The Earth is in imminent peril and nothing short of a planetary rescue will save it from the environmental cataclysm of dangerous climate change, six leading scientists say .
Writing in the Philosophical Transactions of the Royal Society A, the American scientists believe civilisation itself is threatened by global warming.
They also implicitly criticise the UN's Intergovernmental Panel on Climate Change for under-estimating the scale of sea-level rises this century as a result of melting glaciers and polar ice sheets.
Instead of sea levels rising by about 40 centimetres, as the IPCC predicts in one of its computer forecasts, the scientists say the true rise may be as great as several metres by 2100, which is why Earth today is in "imminent peril".
The 29-page scientific paper, Climate Change and Trace Gases, is the product of James Hansen, the director of Nasa's Goddard Institute for Space Studies, Makiko Sato, Pushker Kharecha, and Gary Russell, also of the Goddard Institute, David Lea of the University of California, Santa Barbara, and Mark Siddall of the Lamont-Doherty Earth Observatory at Columbia University in New York.
They say: "Recent greenhouse gas emissions place the Earth perilously close to dramatic climate change that could run out of control, with great dangers for humans and other creatures."
Only intense efforts to curb man-made emissions of CO2 emissions and other greenhouse gases can keep the climate near the range of the past one million years.
The unnatural "forcing" of the climate as a result of man-made emissions of carbon dioxide and other greenhouse gases threatens to generate a "flip" in the climate that could "spark a cataclysm" in the ice sheets of Antarctica and Greenland.
Dramatic climatic flips have occurred in the past but none has happened since the development of complex human societies, which are unlikely to survive the same sort of environmental changes if they occurred now.
"Civilisation developed, and constructed extensive infrastructure, during a period of unusual climate stability, the Holocene, now almost 12,000 years in duration. That period is about to end," the scientists say.
Humanity cannot afford to burn the Earth's remaining underground reserves of fossil fuel. "To do so would guarantee dramatic climate change, yielding a different planet from the one on which civilisation developed and for which extensive physical infrastructure has been built," they say.
Dr Hansen said the world had about 10 years to put into effect the draconian measures needed to curb CO2 emissions quickly enough to avert a dangerous rise in global temperature. Otherwise, the extra heat could trigger the rapid melting of polar ice sheets, made worse by the "albedo flip" - when the sunlight reflected by white ice is suddenly absorbed as ice melts to become the dark surface of open water.
The glaciers and ice sheets of Greenland in the northern hemisphere, and the Western Antarctic ice sheet in the south, both show signs of the rapid changes predicted with rising temperatures.
"The albedo flip property of ice/water provides a trigger mechanism. If the trigger mechanism is engaged long enough, multiple dynamical feedbacks will cause ice-sheet collapse," the scientists say.
"The required persistence for this trigger mechanism is at most a century, probably less."
The study looked back over more than 400,000 years of climate records from deep ice cores and found evidence to suggest that rapid climate change over a period of centuries, or even decades, have in the past occurred once the world began to heat up and ice sheets started melting. However, it is not possible to assess the dangerous level of man-made greenhouse gases.