What is the particularity of grasslands?
Grasslands around the world form a complex biotope. They play a major role in climate change by absorbing and releasing CO2 through its many processes. Today, natural and lightly grazed grasslands account for 80% of the total cumulative carbon sink of the world's grasslands. They are now widely used in industry, especially for grazing: grass fodder feeds nearly half of the world's livestock. However, overgrazing causes serious damage to the soil. Moreover, the animals, the manure or the fertilizers deposited produce greenhouse gases.
In the last century, the number of domesticated ruminants increased from 1.4 to 3.4 billion, leading to a trend towards intensification of grasslands. At the same time, hunting, disease or the division of their habitat has caused a massive disappearance of wild herbivores.
Dr. Jinfeng Chang, now at Zhejiang University in China, attempts to understand the role of grasslands in climate change, and the consequences of human action through his paper “Climate warming from managed grasslands cancels the cooling effect of carbon sinks in sparsely grazed and natural grasslands”, published in Nature Communication. While intensification of grassland management increases CH4 emissions from livestock, the loss of wildlife results in lower emissions.
Image: Complex processes and the greenhouse gas fluxes that are accounted for in “Climate warming from managed grasslands cancels the cooling effect of carbon sinks in sparsely grazed and natural grasslands”
What are the consequences of human actions on grasslands?
Looking at the greenhouse gas balance of the world's grasslands from 1750 to 2012, the researcher and his colleagues note a decline in the leaf area index in the western United States, southern Brazil, Argentina and Australia. This is due to decreased precipitation and overgrazing: plants grow less well because they are eaten, and do not have enough leaves or reserves to grow back.
Since the 1860s, overpopulated areas have steadily increased, and with them methane and nitrogen dioxide emissions, which have more than doubled since 1750. The increase in livestock numbers, manure replenishment, and the addition of nitrogenous fertilizers and minerals are responsible for this.
The study, also relayed by IIASA, shows that greenhouse gas emissions have different origins in different countries: South America and East and Southeast Asia owe their carbon emissions to the transformation of forests into pastureland; carbon emissions due to the transformation of grassland into cropland are caused mainly by North America, Europe and South Asia.
Moreover, the environmental impact is different in different countries. In North America, Russia and Oceania, it is low because grasslands are estimated to store carbon well in the soil. Grasslands in sub-Saharan Africa also have a low impact because of the massive decreased number of wild grazers. In contrast, the high impact of South Asian grasslands is due to the increase in livestock and the conversion of grasslands to cropland. Finally, Latin America produces large methane emissions from livestock and NO2 from fertilizers or manure deposition, but deforestation, which releases large amounts of CO2 into the air, remains their biggest problem.
Understanding the origin of greenhouse gases is a first step to limit them
Research has shown the link between livestock and greenhouse gas emissions. When livestock numbers are reduced, not only are methane and carbon dioxide emissions reduced, but carbon in the soil can also be fixed more efficiently.
Prior to this article, the effects of grasslands on climate change had never been highlighted. Yet the effects of this biotope are essential to understand in order to better comply with the Paris Agreement, whose goal is to limit global warming.
With the increase in demand for livestock products, such as milk for example, comes the increase in methane and nitrogen dioxide released into the air. At this point, overgrazed grasslands have shifted from being a carbon sink or carbon neutral to a source of greenhouse gases due to intensification,mismanagement, and overgrazing, which is why it is important to restore degraded pastures, stop deforestation, and go easy on the consumption of animal products.