TABLE 2 Differences Between Experiment with Volcanic Aerosal Optical Depth Increased to 0.15 and Experiment with Solar Irradiance Reduced by 2 Percent

Latitude (degrees)

T (68 mb)

Surf Temp., Global (°C)

Surf Temp., Land (°C)

Precip., Global (mm/day)

Precip., Land (mm/day)

90

-1

0.8

0

-0.1

0

82

-1

0.3

-0.7

-0.1

-0.1

74

-

0

-0.5

0

-0.1

67

0

0

0

0

0

59

0

0.1

-0.3

0

0

51

1

-0.4

-0.6

0

-0.1

43

1

-0.1

-0.6

0

0

35

2

-0.3

-0.8

0

0

27

2

-0.5

-0.6

0

0

20

2

-0.6

-0.7

0

-0.1

12

3

-0.6

-0.7

0

-0.1

4

3

-0.7

-0.9

-0.1

-0.4

-4

3

-0.7

-0.9

-0.3

-0.4

-12

3

-0.8

-0.6

-0.3

-0.1

-20

2

-0.8

-0.2

0

-0.2

-27

1

-0.4

-0.5

-0.2

-0.1

-35

1

-0.4

0.1

0

0

-43

1

-0.4

0.7

-0.1

0

-51

1

0.8

0.2

0

0.2

-59

0

1

-0.2

0

0.1

-67

0

0.3

0

0

0

-74

0

-0.1

0.3

0

-0.1

-82

-1

0

0

0

0

-90

-1

0.3

0

0

0

GLOBAL

1.4

-0.34

-0.45

-0.04

-0.08

where

A methane reduction from 1.5 ppmv to 0.7 ppmv corresponds to a radiative temperature change of 0.08°C. With the estimated feedback, this gives an equilibrium temperature change of close to 0.3°C.

The observed variation in CO2 and methane in an equilibrium calculation would thus correspond to a temperature change over the last several hundred years of about 1.3°C, with the currently estimated feedback factors. This alone would be sufficient to account for much of the Little Ice Age cooling, without the additional forcing generated by recently added trace-gases (e.g., CFCs). (However, this correspondence also ignores the potential impact of increases in tropospheric aerosols, which, as noted above, may have acted to force the system in the opposite direction, with a magnitude similar to that of the trace-gas effect.) It therefore raises the question of what fraction of this equilibrium response has been observed, i.e., what the response time of the system is. Hansen et al. (1985) discuss the various estimates for this value, noting that it is a function of both ocean mixing (which determines the effective heat capacity of the system) and climate sensitivity (which is indicative of the magnitude of the feedbacks that must come gradually into play; note that if there are no feedbacks, then all the forcing arises instantaneously from the initial perturbation, reducing the delay in system response).

With an effective vertical diffusion through the base of the mixed layer on the order of 1.5 cm2 s-1, as implied by transient ocean tracers (Broecker et al., 1980), and a feedback factor of the magnitude noted above, the CO2-induced warming from 1850 to 1980 amounts to approximately one-third of the equilibrium response (Hansen et al., 1985). For an approximately similar phasing of methane changes, as indicated by the above references, the proportion of equilibrium warming should be similar. Thus we might conclude that only one-third of the equilibrium trace-gas response has been realized, or approximately 0.4°C to 0.5°C. Nevertheless, if the Little Ice Age was 1°C cooler than today, half of this could be a trace-gas effect, with most of the related warming occurring in the twentieth century. Since the trace-gases are globally distributed, the climate changes



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