## 4 Discussion  
The advent of SIF measured from space and the elucidation of the relationship between SIF and GPP constitute a milestone in remote sensing of the terrestrial carbon cycle [41]. We mapped GPP globally with fine spatial (0.05\({}^{\circ}\)) and temporal (8-day) resolutions from our recently produced GOSIF data (a new global, fine-resolution SIF product based on OCO-2 SIF) [31] and the SIF-GPP relationships that we established. Many studies have shown the strong potential of SIF in estimating GPP through analytical models, field measurements, or satellite data in recent years 1 [16; 17; 11; 18; 32; 33; 42; 43]. Some earlier studies showed strong SIF-GPP relationships based on coarse-resolution SIF (e.g., GOME-2 or GOSAT) and gridded GPP products [10; 21; 22; 44]. Several more recent studies found strong ecosystem-level relationships based on OCO-2 SIF and tower GPP [17; 26; 36]. Satellite-based SIF has also been recently used to constrain carbon-related model parameters [45; 22; 46] or identify regions of high productivity crops [47]. However, direct estimation of GPP from satellite SIF at the regional or global scale is challenging mainly due to the following three reasons. First, finer spatial resolution (e.g., several to tens of square kilometers) gridded SIF observations are currently not available. Second, although OCO-2 SIF with much smaller ground size provides a valuable opportunity to directly link satellite-measured SIF with flux tower GPP to examine the SIF-GPP relationship [8; 16; 17; 25; 26], generating finer-resolution GPP is hindered by the sparse global coverage of OCO-2 data. Third, the relationship between GPP and SIF for all the main biome types based on finer-resolution SIF data and high-quality flux tower GPP (rather than gridded GPP products) has not been established until very recently [17]. We mapped GPP globally based on the fine-resolution, OCO-2-based SIF product (GOSIF) that we recently developed [31] and SIF-GPP relationships that we established. We used both a universal SIF-GPP relationship across a wide variety of biomes and biome-specific relationships that we previously established [17]. We also examined how the use of the intercept versus zero-intercept and the spatial scale used to develop the SIF-GPP relationships (site level versus grid cell level) influenced GPP estimates. To account for the uncertainty in the resulting GPP estimates, we produced eight global GPP products based on eight established SIF-GPP relationships consisting of both site and grid cell level relationships and different forms of the SIF-GPP relationship (both universal and biome-specific, both zero and non-zero intercept). This analysis provides insight into how well GPP can be estimated based on the SIF-GPP relationship by validating the resulting GPP estimates against tower GPP from 91 EC flux sites, and also reveals how the uncertainty in GPP estimates propagates to the global scale. The SIF-GPP relationships with different forms had negligible influence on the estimated 8-day GPP at the grid cell level (or at the scale of a flux tower), while it could lead to a difference up to 400 g C m\({}^{-2}\) year\({}^{-1}\) in predicted annual GPP at the grid cell level or up to \(\sim\)20 Pg C yr\({}^{-1}\) in global annual GPP. The relatively large discrepancy in global annual GPP among products based on the different SIF-GPP relationships was mainly caused by the overestimation based on the SIF-GPP relationships with a non-zero intercept, which lead to a widespread increase in annual GPP for regions with lower productivity. For example, the intercept of 0.89 g C m\({}^{-2}\) day\({}^{-1}\)/W m\({}^{-2}\)\(\mu\)m\({}^{-1}\) sr\({}^{-1}\) for the site-level relationship gave rise to a difference of 19. 1 Pg C yr\({}^{-1}\) in global annual GPP compared to the site-level relationship without an intercept. By contrast, applying a universal versus biome-specific relationship resulted in a small difference in global annual GPP. The biases in both OCO-2 SIF and GPP data could alter the SIF-GPP relationships.