地-气间碳通量气候响应的模拟研究II.未来气候变化

Modeling Study of Terrestrial Carbon Flux Response to Climate Change Part II: Future Changes

利用 GCM模式模拟的未来气候状况 ,结合一个简单计算碳通量的模型 ,对未来地 -气间碳交换通量与温度和降水的关系做初步模拟分析。结果显示 ,在未来气候变暖情况下 ,要保持陆地生态系统不成为大气碳源 ,降水量需要有很大的增加幅度 (至少要比 GCM模式模拟的结果要高 )。在不考虑二氧化碳及氮素的“施肥效应”前提下 ,得出要使陆地生态系统的碳通量为零 ,全球平均温度每增加 1℃ ,降水量需要增加 7% (约 5 4 mm)。

Based on a simple soil carbon model and GCM output for future climate, we conducted several experiments in order to study net carbon exchanges between the terrestrial biosphere and atmosphere. The climate situation between 1901 and 2029 was simulated by NCAR CCM model. The simulated land temperature in 2029 will be 17.0℃ which is about 3.8℃ higher than the mean value during 1920~1949, but the simulated precipitation has slight increase. Thus, the GCM predicts a relation with about 5 mm increase in precipitation for 1℃ increase in general. This relation is much lower than the observed relation over the past century which showed 35 mm/1℃. Thus, this GCM predicts a very dry climatic situation in the future. Using the output of GCM for future climate and our simple models, we simulated the carbon exchange between the atmosphere and terrestrial biosphere and found that the biosphere will loss about 200 Pg carbon by 2029, which approximately equals to 100×10 -6 of atmospheric CO-2. This extra carbon dioxide will certainly cause a serious warming. However, we believe that the precipitation increase in the future by this GCM may be under-estimated. If the observed relationship between temperature and precipitation (35mm/1℃) in the past is used to correct the GCM precipitation field, then, the carbon loss reduces greatly, but the net ecosystem productivity (%NEP%) is still negative. Numerical study indicated that in order to let %NEP%=0, 54 mm precipitation is needed for 1℃ increasing. It means that for 1℃ increasing the global precipitation should increase by about 7%. It should be noted that these results were obtained without considering CO-2 and N fertilization effects on NPP, so that the above values may be over estimated. Our model results indicated that the precipitation is a key factor in controlling ecosystem carbon storage.It is obvious that the warm climate may induce more carbon release from biosphere mainly due to the evident effect of temperature on soil respiration. Whereas, the increase in precipitation would greatly stimulate net primary productivity (%NPP%) and then compensate the warm climate-induced carbon release from the biosphere. A cool and wet future climate may induce a net carbon sink by the biosphere, but a warm and dry climate is not favor for ecosystem to absorb atmospheric CO-2.