Which is better for the environment: organic or conventional agriculture? For consumers trying to make sound food choices, it’s an important question. A new international study finds that organic agriculture actually contributes more to climate change than conventional farming does. The study argues that since organic agriculture requires slightly more land for the same yield, organic systems lead to more deforestation, which in turn results in more carbon dioxide emissions. But measuring environmental impact is extremely complicated.

Back in 2010, Nadia El-Hage Scialabba and Maria Müller-Lindenlauf, of the Food and Agriculture Organization of the United Nations (FAO), took an in-depth look at the climate impacts of organic agriculture. F.A.O.’s Codex Alimentarius Commission defines organic agriculture as:

a holistic production management system that avoids use of synthetic fertilizers, pesticides and genetically modified organisms, minimizes pollution of air, soil and water, and optimizes the health and productivity of interdependent communities of plants, animals and people.

Organic agriculture still uses fertilizers and pesticides, but not synthetic ones. And when it comes to climate, the F.A.O study suggests, the fertilizer issue is key.

Conventional agriculture relies on nitrogen fertilizers, produced through a process involving copious amounts of ammonia and methane. Nitrogen fertilizers in turn, release some degree of nitrous oxide, an extremely potent greenhouse gas, with a much greater warming potential per unit released than carbon.

Organic farms bypass the need for chemical fertilizers by planting legumes. Organic farms also tend to store more carbon in the soil, slightly offsetting other greenhouse gas emissions, though, as the F.A.O. study notes, the carbon storage is likely not permanent. Plus, organic farms may burn more fossil fuels through machinery when weeds are removed mechanically.

As for the suggestion that organic agriculture requires slightly more land for the same yield, it depends on the crop. Conventional dairy production, for example, produces much more milk per cow. But there is hardly any yield difference when it comes to organic vs. conventional rice. In some cases, the same crop may have different yields-per-area in the developing world vs. the developed world—organic yields are often higher in developing countries. This is because, as the F.A.O. report notes, some of the ecologically sound practices are difficult to scale up to industrial levels, and work better at smaller scales.

On the other hand, grass-fed livestock requires a lot more space than feedlots, often leading to deforestation.

Many conscious consumers want a definitive answer about whether conventional or organic farming is better, when in fact, both have effects on the climate. There are also other concerns that go into consumer choices, such as animal welfare. And, as Stefan Wirsenius, one of the authors of the international study, says, “The type of food is often much more important. Eating organic beans or organic chicken is much better for the climate than to eat conventionally produced beef.”