Course 5: Reduce Greenhouse Gas Emissions from Agricultural Production (Synthesis)

Ruminant livestock (mainly cattle, sheep, and goats) generate roughly half of all agricultural production emissions. Of these emissions, the largest source is “enteric methane,” generated by microbes in ruminant stomachs.

The same measures needed to increase productivity of ruminants and reduce land-use demands will also reduce methane emissions, mainly because more milk and meat is produced per kilogram of feed. Because the improvements are greatest when moving from the worst-quality feeds to even average-quality feeds, the greatest opportunities to reduce emissions exist in poorer countries. Improving highly inefficient systems causes emissions per kilogram of meat or milk to fall very sharply at first as output per animal increases (Figure 18).

Other strategies to reduce enteric methane emissions rely on manipulating microbiological communities in the ruminant stomach by using vaccines; selectively breeding animals that naturally produce fewer emissions; or incorporating special feeds, drugs, or supplements into diets. These efforts have mostly proved unsuccessful. For example, despite testing thousands of compounds, researchers have found that methane-producing microbes quickly adapt to drugs that initially inhibited them.⁷⁵

More recently, at least one highly promising option has emerged that persistently reduces methane emissions by 30 percent, and may also increase animal growth rates.⁷⁶ So far, this compound—called 3-nitrooxypropan (3-NOP)—requires daily feeding at a minimum, so it is not feasible today for most grazing operations unless dosing can be refined.

Enteric fermentation, atypically, is receiving a reasonable level of R&D funding. It is possible that 3-NOP could eventually pay for itself through reduced feed needs or increased productivity, but these benefits are not guaranteed. However, because the compound will be highly cost-effective for GHG mitigation, we recommend that governments consider three policies:

• Provide incentives to the private sector by promising to require use of 3-NOP or other compounds if and when they are proven to mitigate emissions at a reasonable cost.
• Fund large-scale 3-NOP or related demonstration projects in the short term.
• Maintain public research into compounds to reduce methane from enteric fermentation.

Figure 18 |

More efficient milk production reduces greenhouse gas emissions dramatically

Note

Dots represent country averages.

Source

Gerber et al. (2013).

Manure is “managed” when animals are raised in confined settings and farmers remove the manure and dispose of it. (Manure that ruminants deposit directly on fields is considered “unmanaged” and is addressed in the next menu item.) Managed manure generates both methane and nitrous oxide emissions. Pigs generate roughly half of these emissions, dairy cows just over one-third, and beef cows roughly 15 percent.⁷⁷

The majority of manure is managed in “dry” systems, which account for 40 percent of total managed manure emissions despite low emissions rates.⁷⁸ Estimates of mitigation potential tend to be low, in part because management costs tend to be high per ton of emissions. Even so, the use of dry systems is desirable because emissions from “wet” systems (where farmers do not attempt to dry out manure before storage) can be 20 times higher per ton of manure.⁷⁹ We also believe that separating liquids from solids has underappreciated potential to reduce emissions. Separation technologies range from simple gravity systems to sophisticated chemical treatments. They also reduce hauling costs and make manure more valuable as fertilizer.

Per ton of GHGs reduced, existing wet systems are easier to mitigate because the manure is generating high levels of emissions. Even an extremely sophisticated system for managing pig manure in North Carolina (United States), using a series of treatment tanks to eliminate virtually all air and water pollution, would cost only an estimated $22 per ton of mitigation (CO2e).⁸⁰ This system would add only around 2 percent to the retail price of pork.⁸¹

Digesters, which convert manure into methane for energy use, come in high-technology forms that produce electricity in developed countries and simpler household versions used extensively across Asia. They can help reduce emissions but only if manure would otherwise be managed in wet form, and if strong safeguards are in place to keep methane leakage rates low.

Improving manure management will address a range of environmental pollution, human health, and nuisance concerns. Because measures to mitigate emissions would typically contribute to addressing these other concerns, the mitigation may even be “free” from a socioeconomic perspective. Promising strategies are available:

• Phased regulation of facilities, extending from larger to smaller farms, to encourage innovation.
• Government-funded programs, using competition, to develop the most cost-effective technologies.
• Establishment of government programs to detect and remediate leakages from digesters.

According to standard emissions factors used by the IPCC, nitrogen deposited in feces and urine turns into nitrous oxide at roughly twice the rate of nitrogen in fertilizer. Our 2050 projection already incorporates productivity improvements that lower emissions intensity from manure on pasture by 25 percent compared to 2010, but further increases in feed efficiency could lead to additional modest emissions reductions.

Other studies typically estimate little to no global potential to mitigate this diffuse source of emissions. We are cautiously optimistic, given further development of two nascent technologies. Both work by inhibiting the ability of microorganisms to turn nitrogen from other molecular forms into nitrate, whose further breakdown can release nitrous oxide.

One method involves chemical nitrification inhibitors, which have been found to be quite effective when applied two or three times per year to pastures in New Zealand⁸² and—in a very small number of experiments—when ingested by cows. The other involves biological nitrification inhibition, based on findings in Latin America that manure deposited on one variety of the productive Brachiaria grass generates almost no nitrous oxide emissions.⁸³ To exploit this property more broadly, breeders would have to breed this trait into other planted grasses.

These techniques may be more practicable than they appear. Newer research on nitrogen levels in the atmosphere suggests that current emissions factors for unmanaged manure may be too high and that emissions rates are much higher in some fields (usually in wetter areas) than others.⁸⁴ This difference in emissions would allow more economical targeting of these techniques toward wetter, more intensively grazed fields.

Research funding to address this challenge is vanishingly small. Far greater efforts and resources must be devoted to exploring an array of potential solutions:

• Governments and research agencies should substantially increase research funding into methods for reducing nitrification of manure.
• Governments should commit in advance to implement regulatory or financial incentives to implement these techniques when they do become available, to encourage research and development by the private sector.

Fertilizers applied to crops and pastures (mostly synthetic fertilizers but also manure and other sources) were responsible for estimated emissions of 1.3 Gt CO2e in 2010. Nearly all these emissions result from the manufacture, transportation, and application of nitrogen. We project that these emissions will rise to 1.7 Gt by 2050 in our baseline scenario. Globally, crops absorb less than half the nitrogen applied to farm fields. The rest runs off into ground or surface waters, causing pollution, or escapes into the air as gases, including the potent heat-trapping gas nitrous oxide. Countries, and individual farms, vary greatly in their rates of nitrogen application per hectare and in the percentage of nitrogen that is absorbed by crops rather than lost to the environment (known as “nitrogen use efficiency,” or NUE) (Figure 19).

Mitigation strategies focus on changing agronomic practices. Technically, extremely high rates of efficiency are possible if farmers are willing to assess nitrogen needs and apply nitrogen frequently in just the required amount over the course of a growing season. The challenge is that such intensive management is expensive while nitrogen fertilizer is cheap. Therefore, we believe innovations are required. Nitrification inhibitors and other “enhanced efficiency” fertilizers can increase NUE, reduce nitrous oxide emissions, and increase yields. But they are used with only 2 percent of global fertilizers,⁸⁵ probably because of wide variability in performance and because fertilizer manufacturers today spend little money to improve them. Biological nitrification inhibition is another promising option for crops and pasture grasses but receives little financial support.

We modeled various scenarios of NUE improvement and found that achieving an ambitious global average NUE of 71 percent by 2050 would reduce emissions by 600 million tons, although that would only keep nitrogen emissions close to their 2010 levels. The scope of the nitrogen challenge requires governments to focus on innovative policy approaches:

• Implement flexible regulatory targets to push fertilizer companies to develop improved fertilizers. India provides the closest example to date with its New Urea Policy adopted in 2015.
• Shift subsidies from fertilizers to support for higher NUE, where nitrogen use is excessive.
• Support critical research, particularly into biological nitrification inhibition.
• Fund demonstration projects involving researchers and high nitrogen-using farmers to pursue higher NUE using inhibitors and other innovative technologies.

Figure 19 |

The percentage of applied nitrogen that is absorbed by crops varies widely across the world

Source

Zhang et al. (2015).

The production of flooded or “paddy” rice contributed at least 10 percent of all global agricultural production GHG emissions in 2010, primarily in the form of methane.⁸⁶ Available research suggests high technical potential to mitigate rice emissions, and most mitigation options also offer some prospect of economic gains through higher yields and reduced water consumption. We focus on four main options:

• Increase rice yields. Because methane emissions are tied more to the area than to the quantity of production, exceeding FAO’s forecast rate of yield growth would allow paddy area to remain constant or decrease, reducing emissions.
• Remove rice straw from paddies before reflooding to reduce methane production. Straw may be used for other productive purposes such as growing mushrooms or bioenergy.
• Reduce duration of flooding to reduce growth of methane-producing bacteria. Farmers can draw down water levels for a few days during the middle of the growing seasons, or plant rice initially into dry rather than flooded land.
• Breed lower-methane rice. A few existing varieties emit less methane than others and researchers have shown promising experimental potential,⁸⁷ but these traits have not been bred into the most commercial varieties.⁸⁸

A single drawdown reduces emissions, and multiple drawdowns or dry planting plus one drawdown can reduce methane emissions by up to 90 percent.⁸⁹ In China and Japan, farmers practice at least one drawdown because it increases yields,⁹⁰ though researchers do not find those yield benefits in the United States. Reducing water levels also saves irrigation water, at least at the farm scale.

Yet there are obstacles. Dry planting increases weed growth. Farmers usually will not draw down water unless they are sure they can replace the water and fields are flat enough to ensure no part dries out too much. Another concern is that while drawdowns decrease methane emissions, they tend to increase emissions of nitrous oxide, another powerful greenhouse gas, which encourages joint efforts to use nitrification inhibitors. We propose the following strategies:

• Engineering analyses to determine which farmers have irrigation systems that would allow them to employ drawdowns, followed by programs to reward farmers who practice drawdowns where feasible.
• A major breeding effort to shift to lower-methane varieties.
• Greater efforts to boost rice yields through breeding and management.

Emissions from fossil energy use in agriculture will remain at about 1.6 Gt CO2e/year in 2050 in our baseline. Our assumption, based on past trends, is that a 25 percent increase in energy efficiency is cancelled out by a 25 percent increase in energy use. Mitigation options mirror those for reducing energy emissions in other sectors: they rely on increasing efficiency and switching to renewable energy sources.

Although studies of potential energy efficiency improvements in agriculture are limited, a small number of country-level studies have found large potential for efficiency gains, for example, in alternative water pumps in India⁹¹ or cassava-drying methods in Africa.⁹²

Sixty-five percent of expected agricultural energy emissions in 2050 will result from on-farm energy use. Heating and electrical power can often be provided by solar and wind energy sources, although replacing on-farm coal will require innovative, small-scale solar heating systems. Mitigating the use of diesel fuel by tractors and other heavy equipment will be more difficult and may need to rely on transitions to fuel cells using hydrogen power generated with solar or wind power. Battery-powered equipment and synthetic carbon-based fuels made from renewable electricity may provide alternative technologies.

Renewable sources of hydrogen could also mitigate 85 percent of the emissions from the synthesis of nitrogen fertilizer, currently a highly energy-intensive process. Fortunately, because of the needs of other sectors, substantial research is occurring into production of hydrogen using electricity from solar energy, and costs of solar electricity have been declining rapidly.⁹³

The efficiency gains built into our baseline already require significant effort. We estimate that reducing emissions per unit of energy used by 75 percent, rather than the 25 percent in our baseline, would reduce the GHG mitigation gap by 8 percent. To achieve this goal:

• Governments, aid agencies, and large food purchasers should integrate low-carbon energy sources and efficiency programs into all development efforts and supplier relationships with farmers.
• Research agencies and private investors should continue to fund research into production of nitrogen from renewable electricity and support design of demonstration nitrogen fertilizer plants using renewable electricity. Such research could be linked to ongoing work on developing solar-based production of hydrogen.
• Governments should commit to regulating the emissions from fertilizer manufacturing once viable, low-carbon technologies are available.

Because reducing agricultural production emissions is challenging, much academic and policy attention has focused on strategies to sequester carbon in agricultural soils to offset those emissions. There are only two ways to boost soil carbon: add more or lose less. But recent scholarship and experience indicate that soil carbon sequestration is harder to achieve than previously thought.⁹⁴

Changes in plowing practices, such as no-till, which once appeared to avoid soil carbon losses, now appear to provide only small carbon benefits or no benefits when measured at deeper soil depths than previously measured. No-till strategies must also contend with adverse effects on yields on some lands and the fact that many farmers who practice no-till still plow up soils every few years, probably releasing much of any carbon gain.⁹⁵

Adding mulch or manure are proposed strategies to add carbon to soils but, in effect, double-count their carbon which would have contributed to carbon storage elsewhere. Leaving crop residues otherwise used for animal feed to become soil carbon requires that the animals’ feed comes from other sources, which has some carbon cost because it would often require more agricultural land to grow that feed.

Building soil carbon also generally requires large quantities of nitrogen, which is needed by the microorganisms that convert decaying organic matter to soil organic carbon. Low nitrogen surely limits soil carbon buildup in Africa (Figure 20), where nitrogen additions are insufficient even for crop needs, and probably limits soil carbon buildup elsewhere.⁹⁶

Scientists have come to realize that they do not well understand the factors that lead carbon to remain stored longer in soils rather than being consumed and returned to the air by microorganisms. There is some evidence that croplands are actually losing soil carbon overall in ways neither we nor other researchers count. For these reasons, we do not include additional soil carbon sequestration as a mitigation strategy. We believe efforts are best directed toward stabilizing soil carbon, that is, avoiding further losses, and focusing on no-regrets strategies that provide additional benefits:

• Avoid conversion of carbon-rich ecosystems (e.g., forests).
• Increase productivity of grasslands and croplands, which adds carbon in roots and residues.
• Increase use of agroforestry, which builds above-ground carbon.
• Pursue efforts to build soil carbon, despite the challenges, in areas where soil fertility is critical for food security.

Figure 20 |

Soils in Africa are relatively low in organic carbon

Source

Hengl and Reuter (2009).

Although many opportunities exist for developing cost-effective, or even cost-neutral, GHG mitigation techniques to curb agricultural emissions, the size of the GHG mitigation gap strongly suggests that voluntary approaches will not be sufficient. We recommend a few forms of flexible regulations that should be designed to spur technological development.

In the case of fertilizer, we recommend that countries develop regulatory systems similar to those developed in the United States that require auto manufacturers to increase the fuel efficiency of their fleets over time. Fertilizer manufacturers would be required to sell increasing percentages of their product in a form with “enhanced efficiency,” such as fertilizers incorporating nitrification inhibitors. India has set an example by requiring that fertilizers be coated with neem, which slows nitrogen release.⁹⁷ Phased regulation would provide incentives for manufacturers to develop better products, identify the ideal uses, and market them appropriately to farmers who can most benefit from them. Regulating fertilizer is also consistent with historical regulation of agricultural inputs, such as pesticides.

Manure management is typically subject to weak regulation. Governments should phase in requirements for pollution controls that tighten and reach more sources over time, initially covering new and large existing facilities and extending gradually to medium-sized and smaller farms.

In areas where technologies are underdeveloped, such as enteric methane inhibitors, governments should commit in advance to requiring the use of appropriate drugs or feed supplements if a company develops a system that achieves a certain level of cost-effectiveness in mitigation—for example, $25 per ton of CO2e. Greater market certainty would provide incentives for the private sector to develop needed innovations.

Many of these options may, at least initially, involve additional costs, but they appear cost-effective when compared to climate change mitigation strategies in other sectors. Many options would have large cobenefits, such as reducing water and air pollution, and controlling disease-bearing organisms from poorly controlled livestock waste. Many might eventually more than pay for themselves as technologies evolve, which is likely true of nitrification inhibitors and could be true of additives to curb enteric methane. Yet these technologies do not seem likely to evolve without either strong incentives or some form of regulation designed to advance their development and deployment.