We studied the effects of climate and land-use change on the global terrestrial carbon cycle for the 21st century. Using the process-based land biosphere model (LPJmL), we mechanistically simulated carbon dynamics for natural and managed lands (agriculture and forestry) and for land-use change processes. We ran LPJmL with twelve different dynamic land-use patterns and corresponding climate and atmospheric CO2 projections. These input data were supplied from the IMAGE 2.2 implementations of the IPCC-SRES storylines for the A2, B1, and B2 scenarios. Each of these SRES scenarios was implemented under four different assumptions on spatial climate patterns in IMAGE 2.2, resulting in twelve different Earth System projections. Our selection of SRES scenarios comprises deforestation and afforestation scenarios, bounding a broad range of possible land-use change. Projected land-use change under different socio-economic scenarios has profound effects on the terrestrial carbon balance: While climate change and CO2 fertilization cause an additional terrestrial carbon uptake of 105–225 PgC, land-use change causes terrestrial carbon losses of up to 445 PgC by 2100, dominating the terrestrial carbon balance under the A2 and B2 scenarios. Our results imply that the potential positive feedback of the terrestrial biosphere on anthropogenic climate change will be strongly affected by land-use change. Spatiotemporally explicit projections of land-use change and the effects of land management on terrestrial carbon dynamics need additional attention in future research.