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3 Determining Temperature Changes in Response to CO2 Emissions
Pages 17-34

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From page 17...
... in determining temperature changes is described. Several additional relevant climate metrics that reflect the state of the literature are discussed.
From page 18...
... It does not, however, include other, mostly slower processes, that have not, at least until recently, been represented in coupled global climate models, such as those involving vegetation, land ice, or changes in the carbon cycle; see Figure 3-1. Transient climate response (TCR)
From page 19...
... , the relation nship betwee cumulativ CO2 emiss en ve sions and glo mean te obal emperature change is approximat linear an can be su s tely nd ummarized b a single pa by arameter: th transient he climate response to cumulative carbon emiss r c c sions in the i ndustrial era (TCRE)
From page 20...
... .14 EQUILIBRIUM CLIMATE SENSITIVITY AND TRANSIENT CLIMATE RESPONSE The concepts of ECS and TCR arise, in their simplest form, from the conservation of energy. In equilibrium, the incoming solar radiation absorbed by Earth balances the outgoing longwave infrared radiation emitted by the planet to space.
From page 21...
... NOTES: CO2 concen ntrations incr rease at 1%//year and staabilize either at two time pre-Indust r es trial CO2 in year 70 or fou times pre ur -Industrial CO2 in year 1 C 140. Results from a coup global pled climate model are shown in red; results from a one-box e m m energy balan model ar shown in nce re green.
From page 22...
... . As one example, cloud feedbacks can exhibit state dependence that is represented in atmosphereocean global circulation models and Earth system models but not in the simple climate models that specify a fixed ECS value.15 State-dependent feedbacks can also be related to long-term changes in ocean circulations, land-surface conditions, ocean carbon uptake, and the cryosphere.
From page 23...
... . to elevant proceesses in Earth system mode are a maj area of ac s els jor ctive researc Some sim climate models also ch.
From page 24...
... The AR5 thus reduced the probability of TCR values greater than 3°C from 10 percent to 5 percent. The estimate was based on the good agreement between the range of estimates from observationally constrained simple climate models and the CMIP5 range.
From page 25...
... a The terms "most likely value," "likely," "very likely," and "zero probability" are the keys to translating the uncertainty information into probability distributions representing the IPCC assessments; see Table 3-1 for more details. THE CARBON CYCLE AND TCRE The discussion so far has focused on the response of global mean surface temperature to a particular level or time path of greenhouse gas concentrations in the atmosphere.
From page 26...
... Th multimod he del mean, coomputed by giving each available mo equal w g a odel weight, and th correspon he nding ± two standard deviation ra ange is show by the blac solid line and the gre shading.
From page 27...
... , giving rise to a convex relationship between cumulative carbon emissions and atmospheric CO2 concentrations. When the convex relationship between emissions and concentrations is combined with the concave relationship between concentrations and forcing, the result is a coincidental cancellation that results in a nearly linear relationship between cumulative CO2 emissions and radiative forcing.
From page 28...
... as ed n e, ssociated chaange in global mean surf face temperatu The com ure. mbination of declining at f tmospheric C 2 and inc CO creasing mea ocean an temperatu give rise to a nearly stable globa mean surfa temperat ure e al ace ture after the cessation o e of emissionns.
From page 29...
... . Second, TCRE depen on the warming at th time of CO2 doubling following a gradual inc T nds w he O g crease in concen ntrations, or the TCR.
From page 30...
... . They examined the impact of a 100 Gt C pulse injection of CO2, relative to a baseline scenario in which CO2 concentrations were held constant at 389 ppm following a historical transition to that point in a range of simple climate models and Earth system models of both intermediate and full complexity.
From page 31...
... . or climate mode to generat the near el te linearity of warming in cumulativ carbon em ve missions, as well as the llongevity of the associat f ted warming it is necess g, sary to use ei ither a carbo cycle mod that inclu on del udes the effe of pH an ects nd warming on CO2 solu g ubility (e.g., Glotter et al., 2014)
From page 32...
... and ΔT(t) ; that is, both the baseline temperature response and the long-term temperature changes due to an emissions pulse.
From page 33...
... estim to mates of the SCC allowin for uncer ng rtainty in clim mate and carbo cycle pro on operties plott as a function of ECS, with the bl ted lack line sho owing mean estimate and the shad region sh ded howing 5 to 95 percent r range and do showing individual ots estimates all assumin a discoun rate minus consumptio growth ra of 3 percent and a s, ng nt s on ate quadratic damage fun c nction [the round dot in panel (c)
From page 34...
... Revisions to ECS are therefore relevant to SCC estimation, principally through their possible implications for baseline warming after a century or more. TCR and IPT determine temperature changes over shorter time periods, including the response to a small pulse emission of CO2.


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