Fig.1 Spatiotemporal trends of the NPP caused by urban expansion and climate variability between 2000 and 2010.

The terrestrial biosphere frequently acts as a carbon sink in the global carbon cycle. However, it may release a large amount of carbon dioxide and become a carbon source owing to some disturbances including droughts, urban expansion and other factors. As a crucial indicator of terrestrial ecosystems, terrestrial NPP reflects not only the production capability of vegetation but also the ecological processes directly (e.g., carbon source/sink). It is still uncertain what is the spatiotemporal details of terrestrial NPP change and their mechanisms. Many studies analyzed the responses of terrestrial NPP to urban land expansion and climate driving factors, respectively. Despite it, limited attempts addressed on their coupled impacts of the two factors on terrestrial NPP change, especially at the global scale. To solve this problem, Prof. Xiaoping Liu and his collaborators analyzed the spatiotemporal changes of global urban lands between 2000 and 2010 using a 30-m global land use dataset (i.e., GlobeLand30). In addition, terrestrial NPP was evaluated by combining MODIS NPP products (MOD17A3), the CASA model and the LPJ hydrology model. The relative contribution of global urban expansion and climate variability on terrestrial NPP were then discussed. It is found that global urban land expansion in past decade led to a decrease in terrestrial NPP (22.4 TgC year-1), which offset 30% of the NPP increase that caused by climate variability. The findings are of great significance to understand the complex mechanism of global terrestrial NPP changes.

This work was supported by the National Key Research and Development Program of China (NO. 2017YFA0604404), the Key National Natural Science Foundation of China (NO. 41531176), etc.