If you're paying more for local and organic groceries because you care about the environment, here's some bad news: science shows your efforts won't have much impact on your carbon emissions.

The good news is that scientists have done the math on dietary changes that can make a difference.

With the UN's annual climate conference underway in Bonn, you might be thinking about ways you can do your part to fight climate change.

Many online recommendations for reducing your carbon footprint, including some from the David Suzuki Foundation and the University of Michigan Center for Sustainable Systems, include buying local and organic as a way to reduce your carbon footprint.

So even two scientists who did the calculations, published in separate studies earlier this year, were also surprised.

"There's a …[perception] that organic agriculture is a lot more sustainable than conventional agriculture is, so I guess I was kind of predisposed to believe that too until I looked at the data," said Michael Clark, a PhD student at the University of Minnesota's department of natural resources science and management.

Growing food in greenhouses consumes large amounts of fossil fuels, but the carbon footprint of this type of agriculture could be reduced by using alternative forms of energy. (Erica Berenstein/AFP/Getty Images)

Clark compared the environmental impacts of different food production practices by compiling the energy and land use, as well as other environmental impacts calculated in 164 different scientific papers on 742 food production systems. He published the results in June in the open access journal Environmental Research Letters.

The study found that organic and conventional agriculture "did not differ significantly in their greenhouse gas emissions."

Less energy, more land

Organic agriculture used 25 to 110 per cent more land than conventional agriculture — not ideal — but 15 per cent less energy. That's largely because yields are lower with organic agriculture, but a lot of energy is needed to make synthetic fertilizers and pesticides.

Researcher Michael Clark is vegetarian for health rather than environmental reasons. (Courtesy Michael Clark)

Carbon footprints are similar in the production of organic and conventional foods. Considerable energy is needed to make chemical fertilizers and pesticides, but organic and conventional food production emit about the same total amount of carbons. That's partly because organic fertilizers tend to cause the release of nitrous oxide, a potent greenhouse gas, offsetting the lower emissions from energy use in organic production, the study found.

There was some variation by food group — on average, organic meats tended to have lower carbon footprints, while organic vegetables tended to have higher carbon footprints compared to those that were conventionally produced.

Some of Clark's other findings were:

Grass-fed beef generates 19 per cent more emissions per kilogram than grain-fed beef, largely because grass is less nutritionally dense. Cattle need to eat more grass to get the same nutrition as they would from a smaller amount of grain, they grow more slowly, and must be raised for a longer time before slaughter, generating more emissions.

Trawled fish, especially flat fish, such as sole and halibut, generate an average of 2.8 times more emissions than schooling fish caught with mid-water trawling, seine nets and lines such mackerel and cod.

Crops grown in greenhouse have emissions that are, on average, three times higher than crops grown in field. However, those emissions can be reduced by heating and powering greenhouses with renewable energy.

Trawled fish, especially flat fish, such as sole and halibut, generate an average of 2.8 times more emissions than schooling fish caught with mid-water trawling, seine nets and lines such mackerel and cod.

But what about locally grown foods? Shouldn't it make a difference that your food wasn't flown halfway around the world?

Seth Wynes, a PhD student in the Department of Geography at the University of British Columbia, did a similar analysis to Clark in July, but focused on different recommended "green" lifestyle choices rather than food production. His paper was also published in Environmental Research Letters.

Wynes found that while buying local can have other benefits, such as supporting local communities and knowing where your food comes from, "in terms of your emissions, it's just not a big deal."

The difference is so small that by taking a short drive to pick up local food, you could end up generating more emissions than if you walked to the nearest store to grab something imported.

Go vegetarian or buy a hybrid car?

On the other hand, both Wynes and Clark found that switching to a plant-based diet could make a huge difference. Wynes found going from omnivore to vegetarian could reduce your personal carbon emissions by about 0.8 tonnes per year — a bigger difference than replacing your gasoline-powered car with a hybrid. Going from omnivore to vegan would reduce your emissions by 0.9 tonnes per year.

Over the entire population, that can add up.

Trading your gas-powered car for a hybrid, like the Hyundai Ioniq shown here in a 2016 file photo, would reduce your carbon footprint. But it won't make as much difference as going from an omnivorous diet to a vegetarian one.

Global agriculture currently emits about a third of the world's greenhouse gases. "So it's a very large part of the climate puzzle that isn't often spoken about in terms of government or policy decisions," said Clark. He's vegetarian, but says he gave up meat for health rather than environmental reasons.

In an earlier study using the same data set as his more recent study, he found that global emissions from food production will increase by 80 per cent by 2050, from 2.27 billion to 4.1 billion tonnes of carbon per year, if current dietary and income trends continue. If everyone switched to a vegetarian diet, they would instead decrease by 55 per cent to 1.02 billion tonnes of carbon per year.

Beef generates more emissions than almost all other foods. Reducing this type of meat in your diet can significantly reduce your carbon footprint. (Todd Korol/Reuters)

While some critics question whether individual actions can have a significant impact compared to government policies, the researchers say their numbers show that eating less meat — and especially none at all — will.

"It will absolutely be enough to make a difference," Clark said. He recommends starting by reducing the amount of beef, goat and lamb in your diet, as those by far generate the most emissions.

The David Suzuki Foundation also recommends choosing food "low on the food chain" while the University of Michigan's Center for Sustainable Systems recommends choosing vegetarian foods and replacing some beef consumption with chicken.

Both Clark and Wynes hope their findings will inspire government policies that make it easier to eat sustainably.

Clark said there's an added bonus to shifting toward a plant-based diet – many studies show it's also healthier.