Countries have long been making efforts by reducing emissions of greenhouse gases to mitigate climate change. In the agreements of the United Nations Framework Convention on Climate Change, involved countries have committed to reduction targets. However, carbon (C) sink by natural ecosystems has been difficult to quantify. Using a transient traceability framework, we quantified country-level C sequestration potential by natural terrestrial ecosystems by the middle of the 21st century based on simulations of 12 CMIP5 Earth System Models under RCP8.5. The top 20 countries that have the highest C sequestration potential has the potential to sequester 62 Pg C by the middle of this century. Among the top 20 countries, Russia, Canada, United States, China, and Brazil sequester the most. The dominant forces to drive carbon sequestration are changes in net primary production and C residence time. Our results highlight that model-based estimates of land C sequestration may potentially offset a substantial proportion of greenhouse-gas emissions, especially for countries with a large change in NPP and long inherent residence time.