"The world can still combat climate change but only if nations raise their collective ambition to achieve a carbon-neutral world in the second half of the century," said Christiana Figueres, executive secretary of the UN Framework Convention on Climate Change (UNFCCC).
"We cannot play a waiting game where we bet on future technological miracles to emerge and save the day."
Figueres oversees UN negotiations towards a global pact on curbing climate-altering greenhouse-gas emissions that must be agreed in Paris late next year.
AFP