Humanity needs to act now to avoid threats to human well-being caused by irreversible damage to the Earth, its climate, species and life-supporting systems.
Scientists say it has become essential to define what levels of such human-caused change are 'safe' and which are 'unsafe', and to stay within these boundaries.
The call comes from 28 of the world's most eminent environmental scientists, published today in the world's leading science journal, Nature.
The researchers propose an upper limit of 350 parts per million of CO2 in the atmosphere – a level already exceeded since 1987 - requiring a fast-track reduction in fossil fuels.
"Transgressing the safe boundary of 350ppm will increase the risk of irreversible climate change, such as the loss of major ice sheets, accelerated sea level rise and abrupt shifts in coral reef, forest and agricultural systems," they caution.
"The increasing level of CO2 in the atmosphere and ocean has caused major damage to coral reefs worldwide over the past 25 years. Allowing it to increase to 450 or higher would be irresponsible and hugely detrimental to millions of people who depend on reefs for food and their livelihood" says Australian co-author Professor Terry Hughes of the ARC Centre of Excellence for Coral Reef Studies and James Cook University.
The researchers also propose that safe boundaries be set to other major human impacts, such as species loss, the effect of fertilisers, ozone depletion, ocean acidification, freshwater use and chemical pollution.
The scientists, who come from many of the world's leading research institutions, say human activity is now the main driver of global change and is taking the planet down a path where it may be unable to support humanity's future development.
Human activities – largely resulting from our rapidly growing reliance on fossil fuels and industrialised forms of agriculture – have now reached a magnitude that may trigger irreversible and in some cases abrupt environmental change by damaging the regulatory capacity of the planet, they say.
In the cases of atmospheric CO2, nitrogen fertiliser use and species loss, the Earth has already exceeded the safety zone beyond which irretrievable changes may occur, the paper says.
And we are rapidly approaching the safe upper limits for freshwater use, land clearing and ocean acidification.
"The idea of identifying safe boundaries is based on the tipping-ponts, or thresholds, we see in nature," explains Professor Hughes. "These are natural limits which, if you exceed them, trigger a sudden large change – a grassland to a desert, or a coral reef to a degraded weed-infested system."
"Once you've crossed the line it is very hard, if not impossible, to get back again. The system you knew is gone for good."
The impact of human actions now means that the stable climate and conditions of the last ten thousand years, in which civilization had arisen and flourished, are being replaced by a far more unstable Earth, potentially catastrophic for large parts of humanity, the team warns.
"We felt it was urgent to identify future tipping-points, so we can all start thinking about how to avoid passing the point of no-return," he says.
"Our objective is to define a safe operating space for humanity, to ensure future human wellbeing."
"We cannot continue to ignore the reality of what is happening to our planet, or to keep hoping it isn't happening. The evidence is unambiguous that Earth is changing in profound and serious ways – and that we must take action to change some of the things we do. The time to do this is now."
The scientists argue that humanity faces a choice – between preserving the kind of climate and conditions that cradled the human race as we know it, and an unstable and dangerous future with dire consequences for millions.
"We are beginning to see evidence that some of the Earth's subsystems are already moving outside their stable Holocene state. These include the rapid retreat of the summer sea ice in the Arctic ocean, the retreat of mountain glaciers around the world, the loss of mass from the Greenland and West Antarctic ice sheets and accelerating rates of sea level rise during the past 10-to-15 years."
On extinctions, they say: "The rate of extinction of species today is around 100-to-1000 times more than what could be considered natural."
They also warn that humanity is pulling 35 million tonnes of nitrogen out of the atmosphere for fertiliser production, which is causing havoc in the environment, especially rivers, lakes and coastal seas.
As a starting point they propose ten boundaries which should not be exceeded to avoid crossing dangerous tipping points. These include atmospheric CO2 levels, the rate of species extinction, the over-use of nitrogen and phosphorus in fertilisers, use of fresh water, the clearing of land, ozone depletion, aerosol pollution of the atmosphere and chemical contamination. The detailed rationale for each boundary is explained in a more technical paper at http://www.stockholmresilience.org/planetary-boundaries
The researchers argue that all of the boundaries are interconnected and it is no use observing one but not the others. "If one boundary is transgressed, then safe levels for other boundaries are also under serious risk. For instance, significant land use changes in the Amazon could influence water resources as far away as Tibet," they say.