Tipping elements in the Earth's climate system

Abstract

The term ''tipping point'' commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term ''tipping element'' to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.Earth system ͉ tipping points ͉ climate change ͉ large-scale impacts ͉ climate policy

A safe operating space for humanity

“…However, the growing human pressure on the planet (Vitousek et al 1997;MEA 2005a) necessitates attention to other biophysical processes that are of significance to the resilience ii of subsystems of Earth (Holling 1973;Folke et al 2004;Gordon et al 2008) and the Earth System as a whole. Erosion of resilience manifests itself when long periods of seemingly stable conditions are followed by periods of abrupt, non-linear change, reflected in critical transitions from one stability domain to another when thresholds are crossed (Scheffer et al, 2001;Walker et al 2004;Lenton et al 2008, Scheffer 2009). …”

“…We distinguish between boundaries that are directly related to sharp continental or planetary thresholds, such as the risk of melting of the Greenland and Antarctic ice sheets when permanently crossing a threshold of radiative forcing (Lenton et al 2008;Schellnhuber 2002), and boundaries based on "slow" planetary processes, with no current evidence of planetary scale threshold behavior, which provide the underlying resilience of the Earth System by functioning as sinks and sources of carbon and by regulating water, nutrient and mineral fluxes (Table 1). …”

“…The climate change boundary proposed here aims at minimising the risk of highly non-linear, possibly abrupt and irreversible, Earth System responses (NRC 2002;IPCC 2007b) related to one or more thresholds, the crossing of which could lead to the disruption of regional climates (Lenton et al 2008), trigger the collapse of major climate dynamics patterns such as the thermohaline circulation (Clark et al 2002), and drive other impacts difficult for society to cope with, such as rapid sea level rise. The risk of crossing such thresholds will rise sharply with further anthropogenically driven deviation from the natural variability of the Holocene climate.…”

“…For the global deep ocean to shift to an anoxic state requires strong recycling of P from sediments as bottom waters become more anoxic, thus fuelling increased productivity and amplifying the initial change in a positive feedback loop. In existing models, the resulting dynamics have a 10,000 year timescale due to the long residence time of deep ocean P (Lenton et al 2008). Furthermore, even though humans have greatly accelerated the inflow of P to the oceans, it would still take in the order of 10,000 years to double P in the oceans.…”

“…Here, thresholds are defined as non-linear transitions in the functioning of coupled humanenvironmental systems (Schellnhuber 2002;Lenton et al 2008), such as the recent abrupt retreat of Arctic sea ice caused by anthropogenic global warming (Johannessen 2008). Thresholds are intrinsic features of those systems and are often defined by a position along one or more control variables (Fig.…”