Does Freshwater Runoff in the Arctic change Ocean Circulation to Unlock Methane Hydrate in the Deep Ocean?
Arctic freshwater input into the North Pacific could serve as a catalyst for methane hydrate destabilization, an event suggested as a precursor to the onset of the PETM.
On The Sensitivity Of Ocean Circulation To Arctic Freshwater Pulses During The Paleocene/Eocene Thermal Maximum
Cope, Jesse Tiner (2009)
These results suggest that Arctic freshwater flux into the North Pacific through the Bering Strait may induce circulation patterns similar to those inferred from stable isotope reconstructions during the PETM as well as increase intermediate and deep ocean temperatures and that flow through the Turgay Strait into the North Tethys Ocean would increase surface ocean and atmosphere temperatures. Based upon circulation patterns and temperature increases due to freshwater flux through the Bering Strait, Arctic freshwater input into the North Pacific could serve as a catalyst for methane hydrate destabilization, an event suggested as a precursor to the onset of the PETM.
Past modeling experiments show how alterations to seaway exchanges can have dramatic effects upon sedimentation, global climate, and ocean circulation.Two experiments,one with freshwater exchange between the PETM Arctic and Atlantic Oceans and another between the Arctic and Pacific Oceans, are compared against a reference experiment with exchange between the Arctic and Indian Oceans. [..] Freshwater input into the Pacific Ocean produces the highest temperatures(~12Â°C) in the global ocean in intermediate and deepwaters
Ocean deep-water formation is sensitive to changes in surface buoyancy and momentum fluxes. Changes such as temperature decrease or salinity increase lead to an increase in density relative to underlying waters and causes the water mass to sink (Pond and Pickard, 1983)
Destabilization of methane hydrates, clathrate hydrates, within the oceans depends on temperature and pressure. These hydrates are crystalice structures that contain molecules of CH4 within. As temperature increases or pressure decreases, the ice structures will melt and release the methane, which contains large amounts of carbon. The critical pressure(or depth) of the methane hydrate release depends on the temperature; the higher the temperature the deeper the critical depth of the release (see Dickens et al., 1995 and Figure1 therein). Most hydrates are formed and are stable on the continental margins, specifically the slope and rise that are between 900-2000m (Dickens, 2001). Bice and Marotzke (2002) propose a positive feedback loop responsible for the onset of the PETM due to the release of these hydrates (see Figure9 therein).
They conclude from their ocean model study that an initial increase in CO2 in the atmosphere, caused possibly by volcanic outgassing, would increase the strength of the hydrological cycle. These increases could cause a warming at intermediate depths within the ocean on a regional scale that could induce limited methane hydrate destabilization. They argue that this release of CH4 would then oxidize to CO2 in either the ocean or atmosphere and further exacerbate extremes in the hydrological cycle and eventually switch high southern latitude deep-water formation to high northern latitude deep-water formation. This switch would bring sudden warm water to the ocean bottoms and incite methane release on a global scale. Source
Warming the DeepÂ Ocean
A wedge of cold water at the surface spreading out from the poles would push hotter, saltier water toward the ocean bottom. Fresh water is less dense than salty water, so the fresh water pulses from glaciers and melting ice bergs will act as a wedge, driving the denser, warmer, saltier water toward the bottom The net effect of such changes would be a shallower and weaker ocean circulation system as more warm water is averted toward the ocean bottom near the equator and then spreads northward and as warmer surface waters toward the poles and temperature regions are driven toward the sea-bed.Â Source
The growing bulge of water in the center of the Arctic Circle
The bulge is some 8,000 cubic km in size and has risen by about 15cm since 2002. The team thinks it may be the result of strong winds whipping up a great clockwise current in the northern polar region called the Beaufort Gyre. This would force the water together, raising sea surface height, the group tells the journal Nature Geoscience.
âIn the western Arctic, the Beaufort Gyre is driven by a permanent anti-cyclonic wind circulation. It drives the water, forcing it to pile up in the centre of the gyre, and this domes the sea surface,â explained lead author Dr Katharine Giles from the Centre for Polar Observation and Modelling (CPOM) at University College London. Source
A Looming Climate Shift: Will Ocean Heat Come Back to Haunt us?
âA climate model-based study, Meehl (2011), predicted that this was largely due to anomalous heat removed from the surface ocean and instead transported down into the deep ocean. This anomalous deep ocean warming was later confirmed by observations.
This deep ocean warming in the model occurred during negative phases of the Interdecadal Pacific Oscillation (IPO), an index of the mean state of the north and south Pacific Ocean, and was most likely in response to intensification of the wind-driven ocean circulation.
Meehl (2013) is an update to their previous work, and the authors show that accelerated warming decades are associated with the positive phase of the IPO. This is a result of a weaker wind-driven ocean circulation, when a large decrease in heat transported to the deep ocean allows the surface ocean to warm quickly, and this in turn raises global surface temperatures.
This modelling work, combined with current understanding of the wind-driven ocean circulation, implies that global surface temperatures will rise quickly when the IPO switches from the current negative phase to a positive phase.â Source
Known methane hydrate is located below 270 meters
âThe Arctic is thought to be undergoing some of the most dramatic effects of climate change anywhere in the world. And this particular deposit is just within what scientists call the âmethane hydrate stability zoneâ, the range of pressure and temperature at which gas hydrates are stable. In this region, the stability zone begins at a depth of about 270 m, above which sea temperatures are too warm to ensure the methane remains locked in its water-molecule cageâ Source
Methane Hydrates and Contemporary Climate Change
By:Â Carolyn D. RuppelÂ (U.S. Geological Survey, Woods Hole, MA) 2011Â Nature Education
As the evidence for warming climate became better established in the latter part of the 20th century (IPCC 2001), some scientists raised the alarm that large quantities of methane (CH4) might be liberated by widespread destabilization of climate-sensitive gas hydrate deposits trapped in marine and permafrost-associated sediments (Bohannon 2008, KreyÂ et al. 2009, Mascarelli 2009). Even if only a fraction of the liberated CH4Â were to reach the atmosphere, the potency of CH4Â as a greenhouse gas (GHG) and the persistence of its oxidative product (CO2) heightened concerns that gas hydrate dissociation could represent a slow tipping point (ArcherÂ et al. 2009) for Earth’s contemporary period of climate change. Source
Arctic Ocean freshwater will cause ‘unpredictable changes on climate’
Ice cap meltwater and river run-off could have significant impact on the climates of Europe and North America, say scientists
The water â comprising meltwater from the ice cap and run off from rivers â is at least twice the volume of Lake Victoria in Africa, and is continuing to grow. At some point huge quantities of this water are likely to flush out of the Arctic Ocean and into the Atlantic, which could have significant impacts on the climate. Scientists say they cannot predict when this will happen though.
Ice cap meltwater and river run-off could have significant impact on the climates of Europe and North America, say scientists
Benjamin Rabe of the Alfred Wegener Institute: This could have an influence on ocean circulation, it could have an influence on the Gulf Stream.
At present, the freshwater acts as a “lid”, preventing the warmer salty water below from meeting the ice, which would melt if the two mixed, according to Rabe. But while it is currently stable, this situation is likely to change as atmospheric circulation patterns shift, and as greater quantities of meltwater spill into the “lake”. The Guardian
Climate Change Effects on Hydroecology of Arctic Freshwater Ecosystems (2006)
Spatial pattern of October warming projected for the 2071â2090 time slice. (Note the spatial congruence of warming between the ocean and the adjacent arctic coastal zone and the extension to more southerly latitudes.) Areas where projected temperature increases are particularly pronounced include northern Siberia and the western portions of the Canadian Archipelago. Notably, however, the maximum projected air temperature increases in these areas are about 5C (greatest near the coasts), compared to the almost two-fold greater projected increases in temperature over the Arctic Ocean. Such pronounced potential temperature increases in freshwater systems in October are particularly important because this is typically the time when freshwater lake and river systems along the coastal margins currently experience freeze up.
Thresholds of response: step changes in freshwater systems induced by climate change
Another physical threshold is the onset of stratification in lakes. Once lakes begin to have open water, wind-driven water circulation becomes one of the controls of biological processes. Almost all lakes have a period of complete mixing of the water column immediately after the ice cover disappears. Very cold waters may continue to circulate for the entire summer so that each day algae spend a significant amount of time in deepwaters where there is not enough light for growth. When a lake stratifies (i.e., when only the uppermost waters mix), algae have better light conditions and primary production increases. The higher temperatures in the upper waters increase the rates of all biotic processes. There is a threshold, probably tied to increased primary production, when entirely new trophic levels appear. For example, the sediment record from a lake in Finland shows that Cladocera, a type of zooplankton, began to appear around 150 years ago. Most lakes in the Arctic already exhibit summer stratification, so this threshold will apply mostly to lakes in the far north.
When air temperatures increase above a mean annual air temperature of -2C, permafrost begins to thaw. When the upper layers of ice-rich permafrost thaw, the soil is disturbed; lakes may drain, and ponds form in depressions. In eastern Siberia, newly thawed soils that are rich inorganic matter slump into lakes. Microbial action depletes the oxygen in the lake allowing the bacteria to produce so much methane that the lakes and ponds become a significant source of this greenhouse gas, and enhance an important feedback to the climate system. This threshold is likely to affect lakes in the more southerly regions of the Arctic. Source
Variable Freshwater Input to the Arctic Ocean During the Holocene: Implications for Large-Scale Ocean-Sea Ice Dynamics as Simulated by a Circulation Model Source
Sea ice melting from Cyclone’s and water mixing between surface and lower layers
Sea ice erosion takes place via a pumping process by which the ice is pushed against the ocean surface by the cyclonic wind field. This motion, in turn, stirs up the underlying waters creating a warm, upwelling current. Since the forces occur over broad regions, powerful surface forces allow the upwelling to dredge deep, causing mixing between surface and lower layers. Tendrils and micro-currents of warmer water thus rise to contact the ice. This action can melt the sea ice from below, breaking it into smaller chunks, opening polynas, and riddling the ice with leads. If the storm grows strong enough, large wave action can devour whole sections of ice. Robert Scribbler
The Arctic Ocean has a temperature inversion in which warmer water is deep and colder water is on top. The Eckman pumping process creates upwelling of this warmer water at the center of Arctic Cyclones while it pushes colder water out toward the edges creating down-welling at the edge of the wind field.
This action is hypothesized â by me and others â to result in increased melt rate during powerful storms under current conditions (thin ice, more ocean and atmospheric warming â ie Warm Storm). The storms themselves would mix water, but would not, likely have much affect on hydrates.Â Robert Scribbler
On persistent cyclones
As the persistent Arctic cyclone – or PAC-2013 – of the past couple of weeks winds down, I want to discuss what I’ve found on the subject in a couple of research papers.Â http://neven1.typepad.com/blog/2013/06/on-persistent-cyclones.html
2013 Arctic Arctic Amplification Arctic Circle Atmosphere CH4 Methane Climate Science CO2 Carbon Dioxide Cyclones Freshwater Greenhouse Gases Hydrosphere Methane Hydrate Ocean Currents Ocean Heat Content Ocean Stratification Oceanography Polar Amplification Region Science Talk Sea Level Height Sea Level Rise Storms YearTags