feedback loops
Carbon released from thawing permafrost is now surpassing the emissions of 189 countries and would rank 11th on a list of top emitting countries from 2018. [130] [131] Emissions from the thawing Arctic are expected to increase 41% under a business-as-usual scenario and 17% percent under a moderate mitigation scenario. [132] A warming arctic means more thawing permafrost which releases more carbon, causing a positive feedback forcing on temperature. This mechanism is thought to explain periods of abrupt warming in the past. [133]
Structure of a gas hydrate (methane clathrate) block embedded in the sediment of hydrate ridge, off Oregon, USA. The gas hydrates have been found during a research cruise with the German research ship FS SONNE in the subduction zone off Oregon in a depth of about 1200 meter in the upper meter of the sediment. The shown gas hydrate (white) has been deposited in thin layers into the sediment. [133]
Increased temperatures lead to an increase in organic productivity in the soil, which reduces carbon turnover time (decreasing the time that carbon spends sequestered in the soil), leading to increased CO2 emissions. [134] Increased rainfall in tropical areas is also known to cause an increase in soil carbon turnover. [135] [136] [137]
The predicted Northern Hemisphere September sea ice extent using CMIP5 coupled ocean-atmosphere climate model. Emission scenarios (RCPs) indicated in colour, observations in black, and shaded areas indicate uncertainty (IPCC, 2013). [138]
The collapse of the ice caps is another feedback loop. A lack of sea ice means less solar radiation is reflected back into space and is instead absorbed by the dark ocean, which warms further, increasing the rate of sea ice loss. The arctic ice cap is already almost entirely gone during the summer and we will likely reach a tipping point in this regard by 2022, known as a "Blue Ocean Event," when the ice-free Arctic ocean begins to absorb all incoming solar radiation rather than reflect it back into space. [139] [140]
Monthly November ice extent for 1978 to 2020 shows a decline of 5.1 percent per decade. [141]
Once the Arctic ice sheet collapses, the Arctic will begin to warm far more quickly. This is because the energy that it took to melt the ice now has nowhere else to go but into increasing Arctic ocean temperatures. This is known as the "latent heat" tipping point. A warming Arctic ocean means the collapse of previously-frozen methane deposits buried in the Arctic ocean floor. [143] [144] [145]
Land, atmosphere, and ice heating (red), 0–700 meter OHC increase (light blue), 700–2000 meter OHC increase (dark blue). [146]
Professor Peter Wadhams: “The point about summer conditions is that as long as there is SOME ice present on the sea surface, however thin the layer, then the ocean temperature below it is held to 0 degrees Celsius because the absorbed solar radiation melts the ice rather than warming the water. Also the atmospheric temperature is held to close to 0 degrees Celsius because warmer air melts the surface snow layer on top of the ice and is thereby cooled. The sea ice, even when thinned, continues to act with 100% efficiency as an air conditioning system for ocean and atmosphere alike.”
“BUT”, Prof Wadhams continues, “as soon as the sea ice layer goes, this process ceases and the sea can warm up rapidly (to typically 7 degrees Celsius by the end of summer - which is not much colder than the North Sea), as can the atmosphere (which speeds up Greenland ice sheet melt when that warmed air passes over Greenland). Latent heat is an enormously powerful buffer - the amount of heat that you have to pump in to melt 1 kg of ice will subsequently heat that same amount of melted water to 80 degrees Celsius. So once the ice goes away entirely there is a big jump in temperatures in the upper ocean and atmosphere (with dire consequences for permafrost), and it is very difficult to see how one can ever go back to an ice-covered summer ocean once this has happened.” [147]
Many marine hydrates are locked in place by the icy abyssal currents that are generated by the sinking of cold, dense saltwater in polar regions. These abyssal currents gradually girdle the planet before surfacing and returning to their polar birthplace. This global circulation system is known as the thermohaline current.
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So long as these cold abyssal currents continue to flow, the world’s sea-floor hydrates generally remain frozen and stable. But whenever rising temperatures thaw the polar icecaps to any significant degree, the boyant melt-water pools on the surface, inhibiting the driving mechanism that runs the world’s thermohaline circulation system. This allows wind-driven fingers of warm surface water to reach down to the sea floor and release the methane from its icy cage.
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The last time this appears to have happened was about 55 million years ago, when according to the geological record, a discharge of 1,200–2,500 gigatons of methane caused a temperature spike of 8°–10°C in polar regions and brought lush redwood forest to northern Greenland.
Such polar warmth generated a deep-ocean temperature rise of 5–7°C in high latitudes, and appears to have switched off the the world’s thermohaline circulation starving the inert abyssal waters of oxygen. The carbon-loaded acidified seas caused a mass extinction of marine foraminifera.
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The temperature spike that occurred at this time appears to have been vastly greater and more abrupt than could possibly have been generated by the gradual rise in atmospheric CO2 that preceded it.
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The global temperature has risen almost 30 times faster than this during the past 30 years. [148]
Source: [148]
Warmer Arctic waters lead to more intense storms, which increase vertical mixing in Arctic water columns, increasing the rate of warming. [149]
Negative Arctic Oscillation conditions are associated with higher pressure in the Arctic and a weakened polar vortex (yellow arrows). A weakened jet stream (black arrows) is characterized by larger-amplitude meanders in its trajectory and a reduction in the wave speed of those meanders. [151]
Warming rivers are dumping their contents into the Arctic ocean, exacerbating temperature rise by preventing sea ice formation. [152]
A less severe temperature gradient (due to melting sea and land snow/ice cover) between the poles and the equator leads to a weakening in the polar vortex and jet stream. A slower jet stream has two major effects on global weather patterns. First, storms will tend to linger as ridges and troughs in the jet stream are pushed more slowly. Second, ridges will reach further northward and troughs further south, allowing warm, moist tropical air to penetrate further into the Arctic and bringing polar air into the south. [150] [151]
The image above shows increased sea surface temperature anomalies in the area of the Beaufort Sea where the Mackenzie River is flowing into the Arctic Ocean. [152]
The Greenland ice sheet has recently passed a point of no return, where snowfall amounts are no longer enough to offset the increasing melting rates at the ice sheet margins. [159] [160]
The collapse of ice sheets leads to a phenomenon known as isostatic rebound, when the pressure of the ice sheet is offloaded from the Earth's crust. The crust underneath the former ice sheet uplifts, and surrounding areas subside. This crustal movement leads to earthquakes, which can destabilize methane hydrates on the ocean floor by triggering submarine landslides. This mechanism has been documented in the past. [153] [154]
More shortwave radiation entering Arctic waters will increase the length of the phytoplankton growing season. This in turn will exacerbate Arctic ocean warming through direct biological heating of surface waters. [155]
Warmer waters generally have less nutrient availability, leading to a greater ratio of pico- to microphytoplankton in warmer waters. The increased biomass of picophytoplankton is offset by a decrease in overall phytoplankton biomass, reducing the efficiency of the biological pump that sequesters carbon in the deep ocean. Instead, more carbon is channeled through the upper-ocean microbial loop and back into the atmosphere. [161]
A temperature-dependent biological pump. Illustrative cartoon showing the effects of different water temperatures on organic carbon export and remineralization: more carbon is sequestered when temperature is colder compared to when is warmer. [161]
Higher temperature leads to more humidity in the atmosphere. As a potent greenhouse gas, more water vapor traps more heat at ground level, raising the temperature further. [156]
The impact of cloud formation on the climate is presently uncertain and the source of the greatest uncertainty in climate models. Models tend to run in one direction or the other with an increase in cloud cover. Lower, denser, and brighter clouds increase Earth's albedo and reflect more incoming radiation than they trap (although at night, there is no incoming radiation to reflect and low clouds then act as local insulators). High-altitude, hazy clouds do not reflect much shortwave radiation and instead do more to trap longwave radiation re-emitted by the surface. [157] [158]