# Review of Land Surface Albedo: Variance Characteristics, Climate Effect and Management Strategy
## 2 Literature Analysis of Surface Albedo
### Literature Analysis  
The 1714 papers focused on surface albedo were imported into bibliometric and visual analysis software HistCite [58] and into VOSviewer for citation statistics and analysis with a focus on publication volume, citations, research fields, citation connections, and keywords. Publications and total citations associated with surface albedo from 1954 to 2021 are shown in Figure 2, where the publications and citations show a similar increasing trend, especially after 2008. Increased research interest indicates the importance of surface albedo and strengthens confidence in its further investigation. The percentages of research fields are shown in Figure 3. A paper may be related to one or several research areas, and there are 3031 fields for all the 1714 papers. The meteorology atmospheric sciences is the top field that contributes to surface albedo (i.e.,  
Figure 2: Publications and total cited frequencies associated with surface albedo from 1954 to 2021. 20.0%). Meanwhile, much attention has also been focused with a proportion of larger than 4.7% in the next six fields, including environmental sciences ecology, geology, remote sensing, imaging science photographic technology, engineering and physical geography. Then, 12 fields account for 17.9% of total selected papers with a percentage from 0.5% to 3.0%, where different surface types (e.g., agriculture, water, ocean, forestry and building) and subjects (e.g., geochemistry geophysics, energy, optics, computer and materials) are included. Overall, surface albedo plays an essential role in various research areas, especially for atmospheric and environmental sciences. The relationships between citation connections, research hotspots and institutions were analysed with VOSviewer software. Citation connections are shown in Figure 4, where 538 papers were included with a citation frequency of greater than 30. There are six major cluster networks in total: (1) algorithms, products and validations of surface albedo over various land surfaces at regional and global scales, such as the MODIS surface albedo product [11], and climate analysis, such as urban heat islands (UHIs) [59] and carbon budgets [60] (red dots); (2) definition and inversion algorithms [1,12] (green dots); (3) application to climate analysis, such as drought [61] (blue dots); (4) sea surface albedo [62; 63] (light green dots); and (5) and (6) ice and snow albedo and their effects on climate feedback [64; 65] (cyan and purple dots). Research hotspots are shown in Figure 5, which presents the leading 148 items that occur more than 15 times among all 4304 keywords. We can see that the keyword \"albedo\" achieves the highest density, followed by \"climate\", \"model\" and \"MODIS\", which indicates numerous efforts made in surface albedo-induced climate feedback and inversion algorithms, especially based on the MODIS dataset. The next most significant keywords are \"BRDF\", \"algorithm\", \"product\", \"validation\", \"snow\", \"vegetation\" and \"temperature\". This shows the important role of primary BRDF characteristics in albedo estimation [11; 66; 67], and a series of algorithms and products have been developed in previous studies [13]. Notably, albedos of snow [68] and vegetation [69] surfaces have also attracted much attention in global warming research, and surface albedo-induced climate feedback shows an effect on temperature change. In addition, surface albedo has also connected to various land-atmosphere mass and energy exchange processes, such as soil, ocean, mass balance, carbon, atmospheric aerosol, and precipitation processes. Figure 3: Pie chart of research fields associated with surface albedo. The statistics show that surface albedo has been studied for decades, and attention to the subject has increased, especially since 2008. Studies of various land types have been conducted, as shown in Figures 4 and 5, and the high use of the term \"climate\" indicates its close relationship to surface albedo.