17), and the ice has thinned significantly over a large area. For both summer and winter Arctic sea-ice, the area coverage is declining at present (with summer sea-ice declining more markedly ref. Different stable states for the flow rate through the narrow outlets that drain parts of the Arctic basin have also been found in a recent model ( 16). The instability is not expected to be relevant to Southern Ocean sea-ice because the Antarctic continent covers the region over which it would be expected to arise ( 15). This small ice-cap instability is also found in some atmospheric general circulation models (AGCMs), but it can be largely eliminated by noise due to natural variability ( 14). Energy-balance models suggest that this ice-albedo positive feedback can give rise to multiple stable states of sea-ice (and land snow) cover, including finite ice cap and ice-free states, with ice caps smaller than a certain size being unstable ( 13). Sahara/Sahel and West African monsoon (WAM)Īs sea-ice melts, it exposes a much darker ocean surface, which absorbs more radiation–amplifying the warming. Our collective judgement was used to evaluate condition 4.įeature of system, F (direction of change) Condition 3 was evaluated on the basis of model projections, known shortcomings of the models, and paleodata. Where the proposed ρ crit could be meaningfully related to temperature, condition 2 was evaluated based on an “accessible neighborhood” of global temperatures from the IPCC ( 12) of 1.1–6.4☌ above 1980–1999 that could be committed to over the next T P ∼ 100 years, and on recognition that transient warming is generally greater toward the poles and greater on land than in the ocean. To meet condition 1, there needed to be some theoretical basis (>1 model study) for expecting a system to exhibit a critical threshold (ρ crit) at a subcontinental scale, and/or past evidence of threshold behavior. 1, Table 1), and from literature review and the aforementioned workshop, we identified a short list of candidates that meet conditions 1–4 (top nine rows in Table 1). We considered a long list of candidates ( Fig. Here we focus on policy-relevant potential future tipping elements in the climate system. Then we consider the prospects for early warning of an approaching tipping point. ![]() Results from the expert elicitation are used to rank a subset of these tipping elements in terms of their sensitivity to global warming and the associated uncertainty. ![]() Here we combine a critical review of the literature with the results of the workshop to compile a short list of potential policy-relevant future tipping elements in the climate system. To this end, we organized a workshop entitled “Tipping Points in the Earth System” at the British Embassy, Berlin, which brought together 36 leading experts, and we conducted an expert elicitation that involved 52 members of the international scientific community. ![]() 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 under anthropogenic forcing ( 5) ( Fig. Many of the systems we consider do not yet have convincingly established tipping points.
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