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.
(Image source: NASA)
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 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, Earth albedo changes as ice sheets respond (also a source of additional heat), 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).
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 (CO2 release is another issue that will be explored in another blog).
Methane is a very powerful greenhouse gas. Over twenty years time, it estimated to produce about 105 times the forcing of a similar volume of CO2 (this value is estimated to be about 25 times a similar volume of CO2 over 100 years time). 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 climate change establishes a number of environmental mechanisms created that are likely to result in greater and greater volumes of this store being released over time. These mechanisms may push methane in a slow and gradual way. But, as we proceed down the dangerous path of rapid human-caused warming, there is increasing danger of large, sudden releases.
In addition, the same expanding set of environmental changes 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
(Please ignore the cheesy intro music and proceed on to the interview)
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:
(Image source: Climate State)
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:
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.