有机农业比传统农业更环保吗？

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As someone who has studied the environmental impacts of agriculture for many years, there are a handful of questions that I get over and over. One of them is whether we are right to assume that organic farming is better for the environment than conventional farming.

I think this is a common assumption. When I eat with others, there is often an expectation that I will automatically choose the organic option. When I speak to friends, they are keen to tell me that they’ve started buying organic vegetables or beef. They are looking for a thumbs-up that this was the best choice.

While this choice might seem obvious and simple (organic is good, and conventional is bad), the research is far more mixed. In 2017, I wrote an article looking at the latest studies. I mainly drew on a meta-analysis by Michael Clark and David Tilman, which found little clear difference in some environmental impacts — such as carbon emissions and water use — between organic and conventional methods.1 But it did find some areas where organic performed worse: specifically, water pollution and land use. Organic farming tends to get lower yields and therefore uses more land.

That article has been a popular one since then. But it’s now old and worthy of a refresh. Nine years on, do we have better data on these comparative impacts? How should we think about these differences in farming practices?

In this article, I take a fresh look at these questions.

Some definitionsWhat’s the difference between organic and conventional agriculture?

Organic agriculture refers to the farming of crops or livestock without the use of synthetic inputs. That includes synthetic fertilizers, pesticides, plant growth regulators, nanomaterials, and genetically modified organisms (GMOs).2 This doesn’t mean organic farming is chemical or pesticide-free. Chemicals are often used in organic farming, but most are not synthetically manufactured; there are a few exceptions that have been approved by the National Organic Program of the United States Department of Agriculture (USDA).3 Conventional — sometimes called “industrial” — farming is any agricultural system that uses one or more of the above synthetic inputs.

The methods applied for weed and pest control in conventional and organic systems can also affect planting and tillage techniques. Conventional farming often uses synthetic herbicides to control weeds; this approach is often more conducive to low- or no-till management techniques. Since herbicide applications cannot be widely adopted in organic farming, options for no-till farming can be more limited and place greater emphasis on mechanical controls or mulching.

In crop production, nutrients can be added to the soil as organic matter, such as green compost, animal manure (human sewage sludge is usually banned), or bone meal. For livestock, organic methods mean animals must be fed organically certified feed (or graze on land with no synthetic chemical inputs), and antibiotics cannot be used throughout their lifetime, except in emergency cases such as disease or an outbreak of infection. In conventional livestock production, there are no constraints on feed certification, and antibiotics and growth hormones are often used.

Animal welfare standards for organic certification can vary by country; in many, livestock must be raised with access to the outdoors (i.e., caged hens are not permitted). Conventional livestock farming covers a range of production methods: they can be produced in either “free range” or “caged” conditions. These are typically monitored and labeled as such on product packaging.

Organic farming wins on ecotoxicity

Rather than looking at individual studies one by one, I’ll mostly rely on data from a large meta-analysis by Fatemeh Hashemi and colleagues, published in Communications Earth & Environment in 2024.4 It compiled data from 100 assessment studies.

On some environmental impacts, there was no clear winner. But let me start with two where there was.

Organic agriculture almost always proved better on the metric of “ecotoxicity”. This measures how harmful a farming system’s chemical inputs — which are mostly pesticides — are to plants, insects, and aquatic life in the local environment. Particular chemicals have an ecotoxicity score, which is multiplied by the amount of each substance applied. Across the studies that measured it, organic farming almost always had much lower ecotoxicity than conventional farming.

Since the key distinction between organic and conventional farming is the use of synthetic chemicals, ecotoxicity is really the measure where organic farming nearly always wins. There are a few exceptions, though. Some chemicals, such as copper sulfate, are allowed under organic certification. In a few of the studies where these were used, organic actually had a higher ecotoxicity score.

As a result, organic farming also tends to support greater biodiversity on the land it uses. Studies typically find greater diversity of plant, insect, and bird species on organic land.5 They also find that, on average, these organisms are more abundant.6

These positive results vary widely depending on the scale. Organic tends to perform better in terms of biodiversity at the level of a particular field. But studies have shown that these positive effects diminish when looking at biodiversity across an entire farm or the surrounding area.7

Conventional farming wins on land use

On the other hand, conventional agriculture consistently performed better on land use. It had higher yields of both plant and animal products, so producing one kilogram of food required less land.8

In the chart below, you can see the difference in average land use between organic and conventional products; one dot is equal to one study. The gray line marks where the land use between organic and conventional systems would be equal.

As you can see, almost all of the dots lie above the gray line; this means organic farming almost always uses more land. The median result was that organic practices used 64% more land.

Neither wins on carbon emissions nor on water pollution

Let’s then look at the environmental indicators where the answer is less clear-cut: carbon emissions (often referred to as “global warming potential”) and two metrics that capture water pollution: acidification and eutrophication (the leakage of nutrients into land and water bodies).

The first thing to note is that across all these metrics, organic farming performs better per unit of land. If I grow one hectare of wheat using organic methods, and the same using conventional methods, carbon emissions and water pollution will be lower on the organic hectare.

But as we just saw, organic farms tend to produce less food per hectare. When we look at impacts per kilogram of food (which is ultimately what we care about when it comes to food consumption and our own choices), those gains disappear.

The researchers found that across the 100 assessments, there was no statistically significant difference between organic and conventional farming when it came to greenhouse gas emissions, acidification, and eutrophication. The results were very mixed: in some studies, one won over the other, but almost just as many found the opposite effect.

In the chart below, I’ve shown the outcome of each of these studies. Those in pink on the right-hand side had lower impacts with conventional methods. Those on the left, in purple, found organic was better. You can clearly see the difference between the two methods in the top (ecotoxicity) and bottom (land use) panels, and the mixed results in the three middle panels.

Where do greenhouse gas emissions come from?What are the sources of emissions in organic and conventional farming?

The largest source of emissions in conventional crop farming is the use of synthetic fertilizers. Manufacturing fertilizers is an energy-intensive process. It uses the Haber-Bosch process, in which natural gas serves a