Greenhouse gases in the food system, explained

This past September, experts and policymakers from around the world converged for Climate Week NYC, an annual event held in partnership with the United Nations General Assembly, to talk about the increasing impacts of global warming and to strategize about how to reduce the emissions of the greenhouse gases that cause it. Until fairly recently, food and agriculture were largely left out of these conversations, but this year was different, both at the conference and in the media. Some estimates show the global food system is responsible for a third of global emissions, and as a result, some critics warn it still isn’t getting enough scrutiny.

While agriculture is far from the biggest contributor of greenhouse gases to the atmosphere, it does lead in the production of some of the more potent gases that pose a risk for rapid warming. It’s why making a plan for reducing those emissions has become the focus of many conferences, summits and private initiatives. Where do those gases come from, and why do they attract so much concern? Here, we break down the basics of how greenhouse gases work and where they come from, both on the farm and in the wider food system.

The carbon cycle

In the global carbon cycle, plants harness energy from the sun to convert carbon dioxide from the atmosphere and water into sugars, which store that energy in the chemical bonds between carbon atoms. Plants use that energy directly as food, but they also use these sugars to build leaves, stems, roots and wood. Plankton and seaweeds do the same in the ocean. When living things digest sugars for energy, when plants decompose or when people burn wood for fuel, those sugars get broken back down into carbon that returns to the atmosphere, usually as carbon dioxide or methane.

Living plants store a lot of carbon that would otherwise be in the atmosphere. Dead plants store it too, as do partially decomposed plants that make up the organic matter in soil. Long-dead plants and algae — which become coal, oil and natural gas over millions of years as they become trapped in rocks — effectively remove that carbon from the cycle until humans release it back into the atmosphere by burning it. It’s that release that has the biggest impact on our climate, but other human activities can also dramatically disrupt the carbon balance of ecosystems.

Carbon dioxide

The primary gas people are used to hearing about in climate change discussions is carbon dioxide, or CO₂. Carbon dioxide is produced when living things break down food for energy in an environment with oxygen. This process is called respiration, and it’s performed by every cell in our bodies, as well as by animals, plants, fungi and bacteria.

Respiration-derived emissions are the main source of CO₂ from agriculture, with most of the carbon dioxide emitted by agriculture coming from bacterial and fungal respiration. When soils are disturbed or when crop residues decompose in the field, bacteria and fungi consume organic matter that’s exposed to the air and release that carbon dioxide into the atmosphere. Some level of carbon dioxide emission is normal from any healthy soil as microbes slowly digest organic matter, but aggressively plowing soils can expose more of these organic materials to the air for rapid decomposition.

These direct carbon dioxide emissions from the farm itself are fairly low, accounting for somewhere between 7-11 percent of the warming emissions that come from agriculture. That’s thanks in part to the fact that growing crops absorb some carbon dioxide from the atmosphere, and depending on how crop residues and soils are managed, sometimes this carbon is slow to return to the atmosphere.

Methane

Methane, or CH₄, comes from bacteria breaking down sugars and other organic matter in anaerobic environments, or environments without oxygen. This can happen in a number of places: Bacteria in flooded soils, especially those in rice paddies, produce methane as they break down plant residues. Liquified manure from factory farms also provides an environment for methane-producing bacteria to thrive. In open manure lagoons, that methane escapes straight into the atmosphere, but it can also be intentionally collected in methane digesters, which capture the methane so it can be burned for energy. As the main component of natural gas, methane is already a common fuel source, and since these digesters divert methane emissions from going directly into the atmosphere, factory farm and gas industry lobbyists have marketed this factory farm gas as clean energy. But the reality is that, in sustainable animal farming, manure shouldn’t be accumulating at such high quantities that those methane emissions are generated in the first place.

Nitrous oxide and the nitrogen cycle

One of the most important greenhouse gases produced by agriculture doesn’t involve carbon at all: nitrous oxide. Much like the carbon cycle, the nitrogen cycle that produces nitrous oxide involves bacteria and other organisms that fix nitrogen gas from the atmosphere into compounds like ammonia that can be used by plants. Nitrogen compounds are important components of the proteins that make up living things, and many of these compounds return to soil in concentrated amounts in animal waste or synthetic fertilizers. Depending on temperature and moisture levels, bacteria can release this nitrogen back to the atmosphere in several forms, including nitrous oxide.

Nitrous oxide is also significantly more potent than carbon dioxide: it holds heat about 273 times better than carbon dioxide and is responsible for 49 percent of on-farm emissions in the U.S. But unlike methane, nitrous oxide also has a long life.

If it’s both a potent greenhouse gas and one that will be with us for centuries, why doesn’t it get as much attention? In part, this is because it’s harder to predict what conditions will favor nitrous oxide formation as opposed to the formation of other gases. So while limiting fertilizer application and timing it to the appropriate weather and soil conditions can help, some level of nitrous oxide emissions is hard to avoid from agricultural soils. These emissions also happen from wild soils that haven’t been used for agriculture, though at far lower rates.

Overall, the contributions of nitrous oxide are still minor compared to those from carbon dioxide and methane, but that won’t be true if nitrous oxide concentrations in the atmosphere continue to rise at recent rates. With concentrations rising by 40 percent in the last few decades, largely driven by the expansion of agriculture and an increase in global fertilizer use, nitrous oxide is set to become a bigger contributor to warming.

What about off-farm food system emissions?

The carbon dioxide output of the food system climbs significantly when you include emissions that aren’t always included in the same bucket as agriculture, like the fuel used to power farm equipment or transport food, the energy used in food processing and refrigeration, or the emissions that come from creating food packaging. These emissions account for about 30 percent of the food system’s total footprint.

As it decomposes, food waste generates a lot of emissions, especially methane. Globally, waste in landfills is the third largest source of methane emissions after agriculture and leaks from wells and pipelines. Much of the methane from landfills comes from food waste: A recent EPA analysis estimated that 58 percent of the methane from U.S. landfills comes from food waste, underscoring the potential benefit of composting (which breaks down food waste aerobically to produce carbon dioxide instead) to bring these emissions down.

Changes in land use are also a big driver of emissions, especially carbon dioxide. Forests store a lot of carbon above and below ground, and removing them (especially burning them so they can become cropland or pasture for livestock) is a huge source of carbon emissions. Grasslands also store huge amounts of carbon, especially in the extensive root systems of living and dead plants. Plowing these up or overgrazing them can also release this carbon. Climate scientists refer to this category of emissions as “land use change” and they’re not always counted in the same bucket as agriculture itself. But given that agriculture — especially the expansion of pasture and soybean fields to feed cows — is one of the biggest drivers of land use change globally, these should certainly be counted as emissions that come from the food system.

The food system’s total impact on climate

Overall, counting these emissions alongside those that come directly from the farm gives agriculture a big footprint: About 34 percent of global emissions can be attributed to food systems overall, with 70 percent of these coming from on-farm activities and the conversion of wild land to agriculture. Within that, carbon dioxide accounts for half those emissions, while methane accounts for 35 percent. The remaining 15 percent comes from nitrous oxide. In short, the way we are growing crops in monoculture, clearing land for cattle grazing, and producing too much beef and dairy, is responsible for over a third of the world’s GHG emissions.

Even accounting for all of the emissions that originate from food system-related energy consumption, transportation and land use change, the food system isn’t a top emitter of carbon dioxide. But it is the biggest emitter of both methane and nitrous oxide emissions globally, thanks in large part to the ongoing expansion of factory farming across the world.

While there are a number of practices that claim to reduce these emissions — like feeding cows algae that suppresses methane emissions or capturing manure-generated methane in digesters for use as energy — these are only avoiding excess emissions that shouldn’t be generated in the first place. Only transformational food system change can actually avoid some of these worst impacts.

It’s true that some greenhouse gas emissions are inevitable from the food system thanks to natural soil processes, but divesting from industrial animal agriculture is the only way to curb excess emissions. Slowing the expansion of new cropland to feed animals will go a long way to slowing new carbon dioxide emissions. Applying manure and fertilizers in smaller quantities — rather than pumping out as much corn and other industrial crops for animal feed as possible — will help to limit nitrous oxide emissions. Finally, simply raising fewer cows for meat and milk would cut the single largest source of methane. A recent Greenpeace analysis found that simply switching off of our current trajectory of expanding animal agriculture has the potential to stave off .32 degrees celsius of warming by 2050, no small reduction in a climate change paradigm where even a tenth of a degree can save ice caps and prevent millions of heat-related deaths.

Top image credit: Spiritofamerica via Adobe Stock