过去2000年大气甲烷含量与气候变化的冰芯记录

ATMOSPHERIC METHANE CONCENTRATION AND CLIMATE CHANGE RECORDED IN ICE CORES OVER THE PAST 2000 YEARS

温室气体与气候变化的关系是当前全球变化研究中的一个核心内容。目前关于大气温室气体含量变化只有几十年的实测资料，而冰芯包裹气体中的CH4不仅能反映过去大气CH4含量随时间的变化，而且能很好地揭示陆地CH4向大气中的释放随时间及空间的分布。近年来，极地冰芯研究表明南极和北极过去大气层中的甲烷含量差异很大，北极大气层中甲烷含量远大于南极大气层。科学家们推测，中低纬度地区是全球大气层甲烷含量变化的驱动源。而对这一可能驱动源的甲烷含量变化，很长时间人们一无所知。达索普冰芯记录揭示了中低纬度大气CH4含量与极地冰芯记录相同的变化趋势，并明确显示工业革命以来大气CH4含量的增长。高分辨率达索普冰芯记录的工业革命以来大气CH4含量变化最显著的特征是20世纪两次世界大战期间人类活动CH4排放的减缓使大气CH4含量呈负增长。中低纬度大气CH4含量的恢复使我们有机会与极地冰芯记录进行定量对比研究。0-1850A．D．中低纬度大气CH4的平均含量为782nmol／mol，与格陵兰和南极大气CH4平均含量差分别达66nmol／mol和109nmol／mol，并且其最大自然波动幅度超过200nmol／mol，这是极地冰芯记录从未有过的。达索普冰芯记录表明工业革命前中低纬度为全球大气重要的CH4源区，但最近1000a来，北半球中高纬度的排放有了显著的加强；过去2000a来的自然变化时期，气候变化的纬向差异对北半球不同纬度带CH4排放格局有重要影响。

The relationship between greenhouse gases and climatic change is a pivotal task for the research of global change at present. Greenhouse gas concentrations in the atmosphere have been documented directly only since several decades ago, but they can be reconstructed through ice core analysis. The combined analyses of ice cores from Greenland and Antarctica, have allowed to quantify a remarkable difference of methane mixing ratio between the two polar regions in the past, revealing that there existed a large pole to pole methane gradient, and the methane concentration in north latitudes was higher than in Antarctica. It is suggested that the tropical area was the source area for global methane in pre-industrial time. However, the lacking of constraint on methane mixing ratio in tropical to low-middle latitudes becomes the major limitation for the reconstruction of global methane concentration in the past. This study was designed to solve this problem by analyzing ice cores from the Dasuopu in the Tibetan Plateau. The Dasuopu record shows an obvious methane concentration increase in the low-middle latitudes since industrialization, revealing a similar trend of methane concentration fluctuation recorded in polar ice cores. The high resolution of details of the Dasuopu record over the last century reveals a significant period between the two World Wars when the methane increase caused by human being activities was halted. This sub-tropical methane record also allows quantifying for the first time the difference of methane mixing ratio between polar and sub-tropical latitudes during the pre-industrial time. The average methane concentration in the Dasuopu record is 782nmol/mol in the time interval of 0 - 1850A. D., and the maximum temporal variation exceeded 200nmol/mol which has never been found in polar ice records. In average it is 66nmol/mol higher than Greenland and 109nmol/mol higher than Antarctica, agreeing with the former suggestion that the lower latitude areas are the source areas of global methane in pre-industrial time. Our data shows however that for the past 1000 years, the emission of methane in mid-high latitudes increased largely. During the past 2000 years, the latitudinal difference of climate change in the North Hemisphere had a crucial impact on the terrestrial methane emissions.