Scientists have published research that claims a single loaf produced in the UK contributes as much to global warming as more than half a kilogram (kg) of carbon dioxide.

Growing wheat and especially the use of fertiliser “easily accounts for the biggest slice of the bread’s environmental impact”, according to researchers from the University of Sheffield.

They said ammonium nitrate fertiliser used in wheat cultivation made up 43% of the calculated warming footprint of a typical 800 gram (g) wholegrain loaf.

However, the researchers said there needed to be shared responsibility for the environmental costs of making bread, including fertiliser producers, farmers, millers, bakers, retailers and consumers.

The impact from growing wheat was followed by milling and the bakery stage of the process.

Lead scientist Dr Liam Goucher from the university’s Grantham Centre for Sustainable Futures, said: “We found in every loaf there is embodied global warming resulting from the fertiliser applied to farmers’ fields to increase their wheat harvest. This arises from the large amount of energy needed to make the fertiliser and from nitrous oxide gas released when it is degraded in the soil.”

Taking into account every element of cultivation, production and merchandising, the loaf studied was found to have a “global warming potential” (GWP) equivalent to 0.589 kg (1.29 pounds) of carbon dioxide (CO2), the chief greenhouse gas.

Each loaf required cultivation of just under a square metre (0.72 square metres) of land, using up 42g of granular ammonium nitrate, said the researchers, writing in the journal Nature Plants.

During milling, a total of 520g of flour was produced in processes that consumed about 0.07 kilowatts of electricity.

Baking was calculated to use a total of 0.09 kilowatts and was labelled a global warming “hotspot”, accounting for 9.7% of the loaf’s GWP.

The contribution of farm machinery to GWP was a surprisingly modest 5.2%.

Packaging contributed 3.1%, despite a switch to low-density polyethylene wrapping reducing its environmental impact.

Professor Peter Horton, chief research adviser to the Grantham Centre for Sustainable Futures, said: “Our findings bring into focus a key part of the food security challenge – resolving the major conflicts embedded in the agri-food system, whose primary purpose is to make money, not to provide sustainable global food security.”

The study was carried out in collaboration with a commercial bread and flour producer and a large farming services provider.