The Changing Cryosphere and the Methane Problem
Geoffrey Peterson
Greenhouse gases play an essential part in regulating the Earth’s climate. Water
vapor, carbon dioxide, methane and nitrous oxide are all important greenhouse gases that
absorb infrared radiation. Without the insulating effect of these gases, the average
temperature on the surface of planet would be about 33°C (60°F) cooler (EPA, 2010).
This phenomenon, known as the greenhouse effect, has been predominately regulated by
natural processes for most of Earth’s history. Volcanic eruptions, photosynthesis and
microbial degradation of organic material contribute thousands of teragrams
(1Tg=1000000 metric tons) of greenhouse gases to the atmosphere every year.
Beginning with the Industrial Revolution in the 18th and 19th centuries and continuing
today, the natural greenhouse effect is being modified by anthropogenic activity.
Presently, carbon dioxide is the major driver of climate change. Approximately
63% (1.7W/m2) of heat energy trapped in the atmosphere is absorbed by CO2 (NOAA,
2011). Over the past 50 years, atmosphere CO2 concentrations have risen over 25%
(Keeling, 2010). Many believe that this increase can be directly linked to anthropogenic
CO2 emissions.
The majority of these emissions come from the combustion of fossil
fuels in power plants and motor vehicles. Although CO2 is the main factor in recent
climate change, other greenhouse gases measurably contribute to global warming.
Methane (CH4) is currently responsible for trapping approximately 0.5W/m2 of
the sun’s energy (NOAA, 2011). Like CO2, CH4 comes from a variety of anthropogenic
and natural sources. Wetlands account for the majority (approximately 210 Tg/year) of
natural CH4 emissions (EPA, 2010). High organic content and anaerobic conditions in
wetlands allow methane-producing microbes to thrive. Similar microbial degradation
processes also occur in oceans, lakes and estuaries. Another significant source of CH4 is
the breakdown of methane hydrates in permafrost and deep ocean continental margin
sediments. These natural sources are responsible for approximately one-third of annual
CH4 emissions.
Annual anthropogenic CH4 emissions have risen over 150% since the preindustrial era (Blasing, 2011). This rate of increase is significantly larger than that of any
other greenhouse gas. The agriculture industry is accountable for the majority
(approximately 50%) of anthropogenic CH4 emissions. Cattle, sheep, and other
ruminants release CH4 as a byproduct of their digestive system. Significant amounts of
CH4 also come from the oil and gas industry (approximately 25% of all anthropogenic
emissions). During the process of refining, producing, storing and transporting crude oil,
significant quantities of CH4 (approximately 330Tg/year in the US alone) are released
into the atmosphere. Together, the agriculture, oil and gas industries produce nearly 80%
of all the CH4 that comes from anthropogenic sources.
Increasing CH4 levels can potentially have a much greater impact on climate
change than CO2. Once released in the atmosphere, CH4 is 20 times more effective at
trapping heat energy than CO2. The same measures that have been taken to regulate and
monitor CO2 must be taken with CH4, especially with the threat that climate change poses
to natural methane deposits.
Methane hydrate deposits in continental margins and arctic permafrost regions are
the fastest growing natural
sources of methane gas since
the Industrial Revolution. A
molecule of methane hydrate
contains CH4 that is caged by
water/ice molecules. The
CH4 that becomes methane
hydrate comes from various
sources far beneath the
surface. In deep continental
Figure 1: Methane hydrate ice within a layer of
permafrost
margins, bacteria that live
hundreds of meters beneath the sea floor produce methane as they consume organic
matter from buried plankton (Archer, 2005). Similarly, in arctic regions methane is
produced hundreds of meters beneath the surface from the decomposition of buried
organic matter. After the microbes release the CH4 it seeps slowly through the
subsurface until it reaches a gas hydrate stability zone (Figures 2 and 3). Only under
stable temperature and pressure conditions can methane be trapped in underground
deposits as methane hydrate.
In deep ocean continental margins, methane hydrate formation/deformation is
predominately a function of depth. Assuming
normal salinity conditions (approximately 35
parts per thousand), oceanic methane hydrates
require an ocean depth of at least 300m for
stable formation (NETL). At these depths, the
pressure and temperature of the water
molecules restrict the breakdown of methane
hydrate.
Methane hydrate deposits in arctic
permafrost regions can be found as shallow as
1m beneath the surface. Areas with
continuous permafrost are identified as soil,
sediment or rock that is continuously frozen for
at least two consecutive years. These frozen
Figure 2: Methane Hydrate P-T
stability zone in permafrost regions
regions have ideal conditions for methane
hydrate ice formation. However, when the
permafrost thaws, the trapped methane is
released and can then diffuse through the
subsurface and is emitted to the atmosphere.
Methane hydrate deformation rates are
positively correlated with the rate of
permafrost thawing. Permafrost can melt at
its base due to a geothermal heat flux from
deep in the Earth, or more commonly at the
top from changes in surface or atmospheric
conditions. Melting at the top of the
permafrost is responsible for the majority of
Figure 3: Methane Hydrate P-T
stability zone in continental margins
methane emissions. The rate at which upper permafrost thaws is a factor of climatic
change (global warming), surface disturbances that change the energy balance (e.g.,
fires), coastal erosion that exposes permafrost and construction of roads and buildings.
Between 2003-2007, methane emissions from arctic regions increased by 31% (Bloom,
2010). If permafrost continues to thaw at these rates, methane emissions can kick-start a
feedback that will amplify the current global warming rate. Once this tipping point is
reached, the consequences will likely be irreversible.
Figure 4:
Distribution of
recovered and
inferred gas
hydrate
deposits in
deep
continental
margins.
Recent climate change has introduced new threats to the stability of methane
hydrate deposits. The concentration of carbon in the atmosphere continues to rise with
no end in sight. As more carbon traps the
suns heat energy, global temperatures and
sea levels rise. These climatic changes are
happening faster and more intensely in the
arctic regions. Rising temperatures increase
the rate of methane hydrate breakdown in
arctic permafrost, while rising sea levels
expedite the deformation of
Figure 5:
Distribution of
permafrost in
the Northern
Hemisphere
methane hydrates in seafloor
deposits.
Temperatures in
arctic permafrost have risen as
much as 2º C in the past two to three decades (AMAP). Inland areas are experiencing a
decrease in number of days with snow cover, especially during the spring. Loss of ice
and snow enhances the absorption of the sun’s energy on the surface of the Earth.
Without snow cover surface temperature rise is accelerated, creating a positive feedback
between the cryosphere and the climate. Romanovsky and Osterkamp have been
monitoring permafrost temperatures in northern Alaska for the past few decades. Figure
6 shows trends in subsurface temperatures at 20m depths for five different arctic
Figure 6:
Permafrost
temperatures
for five places
in Northern
Alaska
between 19762010.
permafrost regions. In every locality there has been a 0.5ºC-2ºC increase in permafrost
temperature. Romanovsky and Osterkamp also noticed that relatively cold permafrost
tends to have more warming than relatively warm permafrost. Arctic areas with
continuous permafrost are experiencing a growth in the depth and number of taliks (a
layer of year-round unfrozen ground within permafrost), especially in sandy loam
sediments (Romanovsky, 2010). Development of new, closed taliks is responsible for the
northward movement of the boundary between continuous and discontinuous permafrost
(areas of permafrost that remain frozen for less than two years at a time). The southern
boundary of arctic permafrost is projected to move northward several hundred kilometers
over the next century (CICERO, 2006).
Rising temperatures throughout arctic glaciers, ice caps and the Greenland Ice
Sheet are responsible for over 40% (approximately 3mm/year) of the global sea level rise
between 2003 and 2008 (AMAP). At this rate, sea ice in the Arctic Ocean is likely to
disappear within the next thirty to forty years. Without the albedo of sea ice regulating
water temperature, the sea is able to absorb more of the sun’s energy. Ice cover loss is
the greatest during the summer. During the autumn, the extra solar energy gained by the
sea is released into the lower atmosphere. The depth of the atmospheric layer that is
responsible for the warming of near-surface air is much less in the Arctic than in the
Tropics, thus accelerating the warming rate in the Arctic even more.
Some methane hydrate deposits in deep water continental margins will become
increasingly unstable as rising temperatures cause sea levels to rise. Physical conditions
in the arctic (low temperatures and low salinity levels) allow methane hydrate to exist
closer to the seabed compared to warmer areas. Temperatures in some areas of the arctic
sea are cold enough to allow methane hydrates to exist as shallow as 300m beneath sea
level. These hydrate deposits are shallow enough to be affected by increases in the
surface water temperature alone. The effects of rising sea temperatures on shallow
methane hydrate deposits are shown in figure 7.
Rising sea level increases sedimentation rates on the seafloor, which can
destabilize layers of methane hydrate deposits. Since hydrates prevent sediment
Figure 7: Effects
of temperature
change at the
seabed upon gas
hydrate held within
sediments
below the seafloor.
compaction, weak sediment layers form in areas surrounding the deposit. Methane
hydrates bind together particles in weak sediment layers of the seabed. Piles of loose
sediment that build up around methane hydrate deposits can become enormous. If these
piles of sediment become large enough, the downwards driving stress of the sediment can
exceed the resisting stress of the seafloor. This can cause a submarine landslide capable
of mobilizing hundreds of thousands of km3 of sediment and rock. After such a
catastrophic destabilization, sediment that previously prevented the release of methane
slumps further down the seabed, exposing methane hydrate deposits to unstable
temperature and pressure conditions (figure 8). Plumes of methane gas that are released
after a submarine landslide can have devastating effects on sea life above the methane
hydrate deposit. High enough concentrations of rising methane gas will suffocate any
aquatic life. Methane gas that isn’t consumed by aquatic macro organisms is oxidized by
aquatic microorganisms to form CO2 gas. Although CO2 isn’t as potent of a greenhouse
gas as CH4, the amount of CO2 released in the atmosphere due to submarine landslides
can still have significant effects on global warming (Bosch, 2010).
Figure 8:
Submarine
landslide
over a
methane
hydrate
stability
zone.
Higher CO2 levels in the ocean have a negative correlation with dissolved oxygen
(DO) levels. If DO is scare enough, fish and other aquatic animals can die, creating dead
zones in areas of the ocean. Low DO levels also allow for algal communities to flourish.
Algae blooms (rapid increases or accumulation in the population of algae in an aquatic
system) are a source sink for DO and have the potential to expand at an exponential rate
(Zimmer, 2010). Areas subject to methane plumes from submarine landslides have a
higher risk for low DO levels and algae blooms.
In the summer of 2011, arctic sea ice levels reached the second lowest level in
over 50 years (Borenstein, 2011). At the end of the summer, sea ice covered 1.67 million
square miles, 36% lower than the average minimum of 2.59 million square miles
(Borenstein, 2011). Areas of sea ice that didn’t completely disappear were estimated to
be 40 to 50 percent thinner than average minimum. Methane hydrate deposits that exist
underneath sea ice are stable at pressures above 35 bars (Bosch, 2010). When overlying
sea ice rapidly melts, marine pressure levels can temporarily decrease. If the decrease in
water pressure is enough to expose methane hydrate deposits to pressure-temperature
levels outside the methane
hydrate stability field, the
methane hydrate will
dissociate into methane
gas. A growing number of
methane hydrate deposits
with permanent sea ice
cover are at risk of
dissociation as the
southern boundary of sea
ice continues to move
north.
Rising sea levels
can also have a positive
effect on the stability of
Figure 9: A boat skims through rapidly disappearing
ice off the western coast of Greenland.
deposits. Increased sedimentation due to rising sea levels increases the pressure over on
marine methane hydrate
the seafloor. As long as new sediments are not deposited on continental margins that are
prone to submarine landslides (continental margins that have steep or slippery slopes),
they increase the stability of underlying methane hydrate. The volumetric expansion
seawater (due to increased water temperature and higher sea level) further increases
pressure on top of the sea floor. To some extent, these factors can decrease the rate of
oceanic methane hydrate dissociation as global warming continues to affect our planet.
Cryospheric change will have the most immediate influence on human
populations at the local level. Adaptations must be made to the codes of construction and
Figure 10: Buildings damaged by permafrost thawing.
means of transportation as permafrost and sea ice become increasingly scarce. Buildings
and roads that previously occupied regions of permanent permafrost are being destroyed
as their foundation melts. Local governments need to enforce new building codes and
improve preexisting structures, or else these populations will have to relocate to areas
more suitable for construction. Indigenous arctic people simply do not have the means to
adapt quick enough to the changing cryosphere. Many of these communities have
already seen their homes
deformed by melting
permafrost and have had to
migrate north to find
habitable conditions. Local,
industrialized communities
must help indigenous arctic
people to adapt with these
changes.
The current transportation infrastructure in the arctic will not suffice if global
temperatures continue to rise and permanent permafrost continues to disappear. Many
areas are becoming difficult to access as ice roads melt earlier and freeze later. This
predicament is forcing
Figure 11: An
ice road trucker
becomes a victim
of thinning sea
ice.
industries to restrict ice road
transportation to only the
coldest part of the year.
Communities that rely on ice
roads for year-round imports and exports will not be able to maintain economic viability.
At the global level, all governments must acknowledge the changing climate and
cryosphere as a factor of greenhouse gas emissions. Timely, measured and concerted
action is required to address these emissions. In 2005, ministers of the Arctic Council
took a major step towards the issue. They endorsed numerous policy recommendations
for reducing greenhouse gas emissions. Ultimately, the Arctic Council pledged to limit
long-term emission levels consistent with the objective of the United Nations Framework
Convention on Climate Change (UNFCCC). Other nations outside the Arctic need to
also comply with similar greenhouse gas regulations. Emissions from these nations are
primarily to blame for the rapidly changing cryosphere.
It is difficult to make future projections about the fate of the Arctic. Disappearing
snow/ice cover, thawing permafrost, increased greenhouse gas emissions and rising sea
levels create a feedback between cryosphere and the atmosphere. The effect of the
interaction between these components is difficult to predict in such a new, rapidly
changing, complex system. To reduce the uncertainty of predictions about the Arctic,
more robust observational methods are needed. Satellites provide accurate data about
sea-ice extent and snow cover, but are obsolete for subsurface observations. Sea ice
thickness, snow depth and permafrost thickness require surface-based observations for
proper monitoring. The desolation and obscurity of most of the Arctic makes thorough
monitoring of the subsurface difficult for scientists. Current observational methods need
to expand and improve so humans can adapt accordingly to the transformation of the
cryosphere.
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