Grasslands absorb and release carbon dioxide (CO2), emit methane (CH4) from grazing livestock and emit nitrous oxide (N2O) from soils. Little is known about how the fluxes of these three greenhouse gases, from managed and natural grasslands worldwide, have contributed to past climate change, or the roles of managed pastures versus natural grasslands. Here, global trends and regional patterns of the full greenhouse gas balance of grasslands are estimated for the period 1750 to 2012. A new spatially explicit land 30 surface model is applied, to separate the direct effects of human activities from land management and the indirect effects from climate change, increasing CO2 and regional changes in nitrogen deposition. Direct human management activities are simulated to have caused grasslands to switch from a sink to a source of GHG, because of increased livestock numbers and accelerated conversion of natural lands to pasture. However, climate change drivers contributed a net carbon sink in soil organic matter, mainly from the increased productivity of grasslands due to increased CO2 and nitrogen deposition. 37 The net radiative forcing of all grasslands is currently close to neutral, but has been 38 increasing since the 1960s. Here, we show that the net global climate warming caused by
39 managed grassland cancels the net climate cooling from carbon sinks in sparsely grazed 40 and natural grasslands. In the face of future climate change and increased demand for 41 livestock products, these findings highlight the need to use sustainable management to 42 preserve and enhance soil carbon storage in grasslands and to reduce GHG emissions 43 from managed grasslands .