By Alex Kirby
BBC News Online environment correspondent
The Earth has entered a new era, one in which human beings may be the dominant force, say four environmental leaders.
Uncharted waters: Surprises could be in store (Image: Anne Jennings)
In the International Herald Tribune, they say the uncertainty, magnitude and speed of change in many of the Earth's systems is without precedent.
The four, who include Margot Wallstrom, the European environment commissioner, say uncertainty cannot excuse inaction.
They believe humanity may cross some critical thresholds unawares, setting off changes which cannot be reversed.
Change at a gallop
The other authors are Professor Bert Bolin, founding chair of the Intergovernmental Panel on Climate Change; Professor Paul Crutzen, winner of the 1995 Nobel prize for chemistry; and Dr Will Steffen, director of the International Geosphere-Biosphere Programme (IGBP).
Their article, The Earth's Threatened Life-Support System: A Global Wake-Up Call, marks the publication of an IGBP book, Global Change And The Earth System: A Planet Under Pressure.
They write: "Our planet is changing fast. Change is a fact of life, but in recent decades many environmental indicators have moved outside the range of variation of the last half million years...
"It is the magnitude and rate of human-driven change that are most alarming.
"The human-driven increase in atmospheric carbon dioxide is nearly 100 parts per million and still growing - already equal to the entire range experienced between an ice age and a warm period such as the present.
"And this human-driven increase has occurred at least 10 times faster than any natural increase in the last half million years."
They envisage the possibility, beyond 2050, of "rapid regional climate change, as would be caused by changes in ocean circulation in the North Atlantic, and irreversible changes, such as the melting of the Greenland ice sheet and the concomitant sea-level rise of six metres".
The authors write: "The Earth has entered the so-called Anthropocene - the geologic epoch in which humans are a significant and sometimes dominating environmental force.
"Records from the geological past indicate that never before has the Earth experienced the current suite of simultaneous changes: we are sailing into planetary terra incognita."
They argue for a precautionary approach, partly because natural systems can flip very rapidly from one stable state to another.
The writers say: "We are unsure of just how serious our interference with Earth system dynamics will prove to be, but... there are significant risks of rapid and irreversible changes to which it would be very difficult to adapt."
Dr Steffen told BBC News Online: "It would take about a millennium for the Greenland ice sheet to melt. But we could reach the trigger point that makes the process unstoppable within the next century.
"The book makes the point that this is global change - it looks at the range of effects, at how they're happening simultaneously, and at how they're reinforcing each other.
"It's a synthesis of the science, the best consensus - and it honestly acknowledges the unknowns."