Tipping point (climatology)
A climate tipping point is a somewhat ill-defined concept of a point when global climate changes from one stable state to another stable state, in a similar manner to a wine glass tipping over. After the tipping point has been passed, a transition to a new state occurs. The tipping event may be irreversible, comparable to wine spilling from the glass: standing up the glass will not put the wine back.
The IPCC AR5 report stated with medium confidence that precise levels of climate change sufficient to trigger a tipping point, defined as a threshold for abrupt and irreversible change, remain uncertain, and that the risk associated with crossing multiple tipping points increases with rising temperature.
Global warming proceeds by changing the composition of gases in the Earth's atmosphere and Oceans by the emission of greenhouse gases such as carbon dioxide and methane. As warming proceeds, it brings about changes to the natural environment which may result in other changes. For example, warming may begin to melt the Greenland ice sheet and/or West Antarctic Ice Sheet. At some level of temperature rise, the melt of the entire ice sheet will become inevitable; but the ice sheet itself may persist for millennia. A tipping point may be passed without any immediately obvious consequences, nor any acceleration of the warming process. Carbon dioxide as of May 2012[update] makes up 396.18 ppm of Earth's atmosphere and monitoring stations in the Arctic spring 2012 measuring more than 400 ppm of the heat-trapping gas in the atmosphere. James E. Hansen said that this tipping point had already been reached in April 2008 when the CO2 level was 385 ppm. (Hansen states 350 ppm as the upper limit.) "Further global warming of 1°C defines a critical threshold. Beyond that we will likely see changes that make Earth a different planet than the one we know." He has further suggested potential projections of runaway climate change on Earth, in his book Storms of My Grandchildren.
Some scientists and other specialists sometimes express concern about global warming and irreversible tipping points, and use metaphors such as "the door is closing" and warned of global food and water shortages, hundreds of millions of people being displaced by rising sea levels, famine, unhealthy and unlivable conditions, and storms becoming ever more frequent and severe worldwide.
Others have tried systematically to short-list large scale components of the Earth system that may be subject to tipping points, defining tipping points as a variety of phenomena, including the onset of positive feedback, hysteresis effects, and the possible effect of statistical noise at critical points.
Arctic sea ice
In 2013, a study noted that there is an ongoing debate if Arctic sea ice has already passed a "tipping point", or if it is likely in the future. They pointed out, "Several recent studies argue that the loss of summer sea ice does not involve an irreversible bifurcation, because it is highly reversible in models". The study identified in 2007 abrupt transition in seasonal variability which persisted since then, and made a distinction between bifurcation and non-bifurcation `tipping point'.
- Boreal forest dieback
- Amazon rainforest dieback
- Loss of Arctic and Antarctic sea ice (Polar ice packs) and melting of Greenland and Antarctic ice sheets
- Disruption to Indian and West African monsoon
- Formation of Atlantic deep water near the Arctic ocean, which is a component process of the thermohaline circulation.
- Loss of permafrost, leading to potential Arctic methane release and clathrate gun effect
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