Agricultural ponds emit nearly twice as much methane than is accounted for in national emissions inventories, a new study has found.
Small aquatic habitats emit disproportionately more methane per unit area than larger lakes, say researchers in Australia, who studied agricultural ponds—also called farm dams or impoundments—across their continent and in the United States. Though they cover less than 10% of the global freshwater area, these habitats produce 37% of global methane emissions.
Methane is a climate super-pollutant that carries about 85 times the warming potential of carbon dioxide over the 20-year span when humanity will be scrambling to get climate change under control.
“Many of these small systems are human-constructed to secure water for crops and livestock, and to support the ever-increasing demand for agricultural production,” states the study, published in the journal Nature Communications: Earth and Environment. “These recently discovered emissions are boosted by unusually high concentrations of fertilizer and manure runoffs, which increases organic matter and creates the ideal conditions for methane production.”
The typically shallow ponds also warm rapidly, boosting metabolic rates, bacterial build-up, and methanogenesis—the process by which microorganisms create methane.
Since agricultural ponds “are essential for water security,” and since “their density will continue to grow with rising global food demand,” it is important to develop cost-effective management solutions that can reduce their ecological and environmental impacts, the researchers say.
New guidelines from the Intergovernmental Panel on Climate Change state that agricultural pond emissions must be included in national greenhouse gas (GHG) inventories, but the researchers point to a knowledge gap, with little data on the number and distribution of these ponds. After estimating pond emissions with mapping data and modelling systems, and found that the 2.56 million agricultural ponds in the U.S. cover 420,000 hectares and emit nearly 96,000 tonnes of methane per year. In Australia, 1.79 million ponds cover 291,100 and emit 75,000 tonnes of methane each year. These values were underestimated by half in the countries’ national inventories, the study found.
The researchers say that may be partly because GHG reporting guidelines allow countries to separate methane emissions from animal manure contamination in agricultural ponds. “Unfortunately, national inventories lack details on the methods for accounting for manure in agricultural ponds.”
But “it is important to note that there are several sources of error in our calculations,” they add, including some parameters based on small sample sizes that create uncertainty. They suggest future studies improve current estimates with on-the-ground measurements.
The study did not capture emissions other than methane—such as carbon dioxide or nitrous oxide—and the estimates were based on 10-year temperature averages. Seasonal variability of pond surface areas and temperatures that affect methane emissions were left out of the calculations.
That means “the present work offers an initial assessment of methane emissions from agricultural ponds, but our results should only be taken as a first-order approximation,” the authors say.