Grasslands are managed worldwide to support livestock production, while remaining natural or semi-natural ones provide critical services that contribute to the wellbeing of both people and the planet. Human activities are however causing grasslands to become a source of greenhouse gas emissions rather than a carbon sink. A new study has uncovered how grasslands used by humans have changed our climate over the last centuries.
Grasslands are the most extensive terrestrial biome on Earth and are critically important for animal forage, biodiversity, and ecosystem services. They absorb and release carbon dioxide (CO2), and emit methane (CH4) from grazing livestock and nitrous oxide (N2O) from soils, especially when manure or mineral fertilizers are introduced. Little is known, however, about how the fluxes of these three greenhouse gases from managed and natural grasslands worldwide have contributed to climate change in the past, and about the role of managed pastures versus natural or very sparsely grazed grasslands.
To address this knowledge gap, an international research team quantified the changes in carbon storage and greenhouse gas fluxes in natural and managed grasslands between 1750 and 2012 in their study published in Nature Communications. The study’s comprehensive estimates of global grasslands’ contribution to past climate change illustrate the important climate cooling service provided by sparsely grazed areas, and the growing contribution to warming from quickly increasing livestock numbers and more intensive management ̶ which are in turn associated with more CH4 and N2O emissions ̶ in determining the contemporary net climate effect of the grassland biome.
The study shows that emissions of CH4 and N2O from grasslands increased by a factor of 2.5 since 1750 due to increased emissions from livestock that have more than compensated for reduced emissions from the shrinking number of wild grazers. The net carbon sink effect of grasslands worldwide – in other words, the ability of grasslands to absorb more carbon and pack it in the soil – was estimated to have intensified over the last century, but mainly over sparsely grazed and natural grasslands. Conversely, over the last decade, grasslands intensively managed by humans have become a net source of greenhouse gas emissions – in fact, it has greenhouse gas emission levels similar to those of global croplands, which represent a large source of greenhouse gases.
“The recent trends we see towards the expansion of pasture land and higher livestock numbers lead us to expect that global grasslands will accelerate climate warming if better policies are not put in place to favor soil carbon increases, stop deforestation for ranching, and develop climate-smart livestock production systems,” notes coauthor Thomas Gasser from the International Institute for Applied Systems Analysis (IIASA).
According to the authors, the cooling services provided by sparsely grazed or wild grasslands, makes it clear that countries should assess not only the greenhouse gas budgets of their managed pastures (such as specified in the current national greenhouse gas reporting rules of the UN’s Framework Convention on Climate Change), but also the sinks and sources of sparsely grazed rangelands, steppes, tundra, and wild grasslands. Full greenhouse gas reporting for each country could facilitate the assessment of progress towards the goals of the Paris Agreement and better link national greenhouse gas budgets to the observed growth rates of emissions in the atmosphere.
Image credit: Genta, flickr/Creative Commons
