# Evidence of Carbon Uptake Associated with Vegetation Greening Trends in Eastern China
## 1 Introduction  
Greening trends appear over much of the global land area [1], indicating changes in ecosystem function at regional scales. Atmospheric carbon dioxide (CO\({}_{2}\)) fertilization, temperature, and precipitation changes account for most of the greening of the Northern Hemisphere (NH) over the past two decades [2; 3; 4]. Terrestrial greening in cold climate zones is considered primarily due to warming temperature [5]. Greening may also be caused by the precipitation changes in cool-arid systems like Northern China and Mongolia [6]. In addition to changing climate, vegetation greeningcan also be related to forest regrowth following the abandonment of agricultural lands or plantations [7]. The drivers of greening are fairly well understood and correspond to changes in photosynthetic CO\({}_{2}\) uptake [2; 4]. However, relating these changes in vegetation indices to changes in net carbon fluxes is challenging, because they are largely insensitive to CO\({}_{2}\) release by respiration fluxes [8]. Large-scale land management in certain areas, including agricultural expansion and intensification and afforestation and reforestation efforts, is expected to alter net carbon uptake at the local to regional scales. China's land management policies have promoted both of these practices in recent decades [9; 10; 11; 12; 13]. Globally, agricultural production has intensified from the last century to maintain food production for population growth [14; 15]. Mixed influences on carbon fluxes have been shown, for example, increased carbon uptake due to longer growing seasons [16] or intensification [17] but stronger carbon emissions from the conversion of natural systems to cultivated crops [18]. Sustainable cropland intensification is necessary for China using available arable land and water [10]. If done properly, it has the potential to reduce greenhouse gas emissions, through careful management of soil, nutrients, and water [19]. Forests are significant contributors to the global terrestrial carbon sink [20], absorbing approximately one-quarter to one-third of the anthropogenic CO\({}_{2}\) emissions. Planted forests (afforestation and reforestation) may be used to offset anthropogenic CO\({}_{2}\) emissions [21]. Planted forests across China increased significantly at the end of 20 th century, increasing the carbon density from 15.3 to 31.1 mega-grams per hectare [22]. Forests planted by China's Grain for Green Program, started in 1999, are estimated to have increased natural carbon sequestration, thereby mitigating CO\({}_{2}\) emissions from fossil fuel and cement production [12; 13] by about 3-5% of China's annual carbon emissions (relative to 2010 emissions), which may continue for next 40 years as these forests continue to grow [11]. Even though the precise forest area change is difficult to estimate, the terrestrial carbon sink increase over China is generally accepted [9; 23], with a net sink in the range of 0.26 Pg C per year absorbing 28-37% of the country's cumulated fossil carbon emissions during the 1980s and 1990s [9; 23; 24]. In addition to vegetation indices, atmospheric CO\({}_{2}\) concentrations using column-averaged CO\({}_{2}\) dry air mole fraction (XCO\({}_{2}\)) provide an alternative way to investigate terrestrial biosphere carbon flux changes [25]. XCO\({}_{2}\) represents the total amount of CO\({}_{2}\) in a vertical column through the atmosphere, and therefore is less sensitive to changes in boundary layer height and surface winds that impact a concentration measurement near the surface without necessarily a corresponding change in local CO\({}_{2}\) fluxes. Regional CO\({}_{2}\) fluxes have been inferred using satellite observed XCO\({}_{2}\) and atmospheric transport models, even though some challenges exist, like satellite observations coverage, systematic errors of the retrievals, and systematic errors of the transport models [26; 27]. XCO\({}_{2}\) seasonal cycle changes are strongly correlated with vegetation indices from satellite observations from 2009 to 2014 [28; 29].