Temperature simulation in

Fig.2.9 plotted the differences of simulated temperature between 6 kaPB and 0 kaBP. In the west of Eurasia, annual temperature at 6 kaBP was 00.5 °C warmer than that in the present in the south of 50°N, and colder than present in the north of 50°N (Fig.2.9a). In the East Asia, summer temperature was about 2.0°C warmer at 6 kaBP than the present in the south of 40°N (Fig.2.9b), and winter temperature decreased about 2.0°C (Fig.2.9c) at 6 kaBP comparing with the present. This result agrees with those of PMIP climate simulations at 6ka BP.

PMIP standard simulations at 6 kaBP revealed a typical Post-glacial climate when Quaternary ice sheets were melted in the Northern Hemisphere. In views of climate dynamical mechanisms, solar radiation is the fundamental basic forcing that causes changes in climate warming globally and extensions of African and Indian monsoons regionally. Similar to all of the PMIP simulations at 6 kaBP, the Exp. 1 also showed annual and summer temperature about 1°C increases and significant decrease in winter temperature in the low and middle latitudes (20-50°N) of Eurasian continent.

During the Holocene, the summer solar radiation reached the maximum at 9 kaBP (11 kaBP in calendar year) in the Northern Hemisphere, and was 7-8% greater in summer and 2-3% less in winter than present in middle and low latitude area (south of 50°N). This abnormal pattern had continued to the mid-Holocene (6 kaBP) and becomes a key reason why the PMIP climate simulations and the Exp. 1 modeled the cold temperatures in winter at 6 kaBP.

60N-50N-40N-30N-20N-10N-EQ-10S-

20W 0 20E 40E 60E 80E 100E 120E HOE

60N 50N

40 N

30N 20N

20W 0 20E 40E 60E 80E 100E 120E HOE

20W 0 20E 40E 60E 80E 100E 120E HOE

60N 50N

40 N

30N 20N

20W 0 20E 40E 60E 80E 100E 120E HOE

Fig. 2.9 Differences of simulated temperatures between 6ka BP (Exp. 1) and 0ka BP, (a) Difference of annual temperature (°C), (b) Difference of summer temperature (°C), (c) Difference of winter temperature (°C)

Fig. 2.9 Differences of simulated temperatures between 6ka BP (Exp. 1) and 0ka BP, (a) Difference of annual temperature (°C), (b) Difference of summer temperature (°C), (c) Difference of winter temperature (°C)

Temperature simulation in Exp. 2

In the Exp.2, the vegetation distribution at 6 kaBP is used as land surface coverage in the modeling. The differences of temperature between Exp. 2 and control test (Figs not given) in the East Asia indicate ca 1-2°C warmer in summer and ca 0.5-1.0°C warmer in winter at 6 kaBP than the present. The annual temperature at 6 kaBP increased about 0.5-1.5°C over all of the East Asia. The results imply that Exp. 2 has much more improvement in temperature simulation of 6 kaBP than Exp. 1. To compare the temperature simulations between the two experiments, we computed the temperature differences between Exp. 2 and Exp. 1 in order to understand the net effect of the vegetation on the temperature changes (Fig.2.10a, b and c). The index was shown in Fig. 2.10.

Simulated annual temperature in Exp. 2 was higher than that in Exp. 1 in the north of 50°N in Europe, but it was slightly lower in Exp. 2 than Exp. 1 in Siberian area. In the south of 50°N, annual temperature was about 1-2°C higher than that in Exp. 1 in the eastern and northern China. There was no significant change of summer temperature between Exps. 1 and 2 (Fig.2.10b). However, winter temperature in Exp. 2 was 1°C higher than Exp. 1 in the middle and eastern parts of China in the south of 50°N (Fig. 2.10c)

Exp. 2 did not show a significant temperature change in Tibetan Plateau at 6 kaBP. It was 1-2°C lower in winter in the central Plateau and 1°C

warmer in summer of north plateau than those in Exp. 1. Modern summer warming in the plateau is mainly due to increased solar radiation, which the summer heating is a major contribution to the annual temperature increase due to the specific geographical position and atmospheric optical characteristics of the plateau. Thus the warming effects should not mainly come from the vegetation changes in the plateau region. However, it is necessary to further study the causes of temperature change in the plateau in terms of the specific horizontal and vertical structures, the independent pressure system, and the planetary westerly over the Tibetan Plateau, e.g. using coupled climate-vegetation model to explore the possible mechanisms of climate at 6 kaBP.

(b)

60N 50N 40N 30N 20N 10N EQ 10S

20W 0 20E 40E 60E 80E 100E 120E HOE

Fig. 2.10 Differences of simulated temperature at 6 kaBP between Exp. 2 and Exp.1, (a) Difference of annual temperature (°C), (b) Difference of summer temperature (°C), (c) Difference of winter temperature (°C)

Significant test of simulations

From above analysis we can see that changes of solar radiation and vegetation in the mid-Holocene (6 kaBP) have played very important roles in the climate changes. Here we give the results of significant test for these simulation changes. By using T-student test at 95% confidence limit, two pairs of experiments (0 ka BP and 6 ka BP in Exp. 1, 6ka BP in Exp. 1 and 6 ka BP in Exp. 2) have been performed by T-test processes. Significant areas for each pair of parameters are shadowed in Fig.2.9 and Fig.2.10. In Exp. 1, when solar radiation is considered as a unique forcing factor, both summer warming and winter cooling in the East Asia are significant. The simulation of winter temperature by vegetation forcing in Exp. 2 has the largest significant areas in the east parts of China, and also larger significant area in the subtropics area (western Asia and northern Africa). Significant test showed that increases in annual and summer precipitation are significant in the middle latitude of the East Asia (30°N -50°N), northern Africa, and western China (Figs not shown).

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