Why Agriculture’s Greenhouse Gas Emissions Are Almost Always Underestimated

Agriculture’s contribution to the climate crisis is typically underestimated, according to experts, because of numerous emission sources that are routinely overlooked.
“The classic EPA chart suggested Agriculture is 9 percent of U.S. greenhouse gas emissions. This number’s almost certainly significantly quite low,” said Peter Lehner, managing attorney for EarthJustice, a non-profit environmental law firm. “Most other studies, including by the UN and others, say that agriculture contributes much closer to 15 or 20 percent or more of world greenhouse gas emissions.”
Even those estimates may be low, Lehner argued in a Washington D.C. briefing hosted by the Environmental and Energy Study Institute, because most assessments don’t include these five sources of agricultural emissions:
1. Soil Carbon
When experts assess emissions from agriculture, they rarely consider the carbon emitted by soil disturbance. “Good healthy soil has a lot of carbon in it,” Lehner said, “and when you dig it up you lose all that carbon. It merges with the air and becomes carbon dioxide…. As you continue to convert land—as some programs have incentivized—you continue to lose carbon dioxide carbon stored in the soil.”
2. Lost Sequestration
Most estimates don’t include the carbon sequestration that would be occurring on land had it not been converted to agricultural use. “It doesn't include all the legacy, that lost carbon sequestration, all the land that used to be sequestering carbon but now has been deforested or has been tilled and is no longer sequestering carbon.”
3. Input Footprints
Most estimates neglect to consider the carbon footprint of products consumed by agriculture. “It doesn't include the manufacture of a lot of the input such as fertilizer, which is highly energy intensive.”
4. Difficult Measurements
“It's tougher to measure because unlike burning a gallon of gas or a lump of fossil fuel, agriculture are living biological systems, and so often the measurement of the carbon of the greenhouse-gas impact is just much less certain than with the other sectors of the economy.”
5. Potent Gases
The focus on carbon dioxide, however warranted, neglects more powerful gases emitted in agriculture. “Much of the greenhouse gas effect of agriculture is methane, which is coming from the ruminant emissions of cows. Their stomachs churn away and emit methane. Their waste on the ground converts into methane. Another major source is nitrous oxide. When you put too much fertilizer on the ground, on a field, that which is not taken up by the plants can either run off and cause water pollution or because of microbial action be converted to nitrous oxide, which is 300 times more potent than carbon dioxide.”
Lehner’s list is more modest than one assembled a decade ago by the Worldwatch Institute, which also included livestock respiration in an audit of UN estimates. The Worldwatch Institute included agriculture’s supply chain as well, and it concluded that livestock agriculture is responsible for 51 percent of anthropogenic greenhouse gas emissions.
WWI’s estimate was dismissed by some in agriculture, but a detailed assessment this year found it more right than wrong. That assessment argued for an agricultural contribution of about 37 percent of anthropogenic GHGs. Stanford physicist Steven Chu also argued recently for a higher assessment of agriculture’s contribution, in the neighborhood of 30 percent.
Despite these higher numbers, climate damage from agriculture may be easier to mitigate than pollution from other sectors, Lehner argued, because it could be mitigated with existing technologies. He listed a number of known agricultural practices that reduce emissions, including:
Perennial crops
Crop rotation
Cover crops
No-till, reduced till; prairie strips
Management intensive grazing
Agroforestry & silvopasture (trees)
Dry manure management
Organic fertilizer
Riparian buffers and wind breaks
“These are practices that are well known,” Lehner said. “They have been proven and successfully demonstrated with virtually all crops in most regions of the country at most scales, and they've been found to be both productive at reducing greenhouse gas emissions and increasing soil carbon and increasing productivity and former profitability.”
“The good news,” said Lehner, who authored the Agriculture chapter of “Legal Pathways To Deep Decarbonization In The United States, “is we can actually reach carbon neutral agriculture pretty soon, and we can reach it in a way that is profitable for the farmers and for the communities they live in.”
Just six practices can render agriculture carbon negative, Lehner said, as illustrated by this chart: