摘要
单独用中世纪暖期(公元约1000～1300年)记录进行研究所得到的通常结论是,20世纪目前阶段出现的变暖并不罕见,因此不能把这种变暖看作由温室气体排放所致的强迫气候变化的象征。已发表的北半球气候变化的综合资料不支持上述结论,但由这些综合资料所得出的结论常常被忽视或受到责难。我们在本文中用以前半球资料汇编中没有使用过的另外的综合时间序列来重新探讨这种争论。另一点不同是,我们的重建所用的记录长度只有900～1000年,从而避免了使用不同时段的不同记录所起的潜在不确定性问题。尽管在某些单组记录中有中世纪变暖大于目前变暖的明显证据,但上述新的半球综合资料支持先前作出的半球温度重建的基本结论,而且指出,中世纪变暖的极大值局限于2至3个20～30年的时段,这些时段内的综合值与20世纪中叶变暖时段的值相比仅仅是不相上下而已。由于在确定不同区域变暖时间时存在着显著的偏移即不重合,因此不能证实在综合资料中始终存在着大于目前值的半球变暖;如果忽视这些偏移,就可能在推断中世纪变暖幅度以及说明它与20世纪末变暖的关系时导致严重误差。
A frequent conclusion based on study of individual records from the so-called Medieval Warm Period (-1000-1300 A.D.) is that the present warmth of the 20th century is not unusual and therefore cannot be taken as an indication of forced climate change from greenhouse gas emissions. This conclusion is not supported by published composites of Northern Hemisphere climate change, but the conclusions of such syntheses are often either ignored or challenged. In this paper, we revisit the controversy by incorporating additional time series not used in earlier hemispheric compilations. Another difference is that the present reconstruction uses records that are only 900-1000 years long, thereby, avoiding the potential problem of uncertainties introduced by using different numbers of records at different times. Despite clear evidence for Medieval warmth greater than present in some individual records, the new hemispheric composite supports the principal conclusion of earlier hemispheric reconstructions and, furthermore, indicates that maximum Medieval warmth was restricted to two-three 20-30 year intervals, with composite values during these times being only comparable to the mid-20th century warm time interval. Failure to substantiate hemispheric warmth greater than the present consistently occurs in composites because there are significant offsets in timing of warmth in different regions; ignoring these offsets can lead to serious errors concerning inferences about the magnitude of Medieval warmth and its relevance to interpretation of late 20th century warming.
