A new users' guide is needed to help protect the Earth from dangerous changes such as global warming and extinctions of animals and plants caused by humans, scientists said.

A group of 28 experts suggested nine key areas, such as freshwater use, chemical pollutants or changes in land use, where governments could define limits to ensure a "safe operating space for humanity."

"Today, we are clearly driving blindfolded in the world ," Johan Rockstrom, leader of the study and director of the Stockholm Resilience Centre at Stockholm University, said of a lack of international guidelines.

"We are not considering the risks that there are deep holes we can drive into," he said. The call, for setting "planetary boundaries," was published in the journal Nature.

Dr Rockstrom said there were signs human activities had already pushed the world into the danger zone because of global warming, a high rate of extinctions of animals and plants and pollution caused by nitrogen, mainly used in fertilisers.

Among limits, they suggested capping the percentage of global land area converted to cropland at 15 per cent. At the moment, the percentage is 11.7 per cent, they said.

They added that concentrations of carbon dioxide, the main greenhouse gas, should be limited to 350 parts per million of the atmosphere - below current levels of 387 ppm. Human freshwater use should be capped at 4,000 square kilometres a year - against 2,600 sq km now.

In the editorial Nature journal said the proposed indicators were a "creditable attempt" to quantify limits on human use of the planet. However, it noted, for instance, that fertilizers caused pollution yet helped feed millions of people.

Hans Joachim Schellnhuber, head of the Potsdam Institute for Climate Impact Research and a co-author of the study, said there were growing risks of abrupt and possibly irreversible changes.