The world will in the first 30 years of this century lose some 30m square kilometres of arable land to urbanisation, an area equivalent to the size of Italy, or to that of the key US Corn Belt states of Illinois and Iowa combined, an academic paper said.
"In most parts of the world, urban land is expanding faster than urban populations," the paper, from nine researchers in institutions ranging from US-based Texas A&M University to New Zealand's Canterbury University.
"Whereas urban populations are expected to almost double from 2.6bn in 2000 to 5bn in 2030, urban areas are forecast to triple between 2000 and 2030."
'Acute impacts'
While that loss – which excludes 16m hectares of urban land viewed as being kept on in agriculture - is equivalent to 1.8-2.4% of world arable land, its impact on harvests will be greater, given that the city growth will occur predominantly on higher-yielding land.
"Urban expansion is expected to take place on cropland that is 1.77 times more productive than the global average," said the paper, published in Proceedings of the National Academy of Sciences of the United States of America.
"More than 60% of the world's irrigated croplands are located near urban areas, highlighting the potential competition for land between agricultural and urban uses."
And with the loss of land focused on a few areas, notably in Africa and Asia, "regional impacts will be acute".
'Threat to crop production'
China alone will lose 7.6m hectares of cropland, equivalent to 5.4% of domestic arable area - but enough to cut crop output by 8.7% given that city growth is coming at the expense of relatively productive land.
"Urban expansion in China is taking place in the country's most productive farmland and over large areas," the researchers said.
"Therefore, urban expansion could pose a threat to domestic crop production."
Vietnam, the world's third-ranked rice exporter, and second biggest shipper of coffee, will loss 10.3% of its arable area, equivalent to 15.9% in crop production terms.
'More susceptible to shocks'
However, among nations listed, Egypt will see the biggest impact, in relative terms, losing some 800,000 hectares to cities – equivalent to 34% of its cropland and 37% of output.
While many sub-Saharan countries will be able to offset land loss by bringing fresh areas into output, that is not an option for countries in the likes of Northern Africa and the Middle East, where arable areas have "nearly reached their maximum potential.
"Countries… such as Egypt are likely to resort to trade to compensate for cropland loss, which could make them more susceptible to international food supply shocks."
Egypt is already the top importer of wheat, besides being a major buyer of the likes of soybeans, beef and sunflowerseed oil.
Brazil, India, US outlooks
The US will also lose a substantial amount of cropland, some 1.5m hectares, to cities between 2000 and 2030, but the impact on harvest potential will be limited by urbanisation occurring in the main on less productive land.
Brazil, which will lose 1.0m hectares, and India, where cities will swallow 3.4m hectares, will also be well placed to deal with the trend.
"India, the US, and Brazil will also experience high losses in absolute terms.
"But here urban expansion leaves large expanses of croplands untouched, and is therefore less likely to threaten domestic crop production."
