Runaway climate change

A tipping point in the climate system is a threshold that, when exceeded, can lead to large changes in the state of the system. Potential tipping points have been identified in the physical climate system, in impacted ecosystems, and sometimes in both. For instance, feedback from the global carbon cycle is a driver for the transition between glacial and interglacial periods, with orbital forcing providing the initial trigger. Earth's geologic temperature record includes many more examples of geologically rapid transitions between different climate states. A tipping point in the climate system is a threshold that, when exceeded, can lead to large changes in the state of the system. Potential tipping points have been identified in the physical climate system, in impacted ecosystems, and sometimes in both. For instance, feedback from the global carbon cycle is a driver for the transition between glacial and interglacial periods, with orbital forcing providing the initial trigger. Earth's geologic temperature record includes many more examples of geologically rapid transitions between different climate states. Climate tipping points are of particular interest in reference to concerns about climate change in the modern era. Possible tipping point behaviour has been identified for the global mean surface temperature by studying self-reinforcing feedbacks and the past behavior of Earth's climate system. Self-reinforcing feedbacks in the carbon cycle and planetary reflectivity could trigger a cascading set of tipping points that lead the world into a hothouse climate state. Large-scale components of the Earth system that may pass a tipping point have been referred to as tipping elements. Tipping elements are found in the Greenland and Antarctic ice sheets, possibly causing tens of meters of sea level rise. These tipping points are not always abrupt. For example, at some level of temperature rise the melt of a large part of the Greenland ice sheet and/or West Antarctic Ice Sheet will become inevitable; but the ice sheet itself may persist for many centuries. Some tipping elements, like the collapse of ecosystems, are irreversible. The IPCC AR5 defines a tipping point as an irreversible change in the climate system. It states that the precise levels of climate change sufficient to trigger a tipping point remain uncertain, but that the risk associated with crossing multiple tipping points increases with rising temperature. A more broad definition of tipping points is sometimes used as well, which includes abrupt but reversible tipping points. Tipping point behaviour in the climate can also be described in mathematical terms. Tipping points are then seen as any type of bifurcation with hysteresis. Hysteresis is the dependence of the state of a system on its history. For instance, depending on how warm and cold it was in the past, there can be differing amounts of ice present on the poles at the same concentration of greenhouse gases or temperature. In the context of climate change, an 'adaptation tipping point' has been defined as 'the threshold value or specific boundary condition where ecological, technical, economic, spatial or socially acceptable limits are exceeded.' There are many positive and negative feedbacks to global temperatures and the carbon cycle that have been identified. The IPCC reports that feedbacks to increased temperatures are net positive for the remainder of this century, with the impact of cloud cover the largest uncertainty. IPCC carbon cycle models show higher ocean uptake of carbon corresponding to higher concentration pathways, but land carbon uptake is uncertain due to the combined effect of climate change and land use changes. The geologic record of temperature and greenhouse gas concentration allows climate scientists to gather information on climate feedbacks that lead to different climate states, such as the Late Quaternary (past 1.2 million years), the Pliocene period five million years ago and the Cretaceous period, 100 million years ago. Combining this information with the understanding of current climate change resulted in the finding that 'A 2 °C warming could activate important tipping elements, raising the temperature further to activate other tipping elements in a domino-like cascade that could take the Earth System to even higher temperatures'. The speed of tipping point feedbacks is a critical concern and the geologic record often fails to provide clarity as to whether past temperature changes have taken only a few decades or many millennia of time. For instance, a tipping point that was once feared to be abrupt and overwhelming is the release of clathrate compounds buried in seabeds and seabed permafrost, but that feedback is now thought to be chronic and long term.