Positive feedback occurs when a change (a rise) in one component (global temperatures) of a system (the climate) leads to other changes (such as the melting on the Arctic floating ice) which then "feed back" to amplify it (increased water temperature as the white ice which reflects heat is replaced by dark water which absorbs heat). The result of the first feedback (increased water temperatures) may trigger another change (the beginning of the melting if the Greenland ice sheet) which will itself produce further feedback (rising sea levels with destabilize further parts of the ice sheets) and so on. An unstoppable chain reaction may be set off (runaway global warming), but this far from inevitable: the system may re-stabilize at higher global temperature.
We must prevent that chain reaction from starting. As James Hansen notes, positive feedbacks will be "moderate" but if "global warming becomes larger than that, all bets are off... there seems to be a dichotomy. We either keep the warming small or it is likely to be quite large." (Hansen 2006b)
The example given above is the Albedo effect. It is occurring in the Arctic basin where the summer melting of the floating ice around the north pole is now considered unstoppable. Arctic temperatures will rise much more quickly than the global average: for a global warming of 2°C, the area-mean annual temperature increase over the Arctic (60-90°N) is likely to be between 3.2° and 6.6°C (0.45° to 0.75°C per decade, and possibly even as large as 1.55°C per decade) (New 2006).
Events in the Arctic are already staring to destablize the Greenland ice sheet. At less than a global average 2°C rise there is more than a one-in-three chance that its irreversible melting will have started; at 3°C it is almost certain. Rising Arctic temperatures flowing from floating ice loss are already at "the threshold beyond which glaciologists think the (Greenland) ice sheet may be doomed"; this accelerated melting "is caused by meltwater penetrating crevasses and lubricating the glaciers' flow... The ice is in effect sliding into the ocean on rivers of water", an effect not included in models of the effect of global warming on the Arctic (NS 2006). A recent study found that the Greenland ice cap "may be melting three times faster than indicated by previous measurements" and that "the mass loss is increasing with time" (Young 2006). James Hansen notes that "Ice sheet disintegration starts slowly but multiple positive feedbacks can lead to rapid non-linear collapse" and than "equilibrium sea level rise for ~3°C warming (25±10 m = 80 feet) implies the potential for us to lose control" because "we cannot tie a rope around a collapsing ice sheet" (Hansen 2006a, 2006d).
In a draft paper for publication in 2007, Hansen and fellow researchers warn: "We foresee the gravest threat from the possibility of surface melt on West Antarctica, and interaction among positive feedbacks leading to catastrophic ice loss. Warming in West Antarctica in recent decades has been limited by effects of stratospheric ozone depletion. However, climate projections find warming of nearby ocean at depths that may attack buttressing ice shelves as well as surface warming in the region of West Antarctica. Loss of ice shelves allows more rapid discharge from ice streams, in turn a lowering and warming of the ice sheet surface, and increased surface melt. Rising sea level helps unhinge the ice from pinning points. With GHGs [greenhouse gases] continuing to increase, the planetary energy imbalance provides ample energy to melt ice corresponding to several meters of sea level per century.... Our concern that BAU GHG scenarios would cause large sea level rise this century differs from estimates of IPCC (2001), which foresees little or no contribution to 21st century sea level rise from Greenland and Antarctica. However, the IPCC analyses and projections do not well account for the nonlinear physics of wet ice sheet disintegration, ice streams, and eroding ice shelves, nor are they consistent with the paleoclimate evidence we have presented for the absence of discernable lag between ice sheet forcing and sea level rise." (Hansen et. al 2007).
Loss of the Greenland ice sheet would not only bring a seven metre rise in sea levels, but would start to float the Antarctic ice sheets off their base. Even a one metre of sea level rise from Greenland melt would be devastating. The 2006 Conference of the International Association of Hydrogeologists concluded that rising sea levels will also lead to the inundation by salt water of the aquifers used by cities such as Shanghai, Manila, Jakarta, Bangkok, Kolkata, Mumbai, Karachi, Lagos, Buenos Aires and Lima. "The water supplies of dozens of major cities around the world are at risk from a previously ignored aspect of global warming. Within the next few decades rising sea levels will pollute underground water reserves with salt... Long before the rising tides flood coastal cities, salt water will invade the porous rocks that hold fresh water... The problem will be compounded by sinking water tables due to low rainfall, also caused by climate change, and rising water usage by the world's growing and increasingly urbanised population." (Pearce 2006b)
As the Arctic warms, melting permafrost in the boreal forests and further north in the Arctic tundra is now starting to melt, triggering the release of methane, a greenhouse gas twenty times more powerful than CO2, from thick layers of thawing peat. The West Siberian bog is estimated to contain 70 gigatonnes of CO2, about twice the world's annual total CO2 emissions. The methane is bubbling free into the atmosphere from growing lakes of liquid methane as permafrost underneath liquifies. Prof. Sergei Kirpotin, a botanist at Russia's Tomsk State University, says patches of white lichen on high Siberian ground reflect the sun's rays and help to keep the ground underneath cold, but as the dark lakes expand, more heat is absorbed and more permafrost melts: "As we predicted in the early 1990s, there's a critical barrier... Once global warming pushes the melting process past that line, it begins to perpetuate itself." Some estimates put this methane release in 2006 as high as 100,000 tons a day, "which means a warming effect greater than America's man-made emissions of carbon dioxide." (Connor and McCarthy 2006).
This chain of events will rapidly drive up the temperature, triggering and reinforcing further feedbacks.
Increased mobilisation of organic carbon: Soils and the oceans have historically contributed equally to absorbing atmospheric carbon dioxide. The soil also releases carbon as plant and organic matter decompose. Professor Guy Kirk of the National Soil Resources Institute at Cranfield University has calculated that the increase in carbon lost by UK soil each year since 1978 of 13 million tons of carbon dioxide a year is more than the 12.7 million tons a year Britain saved by cleaning up its industrial emissions as part of its commitment to Kyoto. The loss is likely to be due to plant matter and organic material decomposing at a faster rate as temperatures rise. Soil sinks are predicted to release their carbon at an even faster rate as temperatures increase: "It's a feedback loop," says Kirk. "The warmer it gets, the faster it is happening." (Pickrell 2005, Connor and McCarthy 2006). It is thought that at 2-3°C, the conversion will begin of the terestrial carbon sink to a carbon source due to temperature-enhanced soil and plant respiration overcoming CO2-enhanced photosynthesis, resulting in widespread desertification and enhanced feedback (Sarmiento and Gruber 2003).
Ocean acidity: As more carbon dioxide dissolves in seawater to form carbonic acid, the acidity of the ocean increases. Ken Caldeira of the Carnegie Institution’s Department of Global Ecology says that increased carbon dioxide emissions are rapidly making the world’s oceans more acidic and, if unabated, could cause a mass extinction of marine life similar to one that occurred 65 million years ago when the dinosaurs disappeared. "What we're doing in the next decade will affect our oceans for millions of years... CO2 levels are going up extremely rapidly, and it's overwhelming our marine systems" (Eilperin 2006). Caldeira says “The geologic record tells us the chemical effects of ocean acidification would last tens of thousands of years... But biological recovery could take millions of years. Ocean acidification has the potential to cause extinction of many marine species" (NASA 2006).
Algae extinction: In 2006, Nasa satellites showed earlier that phytoplankton which absorb carbon dioxide are finding it harder to live in the more stratified layers of the warmer ocean, which restrict the mixing of vital nutrients. Since 2000, when the sea surface temperatures began to rise more noticeably, the photosynthetic productivity of phytoplankton have decreased in some ocean regions by 30 per cent. James Lovelock points out that as the ocean surface temperature warms to over 12?, "a stable layer of warm water forms on the surface that stays unmixed with the cooler, nutrient-rich waters below. This purely physical property of ocean water denies nutrients to the life in the warm layer, and soon the upper sunlit ocean water becomes a desert", recognized by the clear azure blue, dead water of 80 per cent of today's ocean surface. In such nutrient-deprived water, ocean life cannot prosper and soon "the surface layer is empty of all but a limited and starving population of algae". Algae, which comprise most of the ocean's plant life, are the world's greatest CO2 sink, pumping down carbon dioxide, as well as contributing to cloud cover by releasing dimethyl sulphide into the atmosphere, gas "connected with the formation of clouds and with climate" (Lovelock 2006: 23), so that warmer seas and less algae will likely reduce cloud formation and further enhance positive feedback. Severe disruption of the algae/DMS relation would signal spiralling and irreversible climate change.