HUMANITY risks causing catastrophic and irreversible environmental damage by crossing ''nine planetary boundaries'', scientists have warned.
In a study from the Resilience Centre at Stockholm University, the scientists found that humans had already stepped over three of those boundaries - climate change, species loss and nitrogen cycles.
The boundaries are levels of environmental damage that humanity cannot cross without changing the Earth's natural functions almost permanently.
Australian National University professor Will Steffen, who was an author of the study, said he hoped the boundaries would spark an ethical and philosophical debate around humanity's role on the planet.
''Really, what we are concerned about is maintaining a planet that humans can thrive on - the sort of planet in the last 10,000 years that we have been able to develop agriculture, villages, cities, civilisations,'' Professor Steffen said yesterday.
Professor Steffen stressed that the boundaries were rough first drafts, which in some cases required further extensive scientific research. Two of the boundaries - chemical pollution and atmospheric aerosol loading such as smog and dust storms - do not yet have a measurable boundary.
The study found boundaries could be crossed for short periods so that humanity could pursue long-term social and economic expansion. But crossing one boundary might trigger consequences in other zones, such as climate change affecting the amount of fresh water on the planet.
Professor Steffen said there had been debate among the scientists about whether any of the boundaries were more important than the others, but climate change and biodiversity loss were clearly the most pressing.
The climate change boundary found that crossing into an atmospheric carbon dioxide concentration of 350-550 parts per million created increasing risk of large polar ice melts. As of March, the concentration was at 387 ppm.
But Professor Steffen said just as crucial was the rate of biodiversity loss, including what is now known as the sixth mass species extinction in Earth's history, and the first caused by humans.
The study said if species extinctions rose above 10 a year for every million on the planet, it would significantly weaken ecosystems, which are essential to natural functioning of the Earth including food growth.
The natural background rate of extinction is up to one species a year for every million. Current rates are estimated to be 100-1000 times the natural rate.
''There needs to be a debate around the philosophical and ethical values of allowing this rate of biodiversity loss to continue,'' Professor Steffen said.
The study will be released in an environmental science journal today.