Changes in global carbon use efficiency in the 21st century and the potential controlling factors

Extensive studies have demonstrated the spatiotemporal changes in carbon use efficiency (CUE) and its driving factors over the past three decades. However, how the global CUE will change and to what extent the CUE is affected by the dominant factor in this century is still unclear. Herein, based on CMIP6 model outputs, we estimated the situation and change trends of CUE in baseline (1982-2014) and future (2015-2100), and identified the controlling factor of CUE variation by boosted regression tree. Further, we predicted the CUE-controlling factor sensitivity (S value, referring to higher/lower controlling factor producing more/less CUE) and its variation under four representative pathways, and revealed the relationship between S value and social economy. Results showed decreased CUE at the end of the 21st century, especially in the SSP5-8.5, its decline rate of CUE is 1.2 x 10-2 +/- 5.2 x 10-4/decade, which is 10 times higher than that in the SSP1-2.6. Spatially, 56.9%, 74.5%, 83.1%, and 88.6% of the global land will exhibit a decreased CUE under the SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios, and primarily concentrates at the middle-high latitudes of the Northern Hemisphere (30 degrees-60 degrees N). Except in Africa, temperature is the controlling factor for CUE variation, and the S value decreases over time, indicating an enhanced inhibitory effect of temperature on CUE. The turning time of S value change will advance with increases in CO2 emission, presenting prolonged high-temperature stress of vegetation ecosystem under high-emission scenarios. A threshold effect can be found between S value change and precipitation, and the precipitation threshold is higher under the SSP5-8.5 scenario. The negative effect of temperature on CUE is attenuated by economic development and population control but this effect diminishes with rising CO2 concentrations; in the future, developing clean energy and formulating population management policies can be used to enhance the carbon sink ability of the global ecosystem.