A group of top international experts on Wednesday urged governments to stick to their promises to combat climate change and said the aim should be to create a "zero-carbon society" by 2050.
To coincide with Earth Day, research institute The Earth League published a statement warning that 2015 would be "a critical year for humanity" ahead of a global warming summit in Paris in December.
World leaders will also meet this year to discuss financing for developing countries and UN sustainable development goals are due to be adopted in September.
"Our civilisation has never faced such existential risks as those associated with global warming, biodiversity erosion and resource depletion," said the statement, unveiled in London by Earth League chairman Johan Rockstrom and climate scientist Brian Hoskins.
Rockstrom said the window of opportunity for keeping warming below the 2.0 degrees celsius (3.6 degrees Fahrenheit) target agreed by politicians was "still open, but barely".
He stressed that nature's capacity to dampen the warming effects of carbon dioxide were unknown, and that "critical thresholds" could be exceeded even before two degrees.
According to the group's calculations, there is a one in 10 chance that temperatures could rise by six degrees by 2100 unless emissions are reduced.
The statement called on world leaders to agree a carbon budget -- the limit of what can still be released into the atmosphere -- which would be around half of what has already been emitted.
It also called for the complete phasing out of greenhouse gases by 2050, a wave of climate-led innovation, measures to build up resilience and the safeguarding of carbon absorbers such as forests.
Rockstrom said carbon dioxide emissions from the world's major energy producers levelled out in 2014, partly fuelled by the US bonanza in shale gas, which produces fewer emissions that coal and oil.
Rich countries would shoulder the burden, the experts said, leading the way in subsidising research of low-emission technologies and financing developing countries in order that they can "leapfrog" the carbon-intensive phase of their evolution.
Jeffrey Sachs, US economist and one of the authors, called Paris conference "the moment of truth" and the last chance to stay within the two degrees upper limit.
"Our studies show this can be accomplished, at modest cost, and with a significant improvement in the quality of life," he wrote.