Grasslands are turning into a source of harmful emissions

According to a new study led by the International Institute of Applied Systems Analysis, human activities are transforming grasslands from a natural carbon sink into a source of greenhouse gas emissions. The researchers describe how grasslands used by humans have changed our climate over the last centuries.

Grasslands absorb and release carbon dioxide (CO2), and emit methane (CH4) from grazing livestock. They also emit nitrous oxide (N2O) from soils, and these emissions increase when manure or mineral fertilizers are introduced. 

Despite their significance, however, little is known about how the fluxes of these three greenhouse gases have contributed to climate change in the past. The researchers also wanted to compare the gas cycling and climate effects of managed pastures to that of natural grasslands.

To investigate, the experts quantified the changes in carbon storage and greenhouse gas fluxes in natural and managed grasslands between 1750 and 2012. 

The findings illustrate the important climate cooling service provided by sparsely grazed areas, as well as the growing contribution to climate warming from continuously grazed areas. These intensively exploited grasslands are associated with more CH4 and N2O emissions.

“We built and applied a new spatially explicit global grassland model that includes mechanisms of soil organic matter and plant productivity changes driven by historical shifts in livestock and the reduction of wild grazers in each region,” said study lead author Jinfeng Chang. “This model is one of the first to simulate the regional details of land use change and degradation from livestock overload.”

“We also looked at the effect of fires, and soil carbon losses by water erosion; CH4 emissions from animals; N2O emissions from animal excrement, manure, and mineral fertilizer applications; and atmospheric nitrogen deposition.”

The study revealed that CH4 and N2O emissions from grasslands increased by a factor of 2.5 since 1750. This is due to higher emissions from livestock that have more than compensated for reduced emissions from the shrinking number of wild grazers. 

The ability of grasslands to absorb carbon has grown over the last century over sparsely grazed and natural grasslands, but these gains are outweighed by emissions from managed pastures. Over the last decade, grasslands intensively managed by humans have become a net source of greenhouse gas emissions.

“Our results show that the different human activities that have affected grasslands have shifted the balance of greenhouse gas removals and emissions more towards warming in intensively exploited pastures, and more towards cooling in natural and semi-natural systems,” explained study co-author Thomas Gasser. “Coincidently, until recently the two types of grasslands have almost been canceling each other out.”

“However, 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.”

“In the context of low-warming climate targets, the mitigating or amplifying role of grasslands will depend on a number of aspects,” concluded study co-author Philippe Ciais. “This includes future changes in grass-fed livestock numbers; the stability of accumulated soil carbon in grasslands; and whether carbon storage can be further increased over time or if it will saturate, as observed in long-term experiments.”

The study is published in the journal Nature Communications.

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By Chrissy Sexton, Earth.com Staff Writer