Our judgment as to the probable values of transient climate response and equilibrium climate sensitivity is summarized in Table 3.1. The values given in this table, used together with the pattern-scaling for regional climate discussed in Chapter 4, will form the basis for our evaluation of the impacts of climate change. The reasoning leading to these estimates, and an exposition of the physical processes involved, are given in Sections 3.2 and 3.3.

At the opposite end of the spectrum of time scales, the third kind of climate sensitivity, called Earth System Sensitivity, incorporates a range of slower feedback processes that can set in during the millennia over which anthropogenic CO2 emissions are expected to continue to affect the climate. These include long-term carbon cycle feedbacks and partial or total deglaciation of Greenland and Antarctica. The human imprint on climate will outlast the fossil fuel era by millennia, because of the long atmospheric lifetime of CO2, and perhaps because of the additional feedbacks the resulting warming may entrain. To future geologists, the fossil fuel era will appear as a boundary between the Holocene epoch and a substantially hotter epoch dubbed the Anthropocene (Crutzen and Stoermer, 2000)—which will be driven by different factors (human factors) than any climate states observed at any time in more than a million years. Will the great ice sheets of Greenland and Antarctica survive the Anthropocene? How much of the world’s present biodiversity will survive the Anthropocene? How will agriculture in an Anthropocene climate feed whatever population may prevail

TABLE 3.1 Transient and Equilibrium Global Mean Warming as a Function of the Atmospheric CO2 Concentration

CO2 (ppm)

Trans. Low

Trans. Prob L

Trans. Med

Trans. Prob H

Trans. High

Eq. Low

Eq. Med

Eq. High

350

0.4

0.4

0.5

0.7

0.8

0.7

1.0

1.4

450

0.8

0.9

1.1

1.5

1.8

1.4

2.2

3.0

550

1.1

1.3

1.6

2.1

2.5

2.1

3.1

4.3

650

1.3

1.6

2.0

2.7

3.2

2.6

3.9

5.4

1000

2.0

2.4

3.0

4.0

4.8

3.9

5.9

8.1

2000

3.1

3.7

4.7

6.2

7.4

6.0

9.1

12.5

NOTE: Warming is given in degrees C relative to a pre-industrial climate with a CO2 concentration of 280 ppm. The equilibrium values were extrapolated logarithmically from data given in Table 8.2 of IPCC, Working Group I, The Physical Science Basis (IPCC, 2007a). The “Eq. Low” column is based on the minimum sensitivity in the ensemble of models, the “Eq. High” is based on the maximum, and the “Eq. Med” is based on the median. The transient climate response estimates are discussed in Section 3.3; the “Prob. Low” column represents the low end of the probable range, the “Prob. High” column the high end of the probable range, and the “Med” column the median value of transient climate response. The stated warming reflects only the influence of CO2 on the climate, but the table can be used to estimate the effect of other greenhouse gases by using the radiative-equivalent CO2e in place of CO2.



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