对青藏高原末次冰盛期降温值、平衡线下降值与模拟结果的讨论

DISCUSSION ON TEMPERATURE LOWERING VALUES AND EQUIBRIUM LINE ALTITUDE IN THE QINGHAI-XIZANG PLATEAU DURING THE LAST GLACIAL MAXIMUM AND THEIR SIMULATED RESULTS

1999年 5期《第四纪研究》刊登的“青藏高原冰期环境与冰期全球降温”论文 ,内容丰富 ,提出了许多重大问题和新鲜讯息 ,发人深思。本文就部分内容进行讨论 ,认为根据新近若干冰期降温值较大的信息 ,就得出冰期降温幅度为过去认识的两倍的普遍性推断 ,还为时过早。青藏高原新近研究表明夏季降温值是较小的 ;冰川平衡线高度取决于以夏季温度为标志所提供的消融热量与全年降雪积累量的平衡 ,单纯就降温值 ,不能决定平衡线下降值 ;全球各地气候、地形差别很大 ,冰期变化也很悬殊 ,不存在全球均一的 1 0 0 0m左右平衡线下降值 ,干旱区的下降值多低于此数。青藏高原末次冰盛期 (LGM)冰川堆积 (终碛、侧碛 )和侵蚀形态(冰斗、槽谷 )一般保存良好、形态鲜明 ,较易识别 ,近年已获取测年资料 ,证明过去地貌法判别的LGM冰川规模 ,并以此决定的LGM平衡线位置基本恰当 ,当然也有误判者。从 2 0世纪 2 0年代起 ,研究者逐渐认识高原内部主要是中西部LGM平衡线下降值较小 ,经过多次改进的LGM平衡线高度分布图显示约有 1 /3左右地区 ,平衡线下降值不足 30 0m ,实为世所罕见。其原因可能主要在高原西北部是全球惟一高海拔寒旱核心区域 ,为极大陆型或极地型冰川分布区 ,以现代西昆仑山崇测冰川、古里雅冰帽为例 。

The paper 'Qinghai Xizang Plateau Glacial Environment and Global Cooling ' by Liu et al . published in Quaternary Research , No.5. 1999 contains abundant contents and new information and put forward several important problems worthy of deep thought. I examine some of the contents and have some different ideas. Liu et al . think that the temperature lowering value during the last glacial may be twice as high as what was believed in the past, but I think it is too early to put forward this summary. Recent studies show that the summer temperature lowering value over the Plateau during the last glacial was small. The equilibrium line altitude (ELA) of glaciers depends on the balance between the ablation heat provided by summer temperature and the annual snow accumulation. Simple temperature lowering values cannot determine the ELA during the LGM. Climates and terrains in different regions differ greatly; and changes of the ELA during the LGM are also quite different. Hence an even global ELA lowering value of about 1?000?m didn't virtually exist and specially in arid zones, and the real ELA lowering values were much lower than this value. Glacial deposits (terminal moraines and lateral moraines) and erosion features (cirque and trough valley) during the LGM in the Qinghai Xizang Plateau are generally well preserved and easy to identify with their conspicuous shapes. Recent dating data demonstrate that the LGM glacier scale and hence the ELA judged previously by the geomorphological method are basically proper, although there are also misjudged ones. Since the 1920s, researchers have gradually understood that the ELA lowering value during the LGM in the interior of the Plateau, especially in the middle and west part, was small. Through a number of investigations on the ELA during the LGM, it is demonstrated that the ELA lowering value was less than 300?m in about one third area of the Plateau. Such a phenomenon is rarely seen in the world. The reason for it may be that the northwestern part of the Plateau is the unique high, cold and arid core in the world, and therefore transforming a region of extreme continental type glaciers or polar type glaciers. Taking present Chongce Glacier and Guliya Ice Cap in the West Kunlun Mountains as the examples, their ELA reaches about 6?000?m a.s.l, the summer temperature is -2°～-3℃, the annual ablation excluding evaporation loss amounts to the water layer thickness of about 200?mm, the annual accumulation is about 250?mm, the glacier flow is very slow and mainly plastic in deformation. Both temperature and precipitation decrease during the last glacial period. Even if the temperature lowering value reaches 9℃, it is almost in the negative zone, and increased the cold storage. So the temperature loweing value has little influence on the ELA lowering value. Because the ELA lowering value is small and the glacial flow is characterized by ice deformation, the expansion of glaciers is largely limited. According to the climate vegetation model developed by Zhang Xinshi and Chen Xudong, as the temperature reduced by 7℃ and the precipitation decreased by 30%, the simulated ice and snow covered zone of the Plateau occupies one fifth of the Plateau's total area. This simulated value is quite close to the actual situation. Both geological data and simulation results rule out the possible occurrence of a unified ice sheet during the last glacial maximum.$$$$