Analyses of simulation Temperature

(1) Characteristics of the temperature change forced by a single factor. The differences between Exp.1 and Exp.2 reflect the possible effect of the decrease in solar radiation output by 0.5%, taking Exp.1 as the comparative standard experiment (control run). Fig. 2.3 shows the difference distribution of annual mean temperature, summer temperature and winter temperature in Eurasian continent between Exp.1 and Exp.2, respectively. The difference of annual mean temperature of the whole Eurasia is abnormally negative (Fig.2.3 (a)), except for the small region of Middle East and the Siberian high-latitude region. And the most remarkable temperature decrease appears in northeastern China and northern Africa, the mean maximum decrease may reach up to 0.6°C, mostly exceeding over 0.2°C. The summer temperature decrease is larger than that of winter. The whole Eurasia of the Eastern Hemisphere suffered from temperature decrease by over 0.2°C, the maximum is up to 1°C (Fig. 2.3 (b)). Compared with that of summer, the temperature decrease of winter is smaller both in region and extent. Temperature still decreases in Northeastern China, whereas the increase occurs in the mid and west part of China. All the results above indicate that the reduced solar radiation, as the forcing mechanism of temperature decrease during LIA, has different effects on the seasonal temperature decrease; the temperature decrease is more evident in summer than in winter. Meanwhile, because of other feedback mechanisms, there is a large regional difference in the winter temperature change. However, the annual mean temperature decrease is the most obvious characteristic.

Table 2.1 Characteristics of LIA simulation experiments

Expt. Solar radiation Optic depth Type of underlain CO2 Remark

Table 2.1 Characteristics of LIA simulation experiments

Expt. Solar radiation Optic depth Type of underlain CO2 Remark

(w/m2)

of volcanic dust

surface vegetation

(ppm)

1

Modern value

Modern

Modern

345

Control ex-

(1367.04)

value

periment

2

-0.5%

Modern

Modern

345

Single-factor

(1360.165)

value

sensitivity

experiment

3

Modern value

+0.15

Modern

345

Single-factor

(1367.04)

sensitivity

experiment

4

Modern value

Modern

Ante- Industrial-

345

Single-factor

(1367.04)

value

Revolution

sensitivity

experiment

5

-0.5%

+0.15

Modern

345

Double-factor

(1360.165)

sensitivity

experiment

6

-0.5%

+0.15

Ante- Industrial-

345

Tri-factor

(1360.165)

Revolution

sensitivity

experiment

7

-0.5%

+0.15

Ante- Industrial-

280

Typical

(1360.165)

Revolution

LIA experi-

ment

Fig.2.4 gives the distribution of temperature difference between Exp.3 and Exp.1, which represents the temperature change effect after increasing the optic depth of stratospheric volcanic dust. The temperature decrease effected by volcanic dust is obvious, and the annual mean value is between 0~0.4°C, with the maximum of 0.4°C being in Northeastern China (Fig.2.4

(a)). The temperature decrease is slightly smaller than that of Exp.2 in both region and extent. Compared with the figure about annual mean, summer mean, and winter mean change of temperature (Fig.2.4), the amplitudes of summer and winter mean change are apparently larger than that of annual mean. Additionally, the change in winter is more remarkable than in summer. The summer temperature decrease in most parts of China is between 0~0.6°C, while that is 0.2~0.8°C in winter, the maximum in the Siberian region is above 1.2°C. The results indicate that volcanic dust plays an important role in the winter temperature decrease. This decrease is opposite to the effect of temperature decrease caused by solar radiation. Moreover, comparing Fig.2.3 with Fig.2.4, the effect of temperature decrease arising from the change of volcanic dust is smaller than that from the reduction of solar radiation, however, their spatial distribution has some similarity.

0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180

Longitude Longitude

Fig. 2.3 The distribution of temperature difference between Exp.1 and Exp.2. (a) Annual mean; (b) summer mean; (c) winter mean.

0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180

Longitude Longitude

Fig. 2.3 The distribution of temperature difference between Exp.1 and Exp.2. (a) Annual mean; (b) summer mean; (c) winter mean.

Fig. 2.4 The distribution of temperature difference between Exp.1 and Exp.3.(a) Annual mean; (b) summer mean; (c) winter mean.

(2) Characteristics of the temperature change forced by double factors. Fig.2.5 shows the temperature effect after reducing the solar radiation and increasing the optic depth of stratospheric volcanic dust. In these maps, annual mean temperature in Eurasia still decreases. The annual mean temperature decrease in China is between 0.2~0.4°C, and that in Europe and most parts of North Africa is above 0.2°C (Fig.2.5 (a)). Comparing with Fig.2.3(a) and Fig.2.4(a), the region of maximum temperature decrease above 0.4°C in Fig.2.5 (a) extends, which reflects that the synchronous function caused by the reduction of solar radiation and the increase of optic depth of volcanic dust has a superposed strengthening effect on the temperature decrease in large regions. The main regions of summer temperature decrease include the whole Eurasia, especially in East China, whose central part is North China, with an amplitude of 0.2~1.0°C. The range of summer temperature decrease is still larger than that of the annual average. In winter the center of temperature decrease in Eurasia moves to the west, the center of maximum temperature decrease approximately above 1°C. And the east part of China turns into an uneven decrease region. The most remarkable temperature decrease occurs in North East and South China, while the slight temperature increase appears in West China.

In the analysis of the results of simulation experiments mentioned above, both the reduction of solar radiation and the increase of the optic depth of stratospheric volcanic dust can result in the air temperature decrease of land surface, but because the mechanism of decrease in temperature and the background of atmospheric circulation are different, the extent of the decrease and the regional distribution are not the same. However, the integrated effect is to decrease the temperature, to make the distribution more even, and to strengthen the average extent of temperature decrease.

(3)Temperature effect resulting from the change of vegetation and CO2 concentration is another important topic in the LIA climate change. Many research and simulation experiments indicate that the change of vegetation plays an important role in the air temperature of land surface in the certain time scale and climatic background. When performing the LIA climate simulation experiments, two types of vegetation are chosen for making comparison, modern vegetation and vegetation before Industrial Revolution. As a whole, the latter has a larger rate of natural vegetation coverage than that of the former. The result of Exp.4 indicates that the change of vegetation field engenders different tendency in temperature change, i.e. the annual mean temperature increased in East Asia, while that of North Africa decreased significantly (figure omitted). Thus the promotion of the vegetation coverage profits the increase of temperature, and the decrease of vegetation coverage contributes to the decrease in temperature of the land surface. Because the LIA climate turned cold, the vegetation coverage in some regions of China became smaller. So, from this point of view the vegetation status has played a certain role in the formation of LIA climate. For lack of the more completed vegetation status in LIA, the simulation experiments with real vegetation coverage in LIA did not perform.

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