Estimation of Global Absorbed Photosynthetically Active Radiation (APAR) using remote sensing data

The photosynthetically active radiation (APAR) absorbed by vegetation is one of the most important parameters for assessing the growth status of vegetation, and it is of great significance to the study of global carbon cycle and climate change. Based on the fraction of photosynthetically active radiation absorbed by vegetation (FPAR) data from MODIS, solar shortwave radiation data (SSRD) data released by ECWMF-ERA5 and CASA model, the photosynthetically active radiation (APAR) absorbed by vegetation with a spatial resolution of 0.05° and a temporal resolution of 8 days during 2003-2019 is estimated, and the results reflect both the temporal and spatial distribution well of photosynthetically active radiation absorbed by vegetation in the global region. The estimated APAR was verified by the flux data and meteorological data from AmeriFlux network. The average $\mathbf{R}^{\mathbf{2}}$ between the estimated APAR values and the site values at different sites was 0.613, the average RMSE was $23.186 \mathrm{w} / \mathrm{m}^{2}$, and the average Bias was $-9.819 \mathrm{w} / \mathrm{m}^{2}$. There’s a significant logarithmic correlation between site air temperature (TA) and the estimated APAR, the average $\mathbf{R}^{\mathbf{2}}$ is higher as $\mathbf{0. 7 7 0}$. On this basis, this paper further analyzed the temporal and spatial changes of global APAR and China APAR, and the results show that: (1) The annual average values of global APAR and China APAR were $33.907 \mathrm{w} / \mathrm{m}^{2}$ and $22.774 \mathrm{w} / \mathrm{m}^{2}$ from 2003 to 2019, respectively. (2) APAR showed an increasing trend in both the global world and China, and the increasing trend in China was greater than global world (the growth rate of APAR was $0.136 \mathrm{w} / \mathrm{m}^{2} / \mathrm{y}$ and 0.046 $\mathbf{w} / \mathbf{m}^{2} / \mathbf{y}$ respectively in China and the global world). (3) The global APAR showed an overall increasing trend during the period of 2003-2019, with significant increases in southern Europe and central Asia, while central Africa and Northern South America is showing a downward trend. Overall, this paper provides a new dataset of global APAR and will be useful for satellite remote sensing research related to APAR.