Abrupt Impacts of Climate Change: Anticipating Surprises. 2013. sponsored by the National Oceanic Atmospheric Administration, the National Science Foundation, and the U.S. Intelligence community.
This was the best summary of the 223 page National Academy of Sciences publication I could find. The first report on abrupt climate change came out in 2002. It is sobering that one-fifth of all fossil fuels that have ever been burned happened since the 2002 report was released.
On Tuesday, the National Research Council published a brand new report, “Abrupt Impacts of Climate Change: Anticipating Surprises,” that lays out what scientists have learned since 2002 about the possibility of sudden climate shifts. There are still plenty of troubling uncertainties, but researchers have learned a fair bit.
The upshot? Earth is already seeing some abrupt changes, like the fast retreat of summer Arctic sea ice. There’s also a real risk that other rapid and drastic shifts could follow in the coming decades if the Earth keeps warming — including widespread plant and animal extinctions and the creation of large “dead zones” in the ocean.
On the flip side, other drastic changes “are now considered unlikely to occur this century.” That includes shifts in Atlantic ocean circulation patterns that could radically alter Europe’s climate, as hyped in the disaster flick “The Day After Tomorrow.” Also unlikely this century: Collapsing ice sheets in West Antarctica that would push sea levels up very quickly, as well as sudden methane eruptions from the Arctic that could heat the planet drastically. Those doomsday scenarios are left to future generations.
The authors do emphasize, however, that scientists still don’t fully understand all the different ways the Earth’s climate can change in short order. There are lots of unknowns here. “Some surprises in the climate system may be inevitable,” they conclude, “but with improved scientific monitoring and a better understanding of the climate system it could be possible to anticipate abrupt change before it occurs and reduce the potential consequences.”
Here’s a longer rundown of some of the abrupt changes the new National Research Council report explores, as well as how probable they are to occur this century (I’ve ordered them from most likely to least likely):
— Sharp increases in extinction rates. A recent study in Science found that the world is on track to warm much faster than it has in the past 65 million years. That could require some species to shift habitats at an unprecedented rate.
This concept is known as the “velocity of climate change,” and the map on the right shows two different estimates of how quickly species would have to shift in order to maintain the climates of their current habitats (assuming they needed to).
Some species will be able to keep up, others likely won’t: There’s only so far up a mountain that pikas can climb to stay cool, for instance. And coral reefs will have difficulty adapting if the oceans keep warming and become more acidic. Add it up, and it raises the prospect of extinctions for many species.
Likelihood this century: Moderate. When you toss in other pressures that many plant and animal species are facing — deforestation, for instance — the report concludes that a mass extinction event “could conceivably occur before the year 2100,” Coral reefs in particular get singled out here: “some models show a crash of coral reefs from climate change alone as early as 2060 under certain scenarios.”
However, the report adds that scientists still need to develop a better understanding of how many species will react to these shifting climates. “It is an open question whether the climatic tolerances of local populations can evolve fast enough to keep up with rapid climate change.”
The report also explores the possibility of an abrupt “collapse” of the Amazon rain forest due to a combination of climate change and deforestation (say, by creating a self-sustaining cycle of fires and dryness). The report concludes that some of these scenarios are “plausible,” but they’re still subject to much intense debate and are very difficult to model the likelihood.
— An abrupt decrease in ocean oxygen. Scientists expect the oxygen content of the ocean to decline as the world warms, due to various chemical and biological changes. And that raises a concern: In some parts of the ocean, it’s possible that this process could accelerate abruptly, creating large “oxygen minimum zones” that are virtually uninhabitable for fish and other organisms.
Likelihood this century: Moderate. Similar “dead zones” are already popping up in many coastal areas around the world, mainly caused by fertilizer run-off and improperly treated wastewater. When combined with other changes in the warming ocean, “the decrease in oxygen availability might become non-linear.”
— Destabilization of the West Antarctic ice sheet. The current scientific consensus is that the world will likely see between 0.4 and 1.2 meters of sea-level rise (1 to 4 feet) by century’s end, depending on how fast emissions rise. This assumes the oceans will expand as they warm and ice caps and glaciers melt at a predictable pace.
But what about surprises? The report notes that the West Antarctic Ice Sheet carries enough ice to raise sea levels by 3 to 4 meters (10 to 13 feet). Right now, that massive ice sheet looks stable. But the geological record that these sheets are capable of shifting very quickly, particularly at the boundary between sea ice and land ice.
“Locations where meltwater forms on the ice shelf surface can wedge open crevasses and cause ice-shelf disintegration—in some cases, very rapidly.”
— Carbon or methane “bombs” released from the Arctic. There’s a lot of carbon that’s locked in frozen permafrost at high latitudes. There’s also a lot of methane stored in the northern oceans, trapped in lattice-like structures known as clathrates. All told, there may be more carbon stored in permafrost and ocean hydrates than their are in known fossil-fuel reserves (Allen et al 2009, IPCC 2007)
So what if the Earth heated up enough that the permafrost melted, the oceans warmed, and these greenhouse gases suddenly got released into the atmosphere? That could, in theory, trigger an extremely large climate shift.
Likelihood this century: Low. A sudden massive release looks unlikely this century. The report concludes that as the Arctic warms, it will gradually release more carbon and methane into the atmosphere, which will “amplify” existing warming. But a very large release is unlikely to happen a short span, say, just one or two decades.
The report cautions, however, that “this conclusion is based on immature science and sparse monitoring capabilities.” Scientists still need better assessments of the long-term stability of those carbon stores. Not out of the clear yet. And the odds here also keep going up if the planet keeps warming after 2100.
— A chaotic disruption of Atlantic ocean circulation patterns. Ever wonder how Western Europe manages to stay relatively warm despite being so far north? Some scientists give partial credit to the Atlantic Meridional Overturning Circulation (AMOC), an ocean pattern that transports warm water into the North Atlantic and Nordic seas. The pattern also plays many other vital roles, like maintaining the ocean’s ability to absorb carbon from the atmosphere.
Back in the early 2000s, scientists raised the prospect of a nightmare climate scenario here. Paleoclimate evidence suggests that the AMOC has changed abruptly in the past due to an influx of cool melting freshwater.
So what if, say, Greenland’s ice sheets melted quickly enough to disrupt this circulation? Would we get a “The Day After Tomorrow” style scenario in Europe, where some coastal areas cool down very rapidly? (Some scientists have argued that a disruption in Atlantic ocean heat circulation may have led to such a cold spell roughly 12,900 years ago.)
Likelihood this century: Low. Fortunately, this doomsday scenario now seems unlikely anytime soon. Climate models broadly agree that an abrupt change to the AMOC “will not occur this century.” Greenland would have to melt at a far faster rate than even the worst-case scenarios. The report does suggest, however, that “it is important to keep a close watch on this system,” to understand both the impact of smaller changes and keep an eye on the remote possibility of big, drastic shifts.
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Table S1 from the report (pp 29-32) summarizing the possibilities
The implications for food, disease, infrastructure, national security, and much more are discussed as well. Here are some more pieces I copied from the NAS paper before I gave up — it’s just too long to summarize.
Abrupt changes—which can occur over periods as short as decades, or even years—have been a natural part of the climate system throughout Earth’s history. One such abrupt change was at the end of the Younger Dryas, a period of cold climatic conditions and drought in the north that occurred about 12,000 years ago. Following a millennium-long cold period, the Younger Dryas abruptly terminated in a few decades or less and is associated with the extinction of 72 percent of the large-bodied mammals in North America.
Abrupt Changes Already Underway
Some abrupt climate changes are already underway, including the rapid decline of Arctic sea ice over the past decade due to warmer polar temperatures. Impacts include disruptions in the marine food web, shifts in the habitats of some marine mammals, and erosion of lation systems of the ocean and atmosphere, changes in the extent of sea ice could cause shifts in climate and weather around the northern hemisphere. The Arctic is also a region of increasing economic importance for a diverse range of stakeholders, and reductions in Arctic sea ice will bring new legal and political challenges as navigation routes for commercial shipping open and marine access to the region increases for offshore oil and gas development, tourism, fishing and other activities.
Increases in Extinction Threat for Marine and Terrestrial Species
The rate of climate change now underway is probably as fast as any warming event in the past 65 million years, and it is projected that its pace over the next 30 to 80 years will continue to be faster and more intense. These rapidly changing conditions make survival difficult for many species. Biologically important climatic attributes—such as number of frost-free days, length and timing of growing seasons, and the frequency and intensity of extreme events (such as number of extremely hot days or severe storms)—are changing so rapidly that some species can neither move nor adapt fast enough
The distinct risks of climate change exacerbate other widely recognized and severe extinction pressures, especially habitat destruction, competition from invasive species, and unsustainable exploitation of species for economic gain, which have already elevated extinction rates to many times above background rates. If unchecked, habitat destruction, fragmentation, and over-exploitation, even without climate change, could result in a mass extinction within the next few centuries equivalent in magnitude to the one that wiped out the dinosaurs. With the ongoing pressures of climate change, comparable levels of extinction conceivably could occur before the year 2100; indeed, some models show a crash of coral reefs from climate change alone as early as 2060
Loss of a species is permanent and irreversible, and has both economic impacts and ethical implications. The economic impacts derive from loss of ecosystem services, revenue, and jobs, for example in the fishing, forestry, and ecotourism industries. Ethical implications include the permanent loss of irreplaceable species and ecosystems as the current generation’s legacy to the next generation.
Abrupt Changes of Unknown Probability
Destabilization of the West Antarctic Ice Sheet
Of greatest concern among the stocks of land ice are those glaciers whose bases are well below sea level, which includes most of West Antarctica, as well as smaller parts of East Antarctica and Greenland. These glaciers are sensitive to warming oceans, which help to thermally erode their base, as well as rising sea level, which helps to float the ice, further destabilizing them. Locations where meltwater forms on the ice shelf surface can wedge open crevasses and cause ice-shelf disintegration—in some cases, very rapidly.
the Greenland ice sheet is not expected to destabilize rapidly within this century. However, a large part of the West Antarctic Ice Sheet (WAIS), representing 3–4 m (10 to 13 feet) of potential sea-level rise, is capable of flowing rapidly into deep ocean basins. Because the full suite of physical processes occurring where ice meets ocean is not included in comprehensive ice-sheet models, it remains possible that future rates of sea-level rise from the WAIS are underestimated, perhaps substantially. Because large uncertainties remain, the Committee judges an abrupt change in the WAIS within this century to be plausible, with an unknown although probably low probability.
Abrupt Changes Unlikely to Occur This Century
More recent research findings have shown that they may be less likely to occur within this century than previously considered possible. These include disruption to the Atlantic Meridional Overturning Circulation (AMOC) and potential abrupt changes of high-latitude methane sources (permafrost soil carbon and ocean methane hydrates). Although the Committee judges the likelihood of an abrupt change within this century to be low for these processes, should they occur even next century or beyond, there would likely be severe impacts.
Potential Abrupt Changes due to High-Latitude Methane
Large amounts of carbon are stored at high latitudes in potentially labile reservoirs such as permafrost soils and methane-containing ices called methane hydrate or clathrate, especially offshore in ocean marginal sediments. Owing to their sheer size, these carbon stocks have the potential to massively affect Earth’s climate should they somehow be released to the atmosphere. An abrupt release of methane is particularly worrisome because methane is many times more potent than carbon dioxide as a greenhouse gas over short time scales. Furthermore, methane is oxidized to carbon dioxide in the atmosphere, representing another carbon dioxide pathway from the biosphere to the atmosphere. According to current scientific understanding, Arctic carbon stores are poised to play a significant amplifying role in the century-scale buildup of carbon dioxide and methane in the atmosphere, but are unlikely to do so abruptly, i.e., on a timescale of one or a few decades. Although comforting, this conclusion is based on immature science and sparse monitoring capabilities. Basic research is required to assess the long-term stability of currently frozen Arctic and sub-Arctic soil stocks, and of the possibility of increasing the release of methane gas bubbles from currently frozen marine and terrestrial sediments, as temperatures rise.
Bark Beetle Outbreaks
Bark beetles are a natural part of forested ecosystems, and infestations are a regular force of natural change. In the last two decades, though, the bark beetle infestations that have occurred across large areas of North America have been the largest and most severe in recorded history, killing millions of trees across millions of hectares of forest from Alaska to southern California. Climate change is thought to have played a significant role in these recent outbreaks by maintaining temperatures above a threshold that would normally lead to cold-induced mortality. In general, elevated temperatures in a warmer climate, particularly when there are consecutive warm years, can speed up reproductive cycles and increase the likelihood of outbreaks.
Climate is not the only stressor on the Earth system—other factors, including resource depletion and ever-growing human consumption and population, are exerting enormous pressure on nature’s and society’s resilience to sudden changes
infrastructure is built with certain expectations of useful life expectancy, but even gradual climate changes may trigger abrupt thresholds in their utility, such as rising sea levels surpassing sea walls or thawing permafrost destabilizing pipelines, buildings, and roads.
VULNERABILITY OF U.S. COASTAL INFRASTRUCTURE TO RISING SEAS
• 39% of the population lives in coastal shoreline counties, by 2020 the percent will rise to 47%. Coastal counties contributed almost half of USA GDP in 2011 ($6.6 trillion dollars)
• 51 million: Total number of jobs in the coastal shoreline counties of the US in 2011.
• $2.8 trillion: Wages paid out to employees working at establishments in the coastal shoreline counties in 2011.
• 3: Global GDP rank (behind the United States and China) of the coastal shoreline counties, if considered an individual country.
• 446 persons/mi2: Average population density of the coastal watershed counties (excluding Alaska). Inland density averages 61 persons per square mile.
It’s likely sea level will rise at least by a meter by the end of the century. For low lying metropolitan areas, such as Miami and San Francisco, such a rise could lead to significant flooding. These areas would be difficult to defend by dikes and dams, and such a large sea level rise would require responses ranging from potentially large and expensive engineering projects to partial or complete abandonment of now-valuable areas as critical infrastructure such as sewer systems, gas lines, and roads are disrupted, perhaps crossing tipping points for adaptation. Miami was founded little more than one century ago, and could face the possibility of sea level rise high enough to potentially threaten the city’s critical infrastructure in another century. In terms of modern expectations for the lifetime of a city’s infrastructure, this is abrupt.
If sometime in the coming centuries sea level should rise 20 to 25 m, as suggested for the Pliocene Epoch, 3 to 5 million years ago (see Figure 2.5), when CO2 is estimated to have had levels similar to today of roughly 400 parts per million, most of Delaware, the first State in the Union, would be under water without very large engineering projects (Figure B). In terms of the expected lifetime of a State, this could also qualify as abrupt.
A study of Earth’s climate history suggests the inevitability of “tipping points”— thresholds beyond which major and rapid changes occur when crossed—that lead to abrupt changes in the climate system. The history of climate on the planet—as read in archives such as tree rings, ocean sediments, and ice cores—is punctuated with large changes that occurred rapidly, over the course of decades to as little as a few years.
The current rate of carbon emissions is changing the climate system at an accelerating pace, making the chances of crossing tipping points all the more likely.
Surprises are inevitable. The question is whether surprises can be anticipated and reduced. That issue is addressed in this report.
Scientific research has already helped us reduce this uncertainty in two important cases; potential abrupt changes in ocean deep water formation and the release of carbon from frozen soils and ices in the polar regions were once of serious near-term concern are now understood to be less imminent, although still worrisome as slow changes over longer time horizons.
In contrast, the potential for abrupt changes in ecosystems, weather and climate extremes, and groundwater supplies critical for agriculture now seem more likely, severe, and imminent.
And the recognition that a gradually changing climate can push both natural systems, as well as human systems, across tipping points has grown over the past decade. This report addresses both abrupt climate changes in the physical climate system, and abrupt climate impacts that occur in human and natural systems from a steadily changing climate.
In addition to a changing climate, multiple other stressors are pushing natural and human systems toward their limits, and thus become more sensitive to small perturbations that can trigger large responses. Groundwater aquifers, for example, are being depleted in many parts of the world, including the southeast of the United States. Groundwater is critical for farmers to ride out droughts, and if that safety net reaches an abrupt end, the impact of droughts on the food supply will be even larger.
it is important to carefully catalog the assets at risk—societies cannot protect everything and will need to prioritize, and without an understanding of what could be lost, such as coastal infrastructure to rising seas, for example, intelligent decisions about what to protect first cannot be made.
Can all tipping points be foreseen? Probably not. Some will have no precursors, or may be triggered by naturally occurring variability in the climate system. Some will be difficult to detect, clearly visible only after they have been crossed and an abrupt change becomes inevitable. Imagine an early European explorer in North America, paddling a canoe on the swift river. This river happens to be named Niagara, but the paddler does not know that. As the paddler approaches the Falls, the roar of the water goes from faint to alarming, and the paddler desperately tries to make for shore. But the water is too swift, the tipping point has already been crossed, and the canoe—with the paddler—goes over the Falls.
Likelihood of Abrupt Changes in ocean oxygen levels
Changes in global ocean oxygen concentrations have the potential to be abrupt because of the threshold to anoxic conditions, under which the region becomes uninhabitable for aerobic organisms including fish and benthic organisms. Once this tipping point is reached in an area, anaerobic processes would be expected to dominate resulting in a likely increase in the production of the greenhouse gas N2O.
OMZs have also been intensified in many areas of the world’s coastal oceans by runoff of plant fertilizers from agriculture and incomplete wastewater treatment. These ‘dead zones’ have spread significantly since the middle of the last century and pose a threat to coastal marine ecosystems
Weather and Climate Extremes
Extreme weather and climate events are among the most deadly and costly natural disasters. For example, tropical cyclone Bhola in 1970 caused about 300,000-500,000 deaths in East Pakistan (Bangladesh today) and West Bengal of India.3,4 Hurricane Katrina caused more than 1,800 deaths and $96-$125 billion in damages to the Southeast U.S. in 2005. Worldwide, more than 115 million people are affected and more than 9,000 people are killed annually by floods, most of them in Asia (Figure 2.9 or see, for example, the Emergency Events Database5). Heat waves contributed to more than 70,000 deaths in Europe in 2003 (e.g., Robine et al., 2008) and more than 730 deaths and thousands of hospitalizations in Chicago in 1995 (Chicago Tribune, July 31, 1995; Centers for Disease Control and Prevention, 1995). Heat waves are one of the largest weather-related sources of mortality in the United States annually.6 According to data collected by the National Climate Data Center, there were 134 weather or climate disaster events with losses exceeding $1 billion each in the United States between 1980 and 2011, an average of more than four per year (Table 2.1). Floods, droughts and wildfires—events that appear to be changing in frequency and severity due to climate change—make up about a third of these and slightly more than a third of the dollar damages (adjusted to 2012 dollars). Droughts are particularly costly, comprising about 12 percent of the events by number, but about double that (23.8 percent) by total cost.
Abrupt Changes at High Latitudes
Potential Climate Surprises Due to High-Latitude Methane and Carbon Cycles
Interest in high-latitude methane and carbon cycles is motivated by the existence of very large stores of carbon (C), in potentially labile reservoirs of soil organic carbon in permafrost (frozen) soils and in methane-containing ices called methane hydrate or clathrate, especially offshore in ocean marginal sediments. Owing to their sheer size, these carbon stocks have potential to massively impact the Earth’s climate, should they somehow be released to the atmosphere. An abrupt release of methane (CH4) is particularly worrisome as it is many times more potent as a greenhouse gas than carbon dioxide (CO2) over short time scales. Furthermore, methane is oxidized to CO2 in the atmosphere representing another CO2 pathway from the biosphere to the atmosphere in addition to direct release of CO2 from aerobic decomposition of carbon-rich soils.
Permafrost
Frozen northern soils contain enough carbon to drive a powerful carbon cycle feedback to a warming climate. These stocks across large areas of Siberia comprise mainly an ice-rich, loess-like deposit averaging ~25 m deep, peatlands, and river delta deposits. Estimates of the total soil-carbon stock in permafrost in the Arctic range from 1,700–1,850 Gt C (Gt C = gigatons of carbon).
To put the Arctic soil carbon reservoir into perspective, the carbon it contains exceeds current estimates of the total carbon content of all living vegetation on Earth (approximately 650 Gt C), the atmosphere (730 Gt C, up from ~360 Gt C during the last ice age and 560 Gt C prior to industrialization), proved reserves of recoverable conventional oil and coal (about 145 Gt C and 632 Gt C, respectively), and even approaches geological estimates of all fossil fuels contained within the Earth (~1,500 – 5,000 Gt C). It represents more than two and a half centuries of our current rate of carbon release through fossil fuel burning and the production of cement (nearly 9 Gt C per year).
it is clear that the time scale for deep permafrost thaw is measured in centuries, not years. Furthermore, unlike methane hydrates (see below), the very large stocks of permafrost soil carbon (i.e., the 1,672 Gt C ) must first undergo anaerobic microbial fermentation to produce methane, itself a gradual decomposition process. There are no currently proposed mechanisms that could liberate a climatically significant amount of methane or CO2 from frozen permafrost soils within an abrupt time scale of a few years, and it appears gradual increases in carbon release from warming soils can be at least partially offset, owing to rising vegetation net primary productivity. Over a time scale of decades, however, a possible self-sustaining decomposition of Yedoma could occur before the end of this century (Khvorostyanov et al., 2008a, 2008b, 2008c). A related idea is the possibility of rising soil temperatures triggering a “compost bomb instability” —possibly including combustion—and a prime example of a rate-dependent tipping point. Such possibilities would represent a rapid breakdown of the Arctic’s very large soil carbon stocks and warrant further research. Even absent an abrupt or catastrophic mobilization of CO2 or methane from permafrost carbon stocks, it is important to recognize that Arctic emissions of these critical greenhouse gases are projected to increase gradually for many decades to centuries, thus helping to drive the global climate system more quickly towards other abrupt thresholds examined in this report.
Methane Hydrates in the Ocean
Under conditions of high pressure, high methane concentration, and low temperature, water and methane can combine to form icy solids known as methane hydrates or clathrates in ocean sediments. The methane derives from biological or thermal degradation of organic matter originally deposited on the sea floor. Although the overall rate of methane production in ocean sediments is fairly slow, over millions of years, substantial reservoirs of methane hydrate have accumulated in the world’s ocean margins. Throughout most of the world ocean, a water depth of about 700 m is required for hydrate stability. In the Arctic, due to colder-than-average water temperatures, only about 200 m of water depth is required, which increases the vulnerability of those methane hydrates to a warming Arctic Ocean. The Arctic is also a focus of concern because of the wide expanse of continental shelf (25 percent of the world’s total), much of which is still frozen owing to its exposure to the frigid atmosphere during lowered sea levels of the last glacial maximum.
The inventory of methane in ocean margin sediments is large but not well constrained, with a generally agreed upon range of 1,000-10,000 Gt C (Archer, 2007; Boswell, 2007; Boswell et al., 2012). One inventory places the total Arctic Ocean hydrates at about 1,600 Gt C by extrapolation of an estimate from Shakhova et al. (2010a) to the entire Arctic shelf region (Isaksen et al., 2011) (see Figure 2.12). The geothermal increase in temperature with depth in the sediment column restricts methane hydrate to within a few hundred meters thickness near the upper surface of the sediments (e.g., Davie and Buffett, 2001). Beneath this stability zone, a layer rich in methane bubbles is often seen in seismic reflection data, called a “bottom simulating reflector.” The areal extent of methane-rich sediments is fairly well known from seismic observations of this feature, but uncertainty in the concentration of methane in those sediments is very large, thus resulting in the large uncertainty in the global inventory of ocean-floor methane.
Potential response to a warming climate
Climate change has the potential to impact ocean methane hydrate deposits through changes in ocean water temperature near the sea bed, or variations in pressure associated with changing sea level. Of the two, temperature changes are thought to be most important, both during the last deglaciation and also in the future. Warming bottom waters in deeper parts of the ocean, where surface sediment is much colder than freezing and the hydrate stability zone is relatively thick, would not thaw hydrates near the sediment surface, but downward heat diffusion into the sediment column would thin the stability zone from below, causing basal hydrates to decompose, releasing gaseous methane. The time scale for this mechanism of hydrate thawing is on the order of centuries to millennia, limited by the rate of anthropogenic heat diffusion into the deep ocean and sediment column. Even on the Siberian continental margin, where water temperatures are colder than the global average, and where the sediment column retains the cold imprint from its exposure to the atmosphere during the last glacial time 20,000 years ago, any methane hydrate must be buried under at least 200 m of water or sediment. Bottom waters at depths of 50 or 100 m might warm relatively quickly with a collapse in sea ice cover, but it would take centuries for that heat to diffuse through the 100- 150 m of sediment column to the hydrate stability zone. Thus the release of 50 Gt C from the Siberian continental shelf in 10 years as postulated by Whiteman et al. (2013) is unlikely.
The proportion of this gas production that will reach the atmosphere as CH4 is likely to be small. To reach the atmosphere, the CH4 would have to avoid oxidization within the sediment column (a chemical trap) and re-freezing within the stability zone shallower in the sediment column (a cold trap). However, the hydrate stability zone thickness decreases to zero near the top of its depth range in the ocean, and an increase in water column temperature there could eliminate the stability zone entirely, potentially providing an easier pathway for methane to reach the sea floor. Episodic and explosive escapes of gaseous methane from the sediment column have been documented by kilometer-scale “wipeout zones” in seismic images, and pockmarks on the sea floor, called eruption craters. However, the processes responsible for these observations are too poorly understood to predict what fraction of deeper CH4 might be released through them.
Most of the methane gas that emerges from the sea floor dissolves in the water column and oxidizes to CO2 instead of reaching the atmosphere. Bubble plumes tend to dissolve on a height scale of tens of meters, although larger plumes, consisting of larger bubbles, do rise farther. However, even in the cold Arctic Ocean, methane hydrate is only stable below about 200 m water depth, making for an inefficient pathway to the atmosphere at best.
The highest oceanic methane fluxes to the atmosphere in the Arctic are probably in the coastal zone, associated with erosion of coastal permafrost. In this region and terrestrial lakes the methane flux to the atmosphere is strongly impacted by ice formation on the water surface, providing another mechanism for climate feedback.
A more abrupt way to transfer methane hydrate from the sediment column to the atmosphere is by way of a submarine landslide. Methane hydrate floats in seawater just as water ice floats, and it also has greater potential to reach the atmosphere than methane bubbles. The largest known submarine landslide (called Storegga) occurred ~8000 years ago, as documented in sediment deposits off Norway. The volume of sliding material multiplied by a reasonable hydrate fraction in the pore space yields a possible methane source of about 1 Gt C. The climatic impact of this quantity of methane would be comparable to that of a volcanic eruption (although warming rather than cooling). As such it would have a significant climate impact, but one that is likely to be smaller than that of the anthropogenic CO2 rise.
Over time scales of centuries and millennia, the ocean hydrate pool has the potential to be a significant amplifier of the anthropogenic fossil fuel carbon release. Because the chemistry of the ocean equilibrates with that of the atmosphere (on time scales of decades to centuries), methane oxidized to CO2 in the water column will eventually increase the atmospheric CO2 burden.
As with decomposing permafrost soils, such release of carbon from the ocean hydrate pool would represent a change to the Earth’s climate system that is irreversible over centuries to millennia. Modeling the response of ocean hydrates to climate change is in its infancy. The largest uncertainty is the concentration of methane hydrate, especially in the shallow sediment column near the sediment water interface.
In summary, the ocean methane hydrate pool has strong potential to amplify the human CO2 release from fossil fuel combustion over times scales of decades to centuries. While anthropogenic warming should accelerate the thawing of offshore permafrost via warming of Arctic Ocean shelf waters, this impact should be considered additive to a broader thawing trend that has been underway for thousands of years.
Impacts of Arctic Methane on Global Climate
Although attention is often focused on methane when considering a potential Arctic carbon release, because methane is a short-lived gas in the atmosphere (CH4 oxidizes to CO2 within about a decade), ultimately a methane problem is a CO2 problem. It does matter how rapidly methane is released, and the impacts of a spike versus chronic emissions are discussed in Box 2.4. As methane emissions from permafrost degradation will also be accompanied by larger fluxes of CO2, Arctic carbon stores clearly have the potential to be a significant amplifier to the human release of carbon.
Speculations about potential methane releases in the Arctic have ranged up to about 75 Gt C from the land and 50 Gt C from the ocean. A release of 50 Gt C methane from the Arctic to the atmosphere over 100 years would increase Arctic CH4 emissions by about a factor of 25, and would make the present-day permafrost area about two times more productive of CH4 on average as comes from wetlands today. Postulating such a methane release over a more abrupt 10-year time scale, the emission rates from present-day permafrost would have to exceed that from wetlands by a seemingly implausible factor of 20, supporting a longer century timescale for this process, and making methane emission from polar regions an unlikely candidate for a tipping point in the climate system.
Nonetheless, as can be seen in Box 2.4, releasing 50 Gt C of methane over 100 years would have a significant impact on Earth’s climate. The atmospheric CH4 concentration would roughly quadruple, with a resulting total radiative forcing from CH4 of about 3 Watts/m2. The magnitude of this forcing is comparable to that from doubling the atmospheric CO2 concentration, but the impact of the methane forcing would be strongly attenuated by its short duration. As concluded above, an increase in Arctic CH4 emissions of more than a factor of 10 is required before it would begin to have a significant impact on Earth’s climate in the short term. Such a strong acceleration of methane degassing from the Arctic would result in measurably higher concentrations of methane in the high northern latitudes.
Summary
Arctic carbon stores are poised to play a significant amplifying role in the century time scale buildup of CO2 and methane in the atmosphere, but are unlikely to do so abruptly, on a time scale of one or a few decades. This conclusion is based on immature science, however, and a truly sparse monitoring capability
What is known about the likelihood and timing of abrupt changes in the climate system over decadal timescales?
• large, abrupt changes in ocean circulation and regional climate;
• reduced ice in the Arctic Ocean and permafrost regions;
• large-scale clathrate release;
• changes in ice sheets;
• large, rapid global sea-level rise;
• growing frequency and length of heat waves and droughts;
• effects on biological systems of permafrost/ground thawing (carbon cycle effects);
• phase changes such as cloud formation processes; and
• changes in weather patterns, such as changes in snowpack, increased frequency and magnitude of heavy rainfall events and floods, or changes in monsoon patterns and modes of interannual or decadal variability.
