过去4000年中国降水与森林变化的数量关系

Quantitative relationship between precipitation and forest changes in the past 4000 years in China

历史时期陆地森林覆被的变化是影响气候变化的重要因素。通过应用历史回推、理论推导和统计分析方法,研究了过去4000a中国降水与森林变化的数量关系。结果表明:近4000a来全国年均降水量由918mm,下降为20世纪70-80年代的612mm,再提高到21世纪初年的628mm。全国森林厚度约由9.38mm,减少到20世纪70年代的1.03mm,再恢复到21世纪初的1.58mm。由于森林相对农田、草原、城市等土地利用类型具有更强大的蒸散作用,加之中国东南季风的特殊气候动力条件,致使中国的森林对降水有重要影响。森林厚度每变化1mm,则影响降水变化37mm。中国20世纪70-90年代出现的罗布泊干涸、青海湖水位降至历史顶点、黄河断流、土地沙漠化扩大等气候干旱现象不是偶然的,从大尺度上说,是我国近4000a来森林资源破坏达于极点而导致气候干旱化达于极点的必然结果。历史上森林破坏时空上的不均匀性可能导致了气候变化的阶段性和突变,280-1230年,受人口南迁和人口数量增长的影响,中国南方亚热带和热带森林受到大规模破坏,致使在此期间中国气候表现为持续干旱化。可见我国南方森林影响全国尤其是北方气候变化的作用值得重视。治理中国北方地区的沙漠化在根本上要靠森林资源的增加,不仅要提高森林覆盖率,更要增加森林厚度。中国土地沙漠化加剧的主要原因在于气候的干旱化,而气候干旱化的主要原因则是中国森林资源的减少。在未来,只要中国森林的厚度增加了,全国年均降水量就会增加起来,中国的沙漠化就会得到治理,西北地区大面积土地的生产潜力就会逐渐得到挖掘和发挥。

The changes in terrestrial forest vegetaton at different historic periods are an important factor related to climate change.With forest thickness as the indicator to measure the change in forest vegetation,the paper studied the quantitative relationship between forest changes and precipitation in the past 4000 years in China by historical data deduction,theoretical derivation and statistical analyses.The results showed that in the past 4000 years the average annual precipitation in China had decreased from 918mm to 612mm in the 1970s-1980s,and then increased to 628mm in the early 21st century,while the forest thickness had decreased from 9.38mm to 1.03mm in the 1970s,and then rose to 1.58 mm at the beginning of the 21st century.As forests have a stronger evapotranspiration capacity compared with other land uses such as farmland,grassland and urban land,and China is located in the area under the special southeast monsoon climate at that,it may be concluded that the forests in China have imposed great impacted on precipitation change.Generally,every 1mm change in forest thickness can lead to 37mm change in precipitation.In such sense,the frequent droughts in the 1970s-1990s did not occur by chance,e.g.Luobu Lake dried up,the historically lowest water level appeared at Qinhai Lake,the Yellow River has saw zero water flow for 22 times,more severe land desertification,etc.At a large scale,these drought can be regarded as necessary results since the extreme forest destruction in the past 4000 years has triggered the climate change and finally the aridification.The heterogeneity of deforestation in temporal and spatial dimension in history may lead to the phase-based occurrence and mutation of climate change.Between the years of 280 DC-1230 DC,the subtropical and tropical forests in the southern part of China had suffered from the large-scale destruction as a result of population migration to the south and demographic growth,and then there appeared a continuous aridification.This example demonstrates that much more attention should be paid to the impact of forests in southern China on climate change in the whole country,especially in the northern part.Aridification has resulted from the decline of forest vegetation,while it is a major factor to worsen land desertification in China.Therefore,to combat the desertification in northern China,it is necessary to vigorously increase forest resources in terms of both the forest coverage and the forest thickness.It is expected that in the future,as long as the forest thickness increases,the average precipitation per year will rise,the desertification in China will be controlled,and the production potential of large-area land in northwestern China will be tapped and developed gradually.