"Society may be lulled into a false sense of security by smooth projections of global change," said the researchers. But man-made climate change may induce abrupt, potentially irreversible, changes.
Writing in PNAS, the team introduce the term tipping element to describe a subsystem of the Earth that is at least subcontinental in scale (around 1,000 km or more) and that can be switched into a qualitatively different state by small perturbations.
"We consider that many of the systems do not yet have convincingly established tipping points," say the researchers. "Nevertheless, increasing political demand to define and justify binding temperature targets, as well as wider societal interest in nonlinear climate changes, makes it timely to review potential tipping elements in the climate system."
The team, from the University of East Anglia, Tyndall Centre for Climate Change Research, Newcastle University and Oxford University, all in the UK, and the Potsdam Institute for Climate Impact Research, Germany, and Carnegie Mellon University, US, classified the Greenland ice sheet and Arctic sea ice as being highly sensitive tipping elements, with the smallest uncertainty.
The scientists found that the West Antarctic Ice Sheet is probably less sensitive as a tipping element than Arctic sea ice and the Greenland ice sheet, but projections of its future behaviour contain a large uncertainly. The same was true for the Amazon rainforest, Boreal (northern) forest, the El Niño phenomenon and the West African monsoon.
"These tipping elements are candidates for surprising society by exhibiting a nearby tipping point," they wrote in their paper.
The final tipping element analysed – the Atlantic thermohaline circulation – has low sensitivity and intermediate uncertainty. There’s up to a 10% chance that it could undergo a large abrupt transition by the end of the century, according to the Intergovernmental Panel on Climate Change (IPCC).
The team stressed the desirability of setting up early warning systems to highlight when an element is approaching its tipping point. "If a forewarning system for approaching thresholds is to become workable, then real-time observation systems need to be improved (e.g. building on the Atlantic thermohaline circulation monitoring at 26.5°N)," write the researchers. "For slow transition systems, notably ocean and ice sheets, observation records also need to be extended further back in time (e.g. for the Atlantic beyond the 150 year sea surface temperature record)."
The researchers anticipate that the scale of potential impacts from tipping elements will "shift the balance towards stronger mitigation and demand adaptation concepts beyond incremental approaches".
The team also proposes study of tipping elements in human socioeconomic systems, with a special focus on whether there is a tipping point that would trigger a move to a low-carbon-energy system.
The analysis follows a workshop on "Tipping Points in the Earth System" attended by 36 climate scientists in Berlin in October 2005 and communications with a further 52 experts.