# Climate, CO2, and Anthropogenic Drivers of Accelerated Vegetation Greening in the Haihe River Basin
###### Abstract  
Vegetation regulates the exchange of terrestrial carbon and water fluxes and connects the biosphere, hydrosphere, and atmosphere. Over the last four decades, vegetation greening has been observed worldwide using satellite technology. China has also experienced a notably widespread greening trend. However, the responsiveness of vegetation dynamics to elevated CO2 concentration, climate change, and human activities remains unclear. In this study, we attempted to explore the impact of natural (precipitation, air temperature), biogeochemical (CO2), and anthropogenic drivers (nighttime light, afforestation area) on changes in vegetation greenness in the Haihe River Basin (HRB) during 2002-2018 at the county-level. We further determined the major factors affecting the variation in satellite-derived normalized difference vegetation index (NDVI) from moderate resolution imaging spectroradiometer (MODIS) for each county. The results indicated that over 85% of the counties had a significantly increased NDVI trend, and the average linear trend of annual NDVI across the study region was 0.0037 per year. The largest contributor to the NDVI trend was CO2 (mean contribution 45%), followed by human activities (mean contribution of 27%). Additionally, afforestation was a pronounced driving force for NDVI changes in mountainous areas, resulting from ecosystem restoration efforts. Our findings emphasize the crucial role of CO2 fertilization in vegetation cover change, while considering CO2 concentration, climate change, and human activities, and shed light on the significant influences of afforestation programs on water resources, especially in mountainous areas. normalized difference vegetation index; afforestation; vegetation restoration; human activities; climate change +
Footnote †: journal: Remote Sens. 0000-0002-0001-9999-2299-29  
with biogeochemistry, climate, and human-induced land-use change [13]. Biogeochemical drivers are expressed by CO\({}_{2}\) fertilization and nitrogen deposition [14].Temperature, precipitation, and radiation are the factors representing regional climate change. Land cover change and land management practices, such as irrigation, fertilization, afforestation, and grazing are important land-use-related drivers [15]. However, because the causes of vegetation greenness change are extremely intricate and involve numerous interacting environmental variables and irregular human land-use patterns, taking all these driving variables into account is still challenging. The Intergovernmental Panel on Climate Change sixth assessment report (IPCC AR6) stated that increases in CO\({}_{2}\) concentration due to anthropological reasons have been unequivocal since around 1750, and this concentration in the atmosphere is rising continuously [16]. As a substrate for photosynthesis, elevating atmospheric CO\({}_{2}\) concentration can accelerate photosynthesis by enhancing the rate of carboxylation [7; 17]. However, the debate about the relative roles of CO\({}_{2}\) fertilization, warming, and land-use management on greening is still ongoing, due to the limited number of models that involve the entire process and consider all the parameters [9; 15; 16]. Moreover, it is difficult to choose indicators that can accurately characterize human activities, due to high variations in anthropological activities. Previous studies often selected gross domestic product (GDP) and population, although they have strong collinearity. A recent study claimed that the best indicator of human activities was nighttime light (NTL) [18]. Generally, NTL is regarded as the indicator that reflected the effects of urban expansion and electricity consumption [19], facilitating a variety of studies related to monitoring long-term dynamics of human and anthropogenic activities. It is insightful and practical to include NTL into the investigation of vegetation greening.