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Pages 83-104

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From page 83...
... The first kind characterizes the equilibrium response to changes in CO2, including relatively fast feedbacks, primarily water vapor, clouds, sea ice, and snow cover. This equilibrium response assumes that the oceans have had time to equilibrate with the new value of carbon dioxide and with these fast feedbacks, an equilibration that is estimated to require multiple centuries to a millennium.
From page 84...
... The equilibrium values were extrapolated logarithmically from data given in Table 8.2 of IPCC, Working Group I, The Physical Science Basis (IPCC, 2007a)
From page 85...
... The discussion of Earth System Sensitivity and other considerations pertaining to very long-term climate change will be deferred to Chapter 6. 3.2 EQUILIBRIUM CLIMATE SENSITIVITY Climate sensitivity is calculated by determining how much Earth's surface and atmosphere need to warm in order to radiate away enough energy to space to make up for the reduction in energy loss out of the top of the atmosphere caused by the increase of CO2 or other anthropogenic greenhouse gases.
From page 86...
... A spring with no stiffness at all would represent a very special case, demanding a specific physical explanation, just as would a case of zero slope of the energy budget line, which corresponds to infinite climate sensitivity. to be closer to the Sun than Earth is, in order to make up for the lack of a greenhouse effect.
From page 87...
... The feedbacks that modify the basic black-body feedback are at the heart of predicting future climate. The combined water vapor and lapse rate feedback increases climate sensitivity by affecting the infrared emission side of the balance.
From page 88...
... , the equilibrium Δ T2x has a minimum of 2.1°C, a maximum of 4.4°C, and a median of 3.2°C. Even the least sensitive model has a higher climate sensitivity than the idealized calculations yield for basic clear-sky water vapor and lapse-rate feedback.
From page 89...
... (2005) , do show that there are physical mechanisms that can operate in climate models, which yield climate sensitivity well above the top of the IPCC range employed in Table 3.1.
From page 90...
... The equilibrium global mean temperature change results shown in Table 3.1 were computed by logarithmic extrapolation from the equilibrium climate sensitivity values reported for the 17 general circulation models in Table 8.2 of IPCC, Working Group I, The Physical Science Basis (IPCC, 2007a) .3 Over the range of CO2 covered in the table, logarithmic extrapola tion is equivalent to assuming temperature change to be linear in radiative forcing.
From page 91...
... This will be taken up in Chapter 6, where it will be shown that the persistent warming computed on the basis of equilibrium climate sensitivity provides a valuable guide as to whether the human imprint on climate is likely to be FIGURE 3.2 Comparison of equilibrium warming based on instantaneous CO2 values with actual modeled Figure 3-2.eps temperatures from Eby et al., 2009. The simulation shown is based on cumulative emission of 3840 Gt bitmap carbon in the form of CO2.
From page 92...
... The transient climate response, or TCR, is traditionally defined in a model using a particular experiment in which the atmospheric CO2 concen tration is increased at the rate of 1% per year. The increase in global mean temperature in a 20-year period centered at the time of doubling (year 70)
From page 93...
... show how the same proportionality constant between forcing and global mean temperature holds in a particular GCM for all of the 21st century forcing scenarios utilized by the AR4 and pictured in Figure 3.3 (IPCC, 2007c)
From page 94...
... . In the CMIP3 archive the value of the radiative restoring relevant on the time scale of the 20th and 21st centuries is larger than the value relevant for the equilibrium response -- by 30% on average, but with considerable model-to-model variations.
From page 95...
... Knutti and Tomassini (2008) use a simple model to fit the 20th century temperature record and ocean heat uptake, using a Bayesian analysis to determine optimal parameters and uncertainties.
From page 96...
... estimate uncertainty by using the variability in 30-year trends of global mean temperature from a particular GCM. Their result is 1.3-2.3°C as the 90% confidence interval around their best estimate of 1.8°C.
From page 97...
... . Paeloclimatic evidence constrains equilibrium sensitivity, and with modeling guidance and heat uptake measurements constraining the ratio of TCR to the equilibrium response, one can also use these to constrain TCR.
From page 98...
... In effect, the carbon-climate reponse is a generalization of the concept of climate sensitivity as it pertains to car bon dioxide forcing. By including the carbon cycle response to emissions in addition to the temperature response to CO2 forcing, the carbon-climate response represents a metric that relates global mean temperature change directly to cumulative carbon emissions.
From page 99...
... (2009) , using a simpler climate model but considering a larger range of climate sensitivity, found a most likely peak temperature response of 2°C/1,000 GtC, with a 5-95% confidence range of 1.3-3.9°C/1,000 GtC.
From page 100...
... Based on the above studies, we select 1.75°C global temperature change per 1,000 GtC emitted to be a representative best estimate for the climate response to cumulative carbon emissions. There is large uncertainty, however, in this estimate of the temperature response to carbon emissions owing both to uncertain carbon cycle response to elevated CO2 and cli mate changes (Section 2.4)
From page 101...
... FIGURE 3.6 Cumulative carbon emissions consistent with global mean temperature changes of 1 to 5°C. Best estimates are based on 1.75°C per 1,000 GtC emitted, taken as a representative best estimate from Matthews et al.
From page 102...
... These scenarios were con Figure3-7.eps structed such that total cumulative carbon emissions were the same for each scenario, the rate of emissions All text converted to outline decline varied from 1.5 to 4. 5% per year relative to the peak emissions, and emissions were constrained to reach zero at the year 2100.
From page 103...
... both the atmospheric CO2 concentration and the associated temperature change can be inferred from cumulative carbon emissions to date. If carbon emissions were subsequently eliminated, atmospheric concentrations would slowly decrease over time, whereas temperature would remain elevated for several 4The temperature responses to cumulative emissions shown in Figure 3.8 include both the carbon cycle and climate sensitivity to emissions, but do not correspond directly to either the transient climate response or the equilibrium climate sensitivity associated with a given CO2 concentration.
From page 104...
... 650 ppm 550 ppm 450 ppm 350 ppm Year FIGURE 3.8 Idealized CO2 concentration scenarios reaching between 350 and 1,000 ppm at the year 2100. At the year 2100, the atmospheric CO2 concentration and global mean temperature change is dependent Figure3-8.eps on cumulative carbon emissions to date, with variation in the rate of emissions over time affecting only the rate of increase of forcing and consequent rate of temperature change.to outline of CO2 concentrations Text on the 3 axes converted Stabilization after the year 2100 would require continued low-level CO2 emissions (leading to increasing cumulative carbon emitted)


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