Hypothetical model illustrating the concept of threshold and tipping point in ecosystems showing alternative stable states (e.g., a diatom-dominated net autotrophic plankton community acting as a CO₂ sink, Regime A, versus a picoplankton-dominated net heterotrophic plankton community acting as a CO₂ source, Regime B; cf. Table 1) in response to pressures (e.g., seawater warming), and the hysteresis effect resulting from different tipping points to shift from one regime to the other and back often observed in these systems (e.g., Duarte et al. 2009). The dotted line with black arrows shows the trajectory of the hypothetical ecosystem when trespassing the tipping point (black vertical line) separating Regime A from B, and the dotted line with red arrows shows the hypothetical trajectory when warming is relaxed, implying a differential tipping point between regimes A and B during warming (black vertical line) and cooling (red vertical line) of the Arctic Ocean

Examples of environmental (a–c) and biological (d–f) tipping elements in the Arctic ecosystem. a Marginal sea ice (Fram Strait, 80°40′N, 1°E); b glacier (Magdalena glacier, Spitzbergen, Svalbard Islands); c permafrost thawing and thermokarst landscape (Sirmilik National Park, Qikiqtaalkuk, Nunavut, Canada); d the Arctic copepod Calanus glacialis; e the polar bear, Ursus maritimus; and f the walrus, Odobenus rosmarus. Changes in either of these keystone species can lead to major reorganization of the Arctic marine ecosystem. Photo courtesya, b Carlos M. Duarte/CSIC; c Isabelle Laurion; d–f Joan Costa

Monthly anomalies in Arctic sea-ice extent (blue line), relative to the monthly averages for the period November 1978–July 2011 and the average sea-ice extent anomaly variance over 16 month running windows (black line). The red lines show the mean sea-ice extent anomaly variance between Nov 1978 and December 2006 [mean (±SE) variance = 0.076 ± 0.002] and between January 2007 and July 2011 [mean (±SE) variance = 0.261 ± 0.016]. Data from Fetterer et al. (2002), accessed August 5, 2011