Widespread concern over livestock produced-methane (CH4) emissions, as a major component of greenhouse gases (GHG)—often depicted in pie charts—, can be misleading, according to a UC Davis waste management specialist. A state inventory analysis indicates that methane associated with agriculture practices is 8% to 9% of the GHG equivalent, while carbon dioxide (CO2) accounts for upwards of 81% to 84.6% in terms of its global warming potential—depending on which data sources are used.
However, dairy and non-dairy livestock (beef and sheep) are responsible for only about 55% of the state’s methane inventory (55% of the 9%, or 4.95%). GHGs can build up in the atmosphere beyond the Earth’s capacity to remove them. This is what scientists say is contributing to global warming, as well as depletion of the ozone layer.
Looking at the composition of the atmosphere as a whole, nitrogen accounts for 78%, oxygen 21%, and argon. 0.9%, with gases like CO2, nitrous oxide, methane and ozone seen as trace gases that account for only about a tenth of 1% of the atmosphere.
Before the industrial revolution in the mid-1700s, natural emissions of heat-trapping gases matched or balanced, what could be absorbed in natural “sinks,” such as when plants absorb carbon dioxide as they grow and release it back into the atmosphere when they die.
However, records show that methane emissions have continued to increase, especially since 1990. While these emissions are expected to remain relatively constant in the future, they are still a significant issue for climatologists, environmentalists, the California Air Resources Board (CARB), and other regulatory bodies that have backed legislation to mandate overall GHG reductions.
“The challenge when discussing GHG is to understand what each piece of the pie charts actually describes and represents,” according to Deanne Meyer, Ph.D., livestock waste management specialist with the U.C. Davis Department of Animal Science.
She said the current approach in California’s SB 1383 legislation (enacted in September 2016) is to reduce emissions of short-lived climate pollutants, with methane targeted for a 40% reduction below 2013 amounts, “Assuming we have good values for 2013 emissions.”
Under Assembly Bill 32, passed in 2006, California must reduce its emissions to 1990 levels (431 million metric tons) by 2020. The 2016 Greenhouse Gas Emissions Inventory published in 2016 showed that California emitted 429 million metric tons of climate pollutants in 2016—a drop of 12 million metric tons, or 3%, from 2015.
In July 2016, CARB announced that GHG pollution in California fell below 1990 levels for the first time since emissions peaked in 2004. That year Governor Brown said, “The next step is for California to cut emissions below 1990 levels by 2030 – a heroic and very ambitious goal.” The 2030 deadline was set by Senate Bill 32 the same year.
For study comparisons, everything is converted to GHG equivalents. Methane is 28 times more potent than CO2, and N2O (nitrous oxide) is 280 times more potent. The big difference is that CO2 and N2O remain in the air far longer, while methane can be oxidized in the atmosphere in eight to 10 years, according to a CARB data inventory. About 90% of non-dairy livestock methane emissions are from enteric fermentation, with remaining emissions from manure management. Landfills are the next largest source followed by natural gas pipeline leaks, oil and gas extraction, wastewater and other industrial and miscellaneous sources.
“Enteric fermentation” is a digestive process by which carbohydrates are broken down by microorganisms into simple molecules for absorption into an animal’s bloodstream producing methane as a byproduct.
Given the level of government and agriculture industry concerns, several steps are being taken to reduce methane levels through a variety of innovative approaches. For example, some innovative dietary and nutritional practices for dairy cows have been shown to reduce methane, according to a PennState Extension report. They include:
1. Addition of ionophores (rumen modifiers) that improve dry matter intake efficiency and suppresses acetate production, thus reducing amounts of hydrogen released;
2. Use of high-quality digestible forages, and increased use of grains;
3. The grinding and pelleting of forages, which can reduce emissions by 40%;
4. Dietary fats have the potential to reduce CH4 up to 37% through biohydration of unsaturated fatty acids, enhanced propionic acid production and protozoal inhibition (removing protozoa can reduce CH4 emissions by 20%).
“The effectiveness of ionophores needs to be managed,” according to Dr. Meyer. “Rumen microorganisms adapt to the ionophore within six to eight weeks. As a result, producers need to take it out of the diet, then wait, then put it back in. The cost of adding grain (to organic dairy cow feed) will likely be very expensive and the methane reduction may not have a financial return for enteric emission reductions. Farmers strive to put up quality forages. However, for cattle grazing, weather conditions will have a huge impact on the quality of forage cattle consume.”
While California leads the nation in reducing GHGs, reductions alone will not be enough to reach targets set by the 2015 Paris climate accord. To accelerate the process, the Muir Institute of the Environment and UC Davis are engaged in a joint project to find shovel-ready solutions for carbon sequestration.
The consortium received a $4.7 million three-year grant from California’s Strategic Growth Council to research scalable methods for storing GHGs, such as CO2, in soil by using soil amendments such as rock, compost, and biochar. The consortium plans to conduct 29 treatment and control sites throughout the state, and it will also determine if this method can improve crop and rangeland productivity and health.
“We found that by adding organic waste, food waste, green waste, livestock manure, as compost amendments could save the equivalent of 28 million tons of CO2 using just 5% of California’s rangelands,” said UC Berkeley Professor Whendee Silver, lead scientist of the Marin Carbon Project, and co-principal investigator. Forests have long served as a critical carbon sink, but decades of fire control, warming temperatures and droughts have increased wildfire risks. Now, experts say forests have been turned from carbon sinks to carbon sources. A UC Davis study launched in 2018 and funded by the National Science Foundation, found that grasslands are more resilient carbon sinks than forests today.
“Our model simulations show grasslands store more carbon than forests because they are impacted by fewer droughts and wildfires,” said lead author Pawlok Dass, a postdoctoral scholar in Professor Benjamin Houlton’s lab at UC Davis.
He said, unlike forests, grasslands sequester most of their carbon underground, while forests store it mostly in biomass and leaves. In a stable climate, trees sequester more carbon than grasslands. When trees burn, stored carbon is released into the air, but when grasslands burn, carbon fixed underground tends to stay in the roots and soil, making it more adaptive to climate change. The study indicated that worldwide, grasslands are the only viable net CO2 sink through 2101.