The box with an arrow in the top left of the figure presents the drivers that were used as input to the ORCHIDEE-GM v3.2. The processes and the greenhouse gas fluxes that are related to grassland are shown as red and blue arrows, respectively. The red-to-blue layout at the bottom of the figure indicates management intensity from high (red; intensively managed grassland) to low (blue; sparsely grazed and natural grassland).
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 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 greenhouse gas, 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. The net radiative forcing of all grasslands is currently close to neutral, but has been increasing since the 1960s. Here, we show that the net global climate warming caused by managed grassland cancels the net climate cooling from carbon sinks in sparsely grazed and natural grasslands. In the face of future climate change and increased demand for livestock products, these findings highlight the need to use sustainable management to preserve and enhance soil carbon storage in grasslands and to reduce greenhouse gas emissions from managed grasslands.
Authors: Jinfeng Chang, Philippe Ciais, Thomas Gasser, Pete Smith, Mario Herrero, Petr Havlík, Michael Obersteiner, Bertrand Guenet, Daniel S. Goll, Wei Li, Victoria Naipal, Shushi Peng, Chunjing Qiu, Hanqin Tian, Nicolas Viovy, Chao Yue & Dan Zhu
Ref.: Nature Communications, 12(1), 1-10, https://www.nature.com/articles/s41467-020-20406-7