# Detection and Attribution of Greening and Land Degradation of Dryland Areas in China and America
## 1 Introduction  
Land degradation is a global issue that affects many countries, and vegetation growth is an important indicator for it [1]. However, regional vegetation dynamics are still poorly understood. Dryland areas, which are home to 41% of the global population, are particularly vulnerable to external forcing, such as anthropogenic climate change (ACC) and land use (LU) actions [2, 3]. These factors have already led to the expansion of arid areas around the world. Additionally, the rising atmospheric CO\({}_{2}\) concentration promotes greening, particularly in water-limited areas [4]. Land degradation is a significant environmental issue in both China and America, with negative impacts on the ecosystems, economy, and society [5]. By studying land degradation in these regions, we can contribute to understanding the underlying causes and potential solutions to this problem. Additionally, both China and America have implemented policies and programs aimed at mitigating land degradation. By studying these regions, we can evaluate the effectiveness of these policies and provide insights for future policy development and implementation [6, 7]. During the past decades, China andAmerica have experienced different degrees of climate change. For China, it has experienced remarkable climatic warming [8]. It has been studied that this climatic warming contributed to Chinese vegetation growth by extending the vegetation growing season and increasing the summer photosynthesis rate [9; 10]. However, climatic warming accelerates surface evaporation, which affects the water availability of vegetation, inducing land degradation especially in dryland areas. For American dryland, grass predominated in the middle and end of the 19 th century in New Mexico. However, the area shifted from grass-dominated to shrub-dominated habitats in the end of the 20 th century [11]. The amount of grass decreased to 7% and this situation led to the change in ecosystem structure and function [12]. Grass depletion resulted in nutrient loss and surface erodibility [13]. Although a number of studies have been conducted to research the drivers of shift, overgrazing along with nutrient depletion is regarded as a cardinal driver for land degradation in Chihuahuan Desert [14]. Research has shown that dryland ecosystems, including those in Chinese and American regions, are not in a state of equilibrium [1]. Vegetation dynamics in these areas tend to be nonlinear and sensitive to external forces, predominantly resulting from two factors: (1) climate change caused by anthropogenic activities, which leads to changes in spatiotemporal precipitation and temperature and affects water availability. Rising atmospheric CO\({}_{2}\) levels also promote vegetation water use efficiency and the greening process of the world [15]. (2) Land use actions such as grazing, cropping, and deforestation [16; 17] have also caused a significant part of land degradation. Anthropogenic climate change and land use have already resulted in the expansion of arid areas worldwide [18]. However, the key driver of increased vegetation productivity is the rising CO\({}_{2}\) concentration. As CO\({}_{2}\) levels increase, plants promote carbon fixation rates, and the non-radiative effects of increasing CO\({}_{2}\) concentration on photosynthesis and biomass production are referred to as CO\({}_{2}\) fertilization effects. Previous studies suggest that these effects have a positive impact on leaf area index (LAI) enhancement, particularly in water-limited areas during the early stages of vegetation development [19; 20; 21]. Currently, the development of satellite-based observational data can readily provide us with spatial and temporal information on vegetation growth [22; 23]. However, quantifying the individual contributions of different driving factors, such as climate change (CC), climate variability (CV), CO\({}_{2}\) fertilization, and land use (LU), remains a challenge.