New maps show that the Earth is rapidly running out of fertile land and that food production will soon be unable to keep up with the world's burgeoning population. The maps reveal that more than one third of the world's land is being used to grow crops or graze cattle.

Scientists at the University of Wisconsin-Madison combined satellite land cover images with agricultural census data from every country in the world to create detailed maps of global land use. Each grid square was 10 kilometres (6.2 miles) across and showed the most prevalent land use in that square, such as forest, grassland or ice.

"In the act of making these maps we are asking: where is the human footprint on the Earth?" said Amato Evan, a member of the University of Wisconsin-Madison research team presenting its results this week at a meeting of the American Geophysical Union in San Francisco.

The current map shows a snapshot of global land use for the year 2000, but the scientists also have land use data going back to 1700, showing how things have changed.

"The maps show, very strikingly, that a large part of our planet (roughly 40%) is being used for either growing crops or grazing cattle," said Dr Navin Ramankutty, a member of the Wisconsin-Madison team. By comparison, only 7% of the world's land was being used for agriculture in 1700.

The Amazon basin has seen some of the greatest changes in recent times, with huge swaths of the rainforest being felled to grow soya beans.

"One of the major changes we see is the fast expansion of soybeans in Brazil and Argentina, grown for export to China and the EU," said Dr Ramankutty.

This agricultural expansion has come at the expense of tropical forests in both countries.

Meanwhile, intensive farming practices mean that cropland areas have decreased slightly in the US and Europe and the land is being gobbled up by urbanisation.

The research indicates that there is now little room for further agricultural expansion.

"Except for Latin America and Africa, all the places in the world where we could grow crops are already being cultivated. The remaining places are either too cold or too dry to grow crops," said Dr Ramankutty.

By continuing to monitor changes in land use the scientists hope that they will be able to highlight problems and help find solutions.

"The real question is, how can we continue to produce food from the land while preventing negative environmental consequences such as deforestation, water pollution and soil erosion?" said Dr Ramankutty.

The next phase of the project is to build an internet-based databank - called the Earth Collaboratory - that would draw on the knowledge of scientists around the world, local environmentalists and members of the general public.

Jonathan Foley, director of the Wisconsin-Madison research team, said: "[The Collaboratory] will truly be a brave new experiment that effectively bridges science, decision-making and real-world environmental practice - collectively envisioning a new way to live sustainably."