecosystems _ Ice
pieces, or wreckage, of ice of many different types (Figure 1(c)). Brash ice is commonly seen along the track of a ship breaking through ice. Brash ice may also be seen near the edge of an ice pack where open water waves and ice are interacting. Pancake ice This type of ice may result when large numbers of formerly broken pieces of floating ice (size may vary as a function of ice thickness and strength, but roughly 0.5–3 m in diameter) are abrading each other’s sides (Figure 1(d)). The result of such abrading action is to cause the ice to lose some of its sharper edges and begin to look like a rounded pancake. Of course, the effects of pure physical abrasion may be noted in nature elsewhere, e.g., rounded stones left in fields after glacial action thousands of years earlier, or simply some beach/shore areas where pebbles and stones have been rolled or pushed around in areas of wave-energy dissipation. In the case of the pebbles and rocks, all sides generally get abraded whereas, in the case of ice, the abrasion is primarily limited to the edges of the floating piece of ice. Lake and river pancake ice may exist as discrete rounded pieces or become frozen into an extended ice sheet. Cake ice Cakes may be variable-sized fragments of ice that are roughly 5–10 m across (Figure 1(e)). Their presence suggests the partial break-up of larger subdivisions of ice such as ice floes (10–2000 m) or ice fields (2–20 km). Pressure ridge This type of ice formation is a generic term for ice that has been squeezed or forced together (Figure 1(f), 1(g), and 1(h)). It is a feature reflective of energy release such that fractured pieces of ice plates may be forced into, upward, or beneath each other in a jumbled mass of ice. Geophysicists can note a surprising similarity of the action of pressure ridges on frozen lakes to the mountain-forming and subduction seen in the plate tectonics on Earth. Being able to break through the thickness of the ice (from one to possibly many meters) at pressure ridges is one of the reasons ice-breaking ships are needed in some water bodies. Foot-ice or ice-foot This term refers to the narrow strip of relatively thick ice observed where the shore meets the lake during freezing conditions (Figure 1(i)). ‘Foot-ice’ is a type of ice seen most commonly along the shores of the larger lakes of the world in the North Temperate Zones, but is generally of little to no consequence in very small lakes. When Arctic or subfreezing air moves across large lakes, which are still ice-free or only partially covered with ice, breaking waves on shore may toss spray into the air. The below freezing air temperatures may cause the spray to freeze in the
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air and/or upon contact with the cold shore. A narrow ‘ice foot’ gradually builds there and thickens with time and freezing temperatures. As the ice builds up, the base of the ice may extend to below the water surface and the heavy strip of foot ice may gradually move lake-ward. The ‘foot ice’ may provide a temporary advantage in that it becomes a seasonal barrier against some types of coastal erosion. A disadvantage may accrue in spring if strong on-shore winds drive heavy pieces of ice into shore and accelerate erosion. Ice volcanoes Ice volcanoes are structures that are sometimes seen to develop at the edge of the foot-ice during freezing conditions (Figure 1(j)). Some sections of a shoreline may have many ice volcanoes side-by-side and other sections may have few to none. Ice volcanoes are an interesting and, depending on the winter and winds, fairly striking feature along the downwind shores of some North American Great Lakes. The eastern end of Lake Erie has stretches of shore line where ice volcanoes are fairly common in average to harsh winters. In some of the areas along the lake-side edge of foot-ice, arriving waves (of a certain energy or wavelength?) are sprayed up into the air through a developing tube-like structure in the foot-ice. The tube may be partially open, on the side facing the open water, or mostly closed except for an opening at the bottom. Some of the wave energy is funneled into the open side of the tube, or into an opening at the bottom of the developing vertical cone. Water and slush may be propelled up through the tube, sprayed out the top, and, when falling back in freezing air or on ice, form a conical top. More spray continues this process and the growing conical ice structure (some many meters high) resemble a small volcano made of ice . . . thus the name of ‘ice volcano.’ Rectilinear thrusts This is a striking feature where lateral stresses on ice may cause portions of a cracked ice sheet to override or slide under one another in a fractured rectangular manner (Figure 1(k)). These beautiful rectangular fractures (not normally seen in small lakes, and observed infrequently in the developing ice of some Laurentian Great Lakes) are probably a reflection of the crystalline structure, strength, and limited thickness of the ice. Barchans Barchans are crescent-shaped structures composed of snow (Figure 1(l)). They lie on top of the ice, where winds can easily transport particles of snow, but they are not necessarily a type of ice by themselves. The crescents are oriented with their widest central portion toward the direction from which the wind is coming, and their two narrower tails are pointed downwind or toward the direction the wind is heading
668 Light and Heat in Aquatic Ecosystems _ Ice
River Ice
It is easy to forget but the departure of river ice, during the spring runoff and increased stream levels, indirectly supported thousands of people as North Temperate Zones were being settled. In areas of original forest in northerly states of the United States and provinces of Canada, huge numbers of people were employed by various lumber industries to harvest the trees. Many people were employed to chop and saw trees in the winter. Other people helped move the felled trees to a river bank by the aid of horses, sleds, tractors, and skidding while the snow and icy logging roads through forests made the task much easier than in summer. Some people were involved in the dangerous job of ‘riding the logs’ down the river, or clearing log jams. Still others were involved in more sustained work at the lumber mills. The rising stream waters of spring not only cleared the ice from the streams, but also served as a vital conveyer belt for transporting cut timber to the lumber mills. The historical and more recent interest in river ice is a reflection of societal concerns about ice jams and flooding as well as interest in navigation and hydroelectric power. Ice jams tend to recur in the same sections of a stream because, in combination with variable flow, some stream and bank morphologies are more favorable for the development of ice jams. Research has shown that ice jams develop most commonly where the slope of the river changes from steep to mild, not necessarily within narrowed reaches of the river. As the flow of the river is lessened floating ice may accumulate and form jams. Ice jams in streams may cause surprisingly rapid ‘over-the-bank’ flooding and subsequent damage to development within flood zones. During the onset of winter, the development of an ice cover on streams may be preceded by water that is supercooled, i.e., cooled a bit below 0 C. If the stream flow is rapid and turbulent, tiny crystals or spicules of ice may form within the vertical flow field and become part of an ice cover, attach to rocks on the bottom, or both. If the stream water moves slowly, or hardly at all, the supercooling at the surface film may cause a thin film of ice to form there and thicken if subfreezing air temperatures persist. Supercooled conditions and variable flows tend to generate a chaotic mix of small crystals and variable-sized chunks of ice such that a jumbled mix at an ice jam may be called ice rubble.
Just as is the case at some lakes, some communities or businesses on the shores of a few rivers hold contests to see who can predict the time when the ice will be gone. The definitions of when the river ice is gone varies among stake holders but, after establishing some local definitions, prediction of the ‘break-up’ date is a social event enjoyed by the happy winner(s) as well as the many losers. Nature retains surprises! Ice as a Climate Indicator
Ice is a powerful indicator of climate change and variability. The ice on lakes and streams and in glaciers, permafrost, and the winter snow cover serve as indicators of climate. On the water and land surfaces, the change in state from water to ice and rain to snow, and visa versa, is obvious. This visible threshold between the hydrosphere and the cryosphere enhances the usefulness because a human observer, whether directly or from photographs or satellites, can personally perceive the transition dates and the geographic extent of frozen water. Ice is perhaps an even more powerful indicator in a less observable way because when glaciers form and thicken over the centuries and millennia, a paleorecord can be obtained from ice cores, e.g., from the Greenland and Antarctic glaciers, that reveal long-term changes in air temperature and atmospheric chemistry for hundreds of thousands of years into the past. Ice and snow cover is very sensitive to warming at the temperatures typical at the Earth’s surface. The shorter snow and ice cover seasons and the melting of glaciers and loss of Arctic sea ice have become graphic images of the current warming and provide the public with strong evidence that global warming is occurring. In the State of Alaska, the end of the snow melt season at Point Barrow has advanced from about
Ice duration on lake mendota 6
19 days less per 100 years
5 4 Months
Barchans are also seen in some of the world’s deserts where blowing sand is transported by wind and may create some crescent-shaped sand dunes.
3 2 1
10 longest ice covers 10 shortest ice covers
0 1850
1900
1950
2000
Figure 2 A long-term record showing the decreasing duration of winter ice cover, with time, for Lake Mendota at Madison, Wisconsin. More evidence of global warming.
Light and Heat in Aquatic Ecosystems _ Ice
16 June to 7 June over the last 40 years and some of the Alaskan glaciers have receded by 30% of their length between 1959 and 1990. The extent of Arctic sea ice has decreased by 15–20% over the last 30 years and, in the Antarctic, sea ice is being rapidly lost around the Palmer Peninsula. Since the 1960s and 1970s, the thickness of the Arctic sea ice cover has thinned from an average of 3.1–1.6 m, or by about 45% across six areas of the floating ice. For the same time period, the thickness of ice at the North Pole has decreased from 3.8 to 2.8 m, or by 25%.
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In Southern Wisconsin, the duration of ice cover on Lake Mendota has declined by an overall average of 19 days per 100 years (see Figure 2), but by about 9 days per decade just since the mid-1970s. The recent general warming since the mid-1800s resulted in later freeze-up and earlier break-up dates on many lakes and rivers. Around the Northern Hemisphere over the last 150 years, the rates of change for freeze-up have been 5.7 days later per 100 years and the break-up 6.3 days earlier per 100 years. These rates of loss of ice cover have been even more rapid since the
Table 1 Evidence of change over time in freeze-up dates, break-up dates, and ice thickness of lakes and rivers at many locations around the world Water body
Measure
Change per C warmer air temperature
Location
Dates
Lake
Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Spring break-up date Fall freeze over date Fall freeze over date Fall freeze over date Fall freeze over date Fall freeze over date Fall freeze over date Fall freeze over date Maximum ice thickness Spring break-up date Spring break-up date Ice thickness
3.8–5.0 days earlier
Finland, n ¼ 3
April air temp. 1836–2002
4.4 days earlier 5 days earlier
N. America, n ¼ 2076 data points from 143 lakes N. Sweden, n ¼ 14
Feb–June air temp. Various and not specified 1961–2002
15 days earlier
S. Sweden n ¼ 14
1961–2002
2 days earlier
Minnesota, n ¼ 6
March air temp. 1960–2004
3 days earlier
Laurentian Great Lake region
Fall to spring 1975–2004
6.4 days earlier
Lake Mendota Wisconsin
Fall and early winter 1855–1990
2.8 days earlier
Grand Traverse Bay, Michigan
8.3 days earlier
Wisconsin, n ¼ 45
Jan–March 1990–1991
5.5 days earlier
N. Hemisphere 24 time series
1846–1995
5.6 days earlier
Lake Mendota Wisconsin, n ¼ 65
2.6–5.1 days later
Finland, n ¼ 3
Model sensitivity to Temp., 1961–1990 Nov. air temp 1836–2002
3.3 days later 3.9 days later
N. America, n ¼ 733 data pts from 143 lakes Lake Mendota Wisconsin
4.9 earlier
Lake Mendota Wisconsin
2.0 days later
Grand Traverse Bay, Michigan
4.8 days later
N. Hemisphere 15 time series
1846–1995
4.7 days later
Fall to spring 1975–2004
3.6 days earlier
Laurentian Great Lake region, n ¼ 65 N. America, n ¼ 831 data points from 143 lakes Finland, n ¼ 1
April air temp. 1802–2002
4.4 days earlier
Maine n ¼ 1
Dec–Feb air temp. 1931–2002
16 cm thinner on 28 Feb
Maine, n ¼ 1
Dec–Feb air temp. 1900–1999
Lake Lake Lake Lake Lake Lake Lake Lake Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river Lake and river River River River
Data compiled from many sources.
6.8 cm thinner
Sept–Dec air Temp. Various and not specified Model sensitivity to Temp., 1961–1990 Fall and early winter
Sept–June Mean temp.
670 Light and Heat in Aquatic Ecosystems _ Ice
mid-1970s. For example, across the Laurentian Great Lakes region of North America, freeze-up and breakup dates for lakes and rivers have been 3.3 days later and 2.1 days earlier, respectively, per 100 years, or about 3–6 times more rapid than for the longer 150 year records. Some specific examples of climateinduced change are given in Table 1. Some of these cryosphere records have been related to temperature and other factors to form a quantitative indicator of temperature change and variability (note changes indicated in Table 1). For lakes and rivers, the midwinter thickness of ice decreases 7–16 cm per 1 C increase in temperature. Dates of freeze-up and break-up are observed and recorded more commonly than is ice thickness. These dates have been changing around the Northern Hemisphere at rates near 4–6 days per 1 C change in temperature, but a greater range from 2 to 15 days per 1 C is observed for some locations. Ice dates are more sensitive to temperature increases in warmer (more southerly) areas of the regions where inland waters become ice covered than in cooler (more northerly) regions. Ice cover on rivers appears to be more sensitive to increases in temperature than is ice cover on lakes. Other explanatory variables such as altitude, latitude, and snow cover relate to ice dates on lakes and rivers as well. Some inland waters that typically have been ice covered are experiencing occasional winters that are warm enough to prevent complete ice formation. When this becomes common, ice cover changes from a continuous variable (day or thickness) to a categorical variable (present or not present). When this happens, ice cover becomes a less precise indicator of further climate change except on some large lakes like the Laurentian Great Lakes where the extent of ice cover can be measured from satellites. The recession of valley glaciers and loss of glacial mass is a visible indication of recent climatic warming. Examples of decreases in the mass of ice characterize the Arctic glaciers for the last 40 years with a loss in the Northern Hemisphere of about 90 km3 year1. The east lobe of Twin Glacier, near the Alexandria Fiord in Greenland, receded about 4 m year1 from 1960 to 1985 and about 8 m year1 from 1985 to 1990. There are many such cases. A few glaciers are growing because glacial growth and loss are complicated by warmth that melts them and increased snow deposition, associated with some of
the warmth that thickens them. Greenland is rapidly losing ice around the edges and thickening centrally for this reason. So glaciers can be a complex indicator of warming with snowfall as a part of the equation. Differences in ice cover from year to year are strong indicators of climate variability as well as of climatic change. For example, lake ice cover dates are related to large-scale climate drivers such as the El Nino Southern Oscillation (ENSO) index, the North Atlantic Oscillation (NAO), and North Pacific Index (NPI). The interactions between ice dates and these drivers vary across the continents with strong signals in some areas and weak signals in others. Their influence also differs with the years being considered, e.g., the influence of El Nino is apparent on break-up dates after 1960 but not before 1950. See also: Physical Properties of Water.
Further Reading ACIA (2004) Impacts of a Warming Arctic; Arctic Climate Assessment. Cambridge University Press. http://www.acia.uaf.edu. Ashton G (1979) River ice. American Scientist 67: 38–45. Huntington TG, Hodgkins GA, and Dudley RW (2003) Historical trend in river ice thickness and coherence in hydroclimatological trends in Maine. Climatic Change 61: 217–236. Korhonen J (2006) Long-term changes in lake ice cover in Finland. Nordic Hydrology 37: 347–363. Magnuson JJ, Robinson DM, Benson BJ, et al. (2000) Historical trends in lake and river ice cover in the northern hemisphere. Science 289: 1743–1746. Erratra 1291–1254. Marshall EW (1966) Air photo interpretation of Great Lakes ice features. Special Report 25, Great Lakes Res. Div., University of Michigan, Ann Arbor, MI. 92 pp. Rondy DR (1969) Great lakes ice atlas. U.S. Lake Survey Research Report 5–6. Dept. of the Army, Corps of Engineers Detroit, MI. (8 pages, but 6 Figures and 31 Plates appended). Shen HT (2003) Research on river ice processes: Progress and missing links. Journal of Cold Regions Engineering 37: 135–142. Stewart KM and Haugen RK (1990) Influence of lake morphometry on ice dates. Verhandlungan International Verein Limnology 24: 122–127. U.S. Department of Commerce (1971) Ice Glossary (No. 75–602). National Oceanographic and Atmospheric Administration. National Ocean Survey, p. 9, Lake Survey Center, Detroit, MI. Weyhenmeyer GA, Meili M, and Livingston DM (2004) Nonlinear temperature response of lake ice breakup. Geophysical. Research Letters 31: L07203, 4. Williams G, Layman KL, and Stefan HG (2004) Dependence of lake ice covers on climatic, geographical and bathymetric variables. Cold Regions Science and Technology 40: 145–164.