Posts
Triggers
to Release the Methane Monster: Sea Ice Retreat, Ocean Warming and
Anoxia, Fires, Sea Level Rise and The Fresh Water Wedge
16
August, 2013
Perhaps the most hotly debated topic among climate scientists, when they are not facing off with the ignorance of underhanded climate change deniers, is the potential rate of Earth Systems response to human caused climate change. In general, the low hanging fruit of climate research is a more easy to puzzle out pace of likely warming due to the direct forcing of human greenhouse gas and CO2 emissions and the more rapid climate feedback coming from increasing water vapor due to increased evaporation. But higher up the tree hang the critical fruits of pace of albedo change and pace of carbon response as the Earth System warms. Understanding these two will provide a much greater clarity to the question of a long term rate of warming given a doubling of atmospheric CO2.
Paleoclimate,
Paleoclimate, and Paleoclimate
Perhaps
the best way to test the accuracy of our long-term Earth Systems
global warming and climate models is to use temperature proxy data
from past ages in Earth’s history. And, based on these proxy
measures, we find that the long term warming from each doubling of
CO2 is at least 6 degrees Celsius. Though the proxies are not
perfect, they are in general agreement on a range of potentials
averaging near this figure. And these measurements can provide some
confidence that the total long-term warming from a doubling of CO2 is
at least twice that caused by a CO2 increase and the related water
vapor rise alone.
More
accurate measures closer to the current day are even less reassuring.
Looking at the ice-age and interglacial transitions over the last
500,000 years, we find that a very small forcing provided by orbital
changes, resulting in a global increase in solar insolation of about
.5 Watts per meter squared combined with changes in the angle at
which sunlight hits the Earth (Milankovitch
Cycles),
is enough to, over the long term, increase CO2 levels by 100 ppm
(from 180 to about 280), increase methane levels by about 300 parts
per billion (ppb) and (here’s the stunning kicker) raise world
temperatures by a whopping 5 degrees Celsius globally and 13 degrees
Celsius at the poles.
A Human Forcing Six Times Greater Than That Which Ended the Last Ice Age
It
should be a serious concern to climate scientists that the initial
forcing of just .5 Watts per meter squared resulted in a relatively
moderate 100 ppm CO2 and 300 ppb methane response which then combined
to force temperatures radically higher. By comparison, the current
human emission of 120 ppm CO2 and 1100 ppb CH4 (methane) and rising,
combine with other human greenhouse gasses such as Nitrous Oxide,
Tropospheric Ozone (human emission), Clorofluorocarbons and Halons to
provide an initial forcing of fully 3 Watts per meter squared or
about 6 times the total forcing that resulted in the last ice age’s
end and ultimately set in place feedbacks that pushed global
temperatures 5 degrees hotter (Data source: Recent
Greenhouse Gas Concentrations).
Earth’s
Own Carbon Stocks are Vast
So
why is was so small an initial solar forcing enough to end an ice age
and, ultimately warm the poles by 13 degrees (C) and the globe by 5
degrees C and what does this mean when the human forcing is now at
least six times greater?
In
short, the Earth holds vast stores of carbon in the form of CO2 in
its oceans, organic carbon in its tundra and frozen beneath land ice,
and in very large stores of methane hydrates on the sea bed. Any
forcing that is large or occurs over a very long period of time will
act continuously on these sources, pushing more and more of the
carbon out until all of the stores newly exposed to that forcing are
emitted, the feedback warming kicks in, and Earth gradually reaches a
new energy equilibrium state.
In
the current day, melting tundra (both land and ocean) in the Northern
Hemisphere holds about 1,500 gigatons of carbon (NSIDC),
the oceans contain between 2,000 and 14,000 gigatons of methane
hydrate (USGS),
and these same oceans hold about 1,000 gigatons of carbon (CO2) in
solution near the surface and 38,000 gigatons of carbon near the sea
floor (University
of New Hampshire: Global Carbon Pools/Fluxes).
USGS
Methane Hydrate
Melting
tundra releases its carbon stores as CO2 in an aerobic/oxygen
environment and as methane in an anaerobic and anoxic environment.
Thawing methane hydrates release methane into the oceans of which
some enters the atmosphere. And warming oceans eventually are unable
to uptake a rising level of atmospheric CO2 and, in extreme cases,
begin emitting CO2 back into the atmosphere.
When
compared to the gentle, though long term, nudge to the Earth’s
carbon stocks generated by orbital changes and a slight increase in
solar insolation that ended the last ice age, the human forcing
equates to a very great and rude shove. And if that much more gentle
nudge was enough to liberate 100 ppm and 300 ppb of methane from the
Earth system into the Earth’s atmosphere, then how much will the
now much faster and harsher human forcing put at risk of liberation?
Methane
Release Sources in the Arctic
That
human greenhouse gas emissions are rapidly warming the Earth at a
rate of about .2 degrees Celsius per decade and that carbon emissions
from the Earth environment are likely to increasingly result from
this rapid and rising rate of warming is a given. At issue is how
fast and powerful an Earth systems response will be. And one critical
issue in understanding the speed of this potential response is rate
of methane release.
Methane
is a very powerful greenhouse gas. Over one year’s time, it
produces about 105 times the forcing of a similar volume of CO2. So
large pulses of this gas could result in a doubling or more of the
total greenhouse gas forcing already acting on the Earth system. Such
catastrophic releases are hypothesized to have acted during other
periods of rapid warming such as during the PETM and Permian
hyperthermals.
The
above, admittedly lengthy preamble, is needed to give context to this
specific issue: potentially large methane releases as a result of
Arctic warming and a number of related release mechanisms that may
increasingly come into play. However, before we drill down to
mechanisms, let’s look at the disposition of potential Arctic
methane sources to give us a basis for our degree of concern.
Thawing
Arctic Permafrost, as
mentioned above, provides a source of 1,500 gigatons of carbon, some
of which will be released as methane as it melts to liberate its
carbon stores to surface, subterranean, and subsea environments. Some
of this permafrost is land-based, some of it is submerged, as on the
East Siberian Arctic shelf. As the permafrost thaws, decay and
release of this carbon into the atmosphere is likely to gradually
build, providing a growing pool of both methane and carbon emissions.
That said, a number of environmental mechanisms created by climate
change are likely to result in greater and greater volumes of this
store being released over time. In addition, a number of
environmental changes are in the pipe which could result in a higher
percentage of this vast store being emitted as methane.
Stable
Sea Bed Clathrates represent
an unknown portion that is likely a majority of the estimated
500-2,000 gigatons of methane hydrates in the Arctic environment.
These clathrates compose methane locked in ice lattice structures
that occur around 200 meters below the sea bed. Release of these
clathrates requires a heat forcing to not only penetrate into the
ocean waters, but for it to also reach the clathrates below hundreds
of feet of rock and mud. Once the clathrates are disassociated, they
must travel through cracks in the rocks and mud, and then through the
water column to reach the ocean surface and the atmosphere. On the
way, some of the liberated methane dissolves in sea water and another
portion is taken in by methane eating organisms. If the pulse is
strong enough, the ocean water saturated enough, and the methane
eating organisms sparse enough, a greater portion of this released
methane will reach the surface.
Ice
Age Relics are
clathrates that have formed as shallow as 20 meters beneath the sea
floor. They are thought to have formed under the glacial cold that
encased the Arctic over the last 2 million years and that occurred
with particular intensity over the last 800,000 years. These ice age
depositions are particularly vulnerable to more rapid release and
their expansion during the last glacial period results in a set of
carbon stocks that are very vulnerable to rapid emission. In this
case, we
find yet one more reason why a rapid rise out of a period of
glaciation is a rather dangerous climate circumstance.
The deposition of carbon stores are placed in regions more vulnerable
to thaw and release once warming is underway.
In
sum, these three represent a majority of potential methane release
sources.
Rumors
of Fire: The East Siberian Arctic Shelf Emission
http://www.youtube.com/watch?v=kx1Jxk6kjbQ#action=share
East
Siberian Sea
Projected
temperature and precipitation change above the Arctic Circle.
During
the 1990s, researchers noticed a methane overburden in atmospheric
regions around the Arctic Circle. This overburden was seen as an
indication that large local methane emissions were occurring in the
Arctic. Subsequent research found methane emissions from thawing
Arctic tundra, from melt lakes and from peat bogs. In addition a
large emission source was identified in the Arctic Ocean.
As
of 2010, reports were coming in from the Arctic that the East
Siberian Arctic Shelf was emitting more methane than the entire Earth
ocean system combined.
By 2011, an expedition to the Arctic found methane emission sources
more than 1 kilometer across over the same region of submerged
permafrost. By 2012, expeditions could no longer be conducted on the
ice surface in the region of the East Siberian Arctic Shelf due to
the fact that the sea ice there had become too thin and unstable to
support research equipment.
Dr.
Natalia Shakhova and Dr. Igor Similetov found that the permafrost cap
over the shallow East Siberian Arctic Shelf seabed had become
perforated. The cap locks a very large volume of methane, estimated
to be about 500 gigatons, under constant cold and pressure. As the
cap perforates, the cold and pressure release and increasing volumes
of methane shoot up from the sea bed saturating the water with
methane with some of the methane releasing to the surface.
Shakhova
and Similetov warn that 1 percent or more of this methane could
release over the course of decades as the sea ice continues to erode
in the region of the East Siberian Arctic Shelf and the undersea
permafrost continues to perforate. Just a 1 percent release would be
enough to double the amount of methane in the Earth’s atmosphere,
resulting in a .5 watt per meter squared forcing from an ESAS release
alone. The researchers also identify the potential for a much larger,
50 gigaton release, which would more than double the current human
GHG forcing over the course of just a few decades.
Such
a large potential release was the subject of a much-debated Nature
article by Peter Wadhams (read
more here).
And it was this article that raised the question of potential
mechanisms that could result in such large releases of methane from
the Arctic in the coming years.
The
Arctic Under Heat: Ever More Powerful Mechanisms For Release
In
examining potential release methane release mechanisms we will start
with those currently acting on the East Siberian Arctic Shelf and
work our way outward to the greater Arctic environment. It is worth
noting that a paper by Carolyn Ruppel recently refuted Shakhova and
Similetov’s findings, but that the Ruppel paper did not study the
region of the East Siberian Arctic Shelf in question, only a related
area of the Beaufort Sea which has not been found to currently show
large, powerful, or widespread methane hydrate release.
(Image
source: Commons)
Taking
the Ice Lid off of a Shallow Sea.
In the case of the East Siberian Arctic Shelf, rapidly warming air
and ocean combine with rapidly retreating sea ice to create what
seems to be a powerful and concerning release mechanism. The East
Siberian Arctic Shelf is a 2 million square kilometer region that
composes some of the Arctic’s densest carbon stores. It represents
about 1/5 the Arctic Ocean area and is thought to contain about 500
gigatons of shallow sea bed methane hydrates. Over the past few
decades, this region has warmed very rapidly, at the rate of about .5
degrees Celsius every ten years. This warming, at about 2.5 times the
global rate, has resulted in a very rapid weakening and retreat of
sea ice from the surface waters of a shallow sea that is, on average,
about 50 meters deep. In recent years, summer sea ice has almost
completely retreated from the ESAS, leaving a dark ocean surface to
absorb sunlight and to rapidly warm. Measurements from the region
show that water temperatures have increased by as much as 7 degrees
Celsius above average once the sea ice pulls away. With the ice now
gone, surface winds provide great mobility and mixing of the water
column, this results in much of the surface water heating being
transported down to the seabed. It also draws methane rich waters up
from below where they can contact the air and release some of the
water-stored methane.
Shakhova
and Simeletov have observed perforations of the subsea permafrost
releasing large volumes of methane from the East Siberian Arctic
Shelf since 2008 and, as noted above, many of the hydrates stored
beneath this permafrost cap are far shallower than is typical for a
normal ocean seabed due to the fact that they are ice age relics.
This combination of mechanisms provides the greatest current risk for
rapid methane release. However, a number of other mechanisms are
increasingly coming into play that may add to the, already concerning
set of risks for rapid ESAS methane release.
Melting
Tundra, Hot Lakes and Arctic Wildfires. NSIDC
has identified about 1,500 gigatons of organic carbon locked in
tundra systems throughout the Arctic. As the Arctic is forced to
rapidly warm, larger and larger portions of this vast carbon store
begin to thaw. Once the tundra melts, this carbon is subject to
breakdown and action by microbes. This process of decay releases CO2
in dry environments and methane in wet, anoxic environments. Much of
the tundra melt is subterranean. As such, this tundra melt is locked
away in moist pockets that have little access to airflow. These
pockets are at risk of being broken down into methane by anaerobic
microbes. In some sections, tundra collapses and fills with water to
form melt lakes. These lakes contact the anaerobic melt regions and
create their own anaerobic bottom systems for carbon breakdown and
release. Many of these lakes are so hot with methane that they
provide emissions with high enough concentration to burn.
As
the Arctic experiences more and more heatwaves, a far greater expanse
of this extreme northern region is subject to wildfires. These fires
are increasingly found to have burned deep into the soil. Reports
from the Arctic find that fires have incinerated as many as 50% of
the stumps of trees in a wildfire zone and consumed the carbon rich
soil to a layer as deep as 3 feet below the surface. The action of
wildfires further breaks open the soil and tundra cap providing
passages to release any methane stored in anaerobic pockets beneath.
With
these tundra regions composing so large a volume of carbon and with
these areas being subject to increasingly rapid melting and
increasingly energetic wildfires, larger and larger methane releases
are entirely likely.
Ocean
Warming, Anoxia, and the Fresh Water Wedge. As
the years and decades progress and Arctic sea ice becomes more
scarce, there is an increasing risk of large freshwater melt pulses
from Greenland to combine with a warming Arctic Ocean to further
amplify methane release. With the increasing removal of sea ice,
Arctic Ocean temperatures surge, spreading a wider and wider area of
heat forcing deeper and deeper into the water column and, eventually,
into the seabed itself.
Some
of this warming is visible in climate models projecting temperature
and precipitation change throughout the Arctic over coming decades:
A
warmer Arctic Ocean is a less oxygen rich environment. The heat
reduces the oxygen in solution, creating more anaerobic environments
for organic carbon to break down as methane. Warmth also creates a
greater sea-bed forcing for spontaneous and long-term release of
methane hydrates.
As
the seas surrounding Greenland warm and the Greenland environment
takes in more of this latent heat, Greenland melt rates will continue
to increase. The large fresh water pulses from Greenland will push
the Gulf Stream further and further south, reducing the mixing of
seawater in and near the Arctic, further reducing oxygen levels.
These pulses will also act as a wedge, forcing warmer, saltier waters
to dive down toward the ocean bottom as a fresh water cap expands
from the Arctic Ocean southward (see Does
Fresh Water Runoff Change Ocean Circulation to Unlock Deepwater
Hydrates?).
This mechanism will create a cool surface, hot depths ocean
environment for the Arctic Ocean and northern latitude regions
surrounding it. Additional fresh water is likely to come from
the continents as rates of precipitation increase, further adding to
the fresh water cap and the creation of a growing region of
stratified ocean with cooler, fresh water at the surface and a
growing pool of warmer water below.
Unfortunately,
large freshwater additions from melting snowcover and increasingly
severe rainfall events, like
the massive Yakutia floods
have already resulted in changes to Arctic Ocean circulation,
creating a large freshwater cap near the Beaufort and resulting in
the risk of fresh water pulses entering the Pacific Ocean. A NASA
animation shows how these changes are already ongoing:
http://www.youtube.com/watch?v=GZvf1pyerEk#action=share
And
we have also noticed
a great increase in ocean bottom heat content concentrated near the
polar regions.
Thus
we have three factors acting in concert to increase methane release.
First, sea ice retreats to warm the Arctic Ocean. Second, increasing
freshwater inflows divert the warmer waters toward the ocean bottom.
Third, the warmer waters are less oxygen rich, creating more anoxic
environments for anaerobic bacteria to break down organic carbon from
thawing permafrost into methane. These anaerobes will receive plenty
of nutrients from the waters washing off of glaciers and continents
and will likely create great blooms over large areas as seas continue
to warm. These combined forcing mechanisms will likely destabilize
the weakest methane hydrate reserves first even as the anaerobes go
to work on the newly liberated organic carbon.
Sea
Level Rise Floods Large Regions of Tundra. A
final mechanism for methane release is the rise of a less oxygenated
Arctic Ocean to flood large sections of coastal tundra in Siberia,
putting it under water and in an oxygen poor environment in which
anaerobic bacteria can act to convert organic carbon into methane. A
wide swath of coastal Siberia is low lying and, in some cases, is
vulnerable to sea level rise for tens or even hundreds of kilometers
inland. Over the years, larger sections of this region will be
claimed by the sea, adding their carbon stores to an oxygen poor
ocean bottom region.
Together,
a rapidly destabilizing ESAS, a rapidly retreating ice sheet,
increasing Arctic Ocean anoxia, increasing fresh water runoff into
the Arctic Ocean, numerous anoxic environments within tundra thaw
regions, increasingly energetic wildfires, expanding regions of
stratified waters with hot ocean bottoms and cooler ocean surfaces,
and seas rising to flood areas of thawing tundra provide sufficient
and numerous mechanisms to be seriously concerned about Arctic
methane release as an amplifier and potential multiplier to human
caused warming.
Links:
No comments:
Post a Comment
Note: only a member of this blog may post a comment.