Download Cryosphere

Justin Faulkner
The Evidence For, and Implications of, Changes in the Cryosphere
and the Relationship with Global Climate Change
This paper is intended to review the actual and potential impacts of global climate change on
snow, ice and frozen ground (the cryosphere). I will examine the basic science of climate change,
observed changes in the cryosphere as well as potential future changes, and how these changes can
affect human beings and the ecosystems in which we live.
Climate is best understood as a system, influenced both by its own internal dynamics as well as
external influences. Radiative forcing is the term climate scientists use to express each individual
factor’s impact on the climate system. Long-life greenhouse gases have a net warming affect on climate,
so we say that they have a positive forcing, because that is the direction an increase in those substances
pushes the global radiative balance (energy coming in vs. energy emitted). A factor that alters the
radiative balance negatively causes greater loss of energy from the Earth to space and thus results in a
net cooling effect. The Intergovernmental Panel on Climate Change, in its Fourth Assessment Report,
expresses climate change as the totality of all internal factors to the climate system in addition to the
net value of all radiative forcings.
Carbon dioxide, methane, nitrous oxide and stratospheric ozone are identified as the most
significant positive forcings affecting the global climate system (IPCC AR4 2.3). We know that the
increase in levels of carbon dioxide are due to human processes first because of the time frame of
increases (as determined by tree ring and ice core data) corresponds to the beginning of the Industrial
Revolution (about 1750), when large-scale fossil fuel burning and forest cutting/burning began. Also,
atmospheric carbon dioxide has a different isotopic composition (more carbon-13 than carbon-12) than
carbon dioxide created through the burning of fossil fuels, which ultimately are derived from ancient
plants.
Other studies also lend support to the idea of radiative forcing and the concept of a planetary
energy imbalance; Hansen et. al. (2005) have calculated that the Earth is now absorbing 0.85±0.15
W/m2 more energy from the Sun than it is emitting into space. This determination was made by
precisely measuring ocean heat content over a period of ten years. Other studies have also confirmed
that the oceans are warming and that partly this is a function of greenhouse gases preventing infrared
radiation from escaping the thin 1mm “skin” that is directly in contact with the atmosphere (Willis et al,
2004; Levitus et al, 2005; Lyman et al, 2006).
It is in the context of a warming planet that I examine the cryosphere, the component of the
hydrological cycle most sensitive to climate change. The components of the cryosphere include river
and lake ice, sea ice, glaciers, ice caps, ice shelves, ice sheets and frozen ground. Approximately ten
percent of the Earth’s surface—and seven percent of the oceans--is presently covered by ice. There is
abundant and significant evidence that the cryosphere is shrinking, partly in response to global warming,
and this is already beginning to impact human beings in serious ways.
There have been observable changes in snow cover on land and ice; decreases in snow cover
have been observed in North America, Asia and Europe in the latter half of the 20th century that are
attributable to warming through regression analyses (AR4 WG2 4.2.2, Groisman et. al. 2004, Stewart et.
al. 2005). It is difficult to accurately measure snow cover in the southern hemisphere because so few
areas outside Antarctica receive any snow cover at all. Long term data on snow cover is limited, making
it difficult to say conclusively that these decreases are the result of an increase in surface temperatures.
However, regardless of the causes, less snow covering the Earth’s surface will translate into less surface
albedo. The albedo effect is the reflection of the sun’s radiative energy away from the surface by snow
or ice. More fresh snow on the ground (not covered In black carbon) reflects more heat, and thus under
conditions of decreased snow cover the ground absorbs more energy and speeds melting (Hanna et al.,
2005, 2006; Box et al., 2006). Paradoxically, Russell (1993) predicts that a greenhouse future will result
in as much as 40% increased snowfall in the Arctic.
River and lake ice have been exhibiting varying patterns which could be associated with
warming, but the data is not as conclusive. Some studies (Magnuson et. al. 2000) have produced data
that suggests trends in northern lakes and rivers toward later freeze-up and earlier breakup in the
spring, however a larger study of Canadian rivers (Zhang et. al. 2001) and one of Russian rivers (Smith,
2000) revealed either mixed trends or trends towards both earlier freeze-up and breakup.
Sea ice, or frozen seawater, is one of the more important indicators of climatic conditions
because the thickness of sea ice is the result of past growth, melt and deformation. Sea ice performs
three very important functions related to the climate system: it contributes to the albedo effect,
modifies exchanges of heat, gases and momentum, and redistributes freshwater, which affects water
buoyancy within the context of oceanic temperature currents. Antarctic sea ice has changed very little
on average, even demonstrating an insignificant positive trend, but for the Arctic multiple studies have
all shown a shrinkage in ice extent by approximately 33,000 km2 every ten years (2.7% shrinkage per
decade) between 1979 and 2005 (Cosimo, 2003). Further, a study of ice that survives the summer—
referred to as the minimum sea ice extent— over that same period demonstrated a decrease of 60,000
km2 (7.4%) per decade since 1979 (Belchansky et. al. 2005). It is also very likely that the average sea ice
thickness decreased by up to one meter in the Arctic region between the late 1980’s and the late 1990’s
(AR4 WG2 4.4.3.7, p.354). Sea ice, unlike glaciers and ice caps, does not contribute to sea level rise but
as mentioned above it does play a role in regulating air and ocean currents. From September 3-9, 2007,
69,000 square miles of sea ice melted—an area the size of Florida—leading climate researchers to
wonder if the Arctic will be sea ice-free much sooner than previously thought (Sandell, 2007).
Decreasing sea ice also translates into less ice albedo effect, causing ocean temperatures to rise more
quickly and ice to melt at a more rapid pace.
One of the most visible aspects of climate change, and one of the most striking in the
cryosphere, is the rapid melting of glaciers and the polar ice caps. Approximately 10% of the Earth’s
surface is covered by glaciers, mostly concentrated in Antarctica and Greenland (USGS, 2005). They are
one of the world’s primary sources of freshwater. Glaciers and ice caps form on a timescale of
centuries, as opposed to the course of one year or less as with snow, river and lake ice, and sea ice. It is
useful to think of glaciers and ice caps in terms of their mass balance, the gain or loss of snow and ice
over a hydrological cycle. Changes in the climate affect when and if accumulation (expansion) and
ablation (shrinkage) occur through variations in temperature, wind, precipitation and seasons. If
temperature increases then a glacier, after a delay, will adjust towards a mass-balance equilibrium of
zero (no addition or loss), though this equilibrium, is actually never fully attainable.
Yamada (1998) and Mool et. al. (2001) have documented the formation of large lakes as glaciers
gradually retreat (melt at the edges) in the Himalayas, and this has also been documented on glaciers in
the Alps and the Andes mountains (AR4 WG1, 1.3.1.1, p.86). Retreat of glaciers in the past 150 years is
unequivocal, but estimates vary. Mass balances in Patagonia, the northwest USA and southwest
Canada, and Alaska have witnessed the strongest negative trends (Dyurgerov and Meier, 2005). The
IPCC concludes, using data from three discrete periods, that glacier wasting is accelerating more rapidly
as time progresses. Glacier and ice cap melt was faster, for example, during the period from 1990-2004
than the period from 1960-1990 (AR4 WG1, p.358). Studies estimate that glacier and ice cap melting
has contributed an average of 0.5 mm per decade to sea level rise between 1961 and 2004 (AR4 WG1,
p.356), and Greene (2005) has concluded with some confidence that the glacier melt of the 1990’s was
in response to a general warming after 1970. In terms of local conditions, glacial melting increases
runoff from mountains, often leading to flooding (and incidentally increased crop yields), however in the
long run, as glaciers melt past a critical point, runoff begins to decrease then fall sharply (Jannson, et. al.,
2003), threatening the fresh water supply of anyone living in the area.
The great ice sheets form from accumulated snow over a timescale even larger than glaciers or
ice caps, that of centuries and millenia. Ice sheets are defined as a mass of glacier ice larger than 50,000
square kilometers (American Geological Institute, 1987). To understand the scale of the water
contained within the world’s ice sheets, it is useful to note that they contain enough water to raise
worldwide sea level, if melted, by approximately 64 meters. Greenland’s ice sheet contains enough
water to raise sea levels by approximately 7m (23 ft). A recent study concluded that by the end of this
century, even under optimistic scenarios, the ice in Greenland could melt completely. The reason for
this Is the delay associated with such large masses of ice moving towards their mass-balance equilibrium
(Handwerk, 2004). Also, large-scale melting of glaciers can trigger a series of climactic changes by
altering the water salinity and thus ocean temperature currents (Lovgren, 2003).
In terms of total surface area, frozen ground comprises the largest part of the cryosphere. Due
to rising temperatures, areas of ground that were permanently frozen are beginning to melt and,
incidentally, to release stored carbon. The Arctic contains more carbon stored in frozen ground than
exists in the atmosphere today, and scientists are warning that its release (especially as methane) will
act as a positive feedback mechanism, accelerate global warming (Zimov et. al., 2006, Walter et. al.,
2006).
Zimov. http://www.sciencemag.org/cgi/content/summary/312/5780/1612
Walter. http://www.nature.com/nature/journal/v443/n7107/abs/nature05040.html
Lovgren. http://news.nationalgeographic.com/news/2003/03/0317_030317_iceshelf.html
Handwerk. http://news.nationalgeographic.com/news/2004/11/1109_041109_polar_ice.html
Don E. Russell, 1993. "Effects of global warming on the biology and management of the Porcupine
caribou herd", in G. Wall (ed.), Impacts of Climate Change on Resource Management in the North, Dept.
of Geography, Occasional Paper No. 16, University of Waterloo, pp. 91-97.
http://news.nationalgeographic.com/news/2007/09/070917-northwest-passage.html
http://www.abcnews.go.com/Technology/GlobalWarming/story?id=3582433&page=1
http://www.realclimate.org
http://www.ipcc.ch/ipccreports/ar4-wg1.htm
http://www.ipcc.ch/ipccreports/ar4-wg2.htm