The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report on Climate Change reported that during the past half-century, almost all regions across the globe experienced warming [1
], with the fastest warming region shown to be the mid-latitudes of the Northern Hemisphere [2
]. The report further showed that global climate change is caused by a combination of both natural and man-made factors. However, human activity is very likely to be the main reason for global warming since the mid-20th century, with a likelihood of more than 95% [1
]. Rising temperatures accelerate global water and carbon cycles, exacerbate extreme hydrological and climatic events and lead to worldwide redistributions of water resources and ecosystems at various scales [3
]. Moreover, ecological conservation and restoration are necessary to mitigate the effects of environmental degradation, and China has contributed significantly to such actions [4
]. Determining exactly how global climate change and ecological conservation affect the eco-environmental balance has been an important research focus for the IPCC, the International Geosphere-Biosphere Program (IGBP), the Future Earth-International Council for Scientific Unions (ICSU) and other international organizations and projects. In China, during the past few decades, the regional climate in the western region has become warmer and wetter [5
], and this trend is likely to continue [3
]. Owing to the contribution from meltwater, the water system in the arid and semi-arid regions of western China is highly fragile in the context of global and regional warming. In addition to the impact on water resources, climate change can influence other aspects of eco-environments, such as land degradation and soil erosion [8
], carbon cycles [9
], vegetation growth and productivity [10
], glaciers, permafrost and snow cover melt [3
The Three-Rivers Headwater Region (TRHR) on the Tibetan Plateau, China, is located in the mid-latitudinal regions of Eurasia and is one of the most globally-sensitive areas regarding climate change responses [3
]. Owing to its unique geographical location, rich natural resources and ecological importance, this region serves as a critical natural buffer on the Tibetan Plateau, China [8
]. However, owing to the high altitude and harsh natural conditions, its eco-environment is highly fragile. Recently, this eco-environment has undergone significant changes owing to the impact of climate change and increased human activities. For example, glacial retreat, rising snow lines, grassland degradation and a decline in water conservation capacity pose direct threats to the ecological balance in the TRHR [8
]. In view of this background, the state council promulgated and implemented the Overall Planning of Eco-environment Protection and Construction in TRHR, also known as the TRHR Project, in 2005.
Domestic and international researchers have conducted systematic and in-depth studies at both regional and local scales on eco-environmental changes in the TRHR, including studies on grassland degradation [8
], vegetation cover change [10
], land use and cover change [17
], soil organic carbon [18
] and streamflow regulating functional change [19
]. However, this research has focused on eco-environmental changes after the implementation of ecological projects. In other words, although dynamic monitoring and effectiveness assessments of single and multiple ecological projects have been conducted, few analyses exist that have focused on the causes of eco-environmental change and on how results from regional-scale variations might be understood in the context of larger-scale climate change. In addition, most of the research has focused on recent years, with little information available on eco-environmental changes prior to 2005 and few studies encompassing more than 30 years of data. Several important eco-environmental phenomena in the TRHR, such as soil erosion, sandstorm events and water temperature changes, have not been adequately analyzed and understood. Therefore, there is an urgent need for in-depth investigation on eco-environmental fluctuations and responses to climate change, which will play a crucial role in future sustainable development.
Therefore, the objectives of this study are: (1) to quantitatively assess the eco-environmental changes over the past half-century in the TRHR through observations and model simulations of, for example, water resources, the water environment, soil erosion, sandstorm events, vegetation growth and glacier and snow-cover melting; (2) to investigate the influential factors for eco-environmental changes; and (3) to discuss the policy and practical implications of eco-environmental changes and to provide a reference for eco-environmental policy-making and planning. The results of these analyses will allow an assessment of the following factors: (1) the coexistence of positive and negative eco-environmental impacts in the TRHR due to climate change; (2) the eco-environmental changes in the TRHR most relevant to climate fluctuations; and (3) the heterogeneity of large-scale climatic and eco-environmental change.
6. Conclusion and Implications
The results of this study are summarized in the following points:
(1) In the TRHR, temperature and precipitation experienced sharp increases during the past 57 years. The precipitation trend changed in 1990 and has since been in a state of high volatility. The rate of the temperature increase, initially observed in 1990, has remained highly volatile and has accelerated. The dramatic rise in winter temperatures is an important reason for the increase in the average annual temperature.
(2) During the past 57 years, annual runoff in the LRB and YARB showed increasing trends, whereas the trend in the YRB declined slightly. Runoff is mainly influenced by rainfall, which is jointly controlled by several monsoon systems. The water temperature anomalies at Tangnaihai station in the YRB and at Batang station in the YARB increased significantly (p < 0.001) during 1958 to 2007, driven by air temperature changes.
(3) Owing to warming and wetting trends in the TRHR, which provide sufficient water and heat for vegetation growth, the NPP during 1956 to 2012 and the NDVI during 1982 to 2012 showed significant increasing trends with slopes of 13.53 kg C/(hm2 a) (p < 0.001) and 0.0025/a (p < 0.001), respectively.
(4) During 1956 to 2012, the rainfall erosivity presented a significant increasing trend (p < 0.01). The water erosion amount increased in 2000 to 2012, as did the erosion per unit area (500 to 2500 t/km2, an increase of about 10%). The wind speed saw a significant declining trend (p < 0.05), which caused wind erosion to decline and reduced the frequency and duration of sandstorm events.
(5) During 1956 to 2012, the >0 °C annual cumulative temperature and summer FLH increased significantly, showing slopes of 16.28 m/a and 7.30 °C/a (p < 0.001), respectively. The glacier runoff therefore presented an obvious increasing trend, with the highest value appearing in the 2000s. The contributions of the glacier runoff to the total runoff were 6.7% in the LRB, 1.6% in the YRB and 11.7% in the YARB.
Considering the climate variability and vulnerability in the TRHR eco-environment, this region is of great significance for investigating the regional eco-environment under the conditions of climate change. The hydrological cycle in this region has particular importance. Thus, adequate planning is necessary, as is active response and adaptation to the possible effects of future climate change, particularly the effects on water resources, to ensure sustainable development and ecological safety for the TRHR. Moreover, the current ecological restoration is the joint effect of ecological engineering and climate fluctuations, but it is a purely local and temporary restoration, not an overall or a fundamental improvement. In order to secure informed policy-making in the layout, development, utilization, protection and management of natural resources in a reasonable and sustainable way, environment managers should pay particular attention to the eco-environmental changes caused by climate change.