Around a third of human-generated greenhouse gas emissions comes from the global food system, and lost or wasted food is known to contribute some amount—but it has never been clear to exactly what degree. Now, by following specific foods through their entire life cycle, researchers have determined just how much this wasted food adds to emissions through phases such as harvest, transportation and disposal.

For a study in Nature Food, Xunchang Fei of Singapore’s Nanyang Technological University and his colleagues used 164 countries’ food supply data from 2001 to 2017 to estimate emissions across 54 food commodities and four categories: cereals and pulses; meat and animal products; vegetables and fruits; and root and oil crops.

Roughly a third of food is lost during harvest, storage and transportation or is wasted by consumers. The team found this food was responsible for greenhouse gases equivalent to 9.3 billion metric tons of carbon dioxide—about half the global food system’s total emissions—in 2017. Four countries (China, the U.S., India and Brazil) contributed 44.3 percent, mainly owing to their dietary habits and large populations. Of the four food categories, meat and animal products were the source of almost three quarters of emissions that occurred during the supply-chain phase for food that was ultimately lost.

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