BOX 3.1 Tipping Points and also the allude of No Return
There space sound theoretical factors to think that tipping points across climatic thresholds exist (Gladwell, 2000; NRC, 2002). Instances of threshold habits include thermohaline circulation modifications, ice sheet instabilities, sea ice cream instabilities, soil-moisture feedbacks, and the start of high-latitude convection and associated high-level cloud forcing. Hansen et al. (2008) presented the term “tipping element” to explain subcontinental-scale subsystems the the planet system that room susceptible to being forced into a new state by tiny perturbations. Tipping level—the magnitude of climate forcing beyond which, if sustained, abrupt climate change will at some point occur—is distinguished from “point of no return.” If the tipping level is surpassed for just a brief duration of time, the original state the the system have the right to be restored. Much more persistent forcing deserve to push the system to the “point of no return,” whereby a palliation of the forcing listed below the tipping level is ineffective in halting the climate change (Figure 3.1). This irreversibility of the system response is referred to as hysteresis (NRC, 2002).
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FIGURE 3.1 Equilibrium claims of a “system” (valleys) in an answer to steady anthropogenic CO2 forcing (progressing from dark to light blue). The curvature of the sink is inversely proportional to the system’s response time (τ) to little perturbations. A threshold is reached once the valley becomes shallower and also finally vanishes resulting in the sphere to abruptly roll to a brand-new state (to the left). SOURCE: Lenton et al. (2008), ©National Academy of Sciences, U.S.A.
FIGURE 3.2 Relationship between atmospheric CO2 (A) and also climate (B) with the Cenozoic. The top panel shows rebuilded pCO2 from marine and lacustrine proxy records; the dashed heat is best pCO2 because that the Neogene estimated by equilibrium calculations making use of lacustrine mineral phases (Lowenstein and Demicco, 2006). The climate curve in the reduced panel is a composite that deep-sea benthic foraminiferal oxygen-isotope records, smoothed utilizing a five-point running average (Zachos et al., 2001a, 2008). The temperature scale on the appropriate axis was calculated for an “ice-free ocean,” and also is for this reason applicable solely to the pre-Oligocene part of record. SOURCE: Zachos et al. (2008), reprinted by permission that Macmillan Publishers Ltd.
FIGURE 3.3 maritime stable isotope and also seafloor sediment CaCO3 records compiled using several s drilling sites because that the PETM, a hyperthermal through some parallels to contemporary greenhouse gas-driven an international change. (A) δ13C time series developed from benthic foraminifera depicting ~2.5 part per thousands (‰) excursion at ~55 Ma. (B) δ18O time collection and inferred temperatures record the prolonged period of ocean warming (~70-80 ky) and also its large magnitude. Over there may have been several occasions of greenhouse gas release throughout the PETM that produced the large, abrupt alters in ocean temperatures. (C) document of seafloor calcium carbonate contents from the south Atlantic documents the far-ranging reduction because of dissolution and also deep-ocean acidification throughout the PETM. The evident onset of CaCO3 dissolution before the onset of the carbon isotope excursion reflects the substantial dissolution of uppermost Paleocene sediments through acidic waters throughout the PETM. SOURCE: Zachos et al. (2008), reprinted through permission the Macmillan Publishers Ltd.
FIGURE 3.4 early carbon pulse because that the PETM (red curves), approximated to be 3,000 Pg carbon using published carbon isotope and observed deep-sea lead carbonate dissolution records, and also a carbon cycle design (LOSCAR; Zeebe et al., 2008, 2009). The size of the intake carbon mass to be inferred from carbonate dissolution records, with the δ13C the the carbon pulse (≤ –50‰) constrained by inquiry the version outcome to complement observed deep-sea δ13C records. The model assumes a huge initial intake of carbon over 5 ky, followed by further smaller pulses and also a low continuous carbon release (an added 1,500 Pg) throughout the PETM main event. Alters in calcite saturation in the surface s (lower diagram) are estimated for the PETM (red curve) and for the future (black curve), based upon the inferred size of the carbon pulse to atmosphere.
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SOURCE: Courtesy that R.E. Zeebe, personal communication (2010).