Now it seems, there's a growing consensus that the next 18 months will be critical in dealing with the global heating crisis, among other environmental challenges.
Last year, the Intergovernmental Panel on Climate Change (IPCC) reported that to keep the rise in global temperatures below 1.5C this century , emissions of carbon dioxide would have to be cut by 45% by 2030.
By Matt McGrath,
But today, observers recognise that the decisive, political steps to enable the cuts in carbon to take place will have to happen before the end of next year.
The idea that 2020 is a firm deadline was eloquently addressed by one of the world's top climate scientists, speaking back in 2017.
The climate math is brutally clear: While the world can't be healed within the next few years, it may be fatally wounded by negligence until 2020," said Hans Joachim Schellnhuber, founder and now director emeritus of the Potsdam Climate Institute.
The sense that the end of next year is the last chance saloon for climate change is becoming clearer all the time.
"I am firmly of the view that the next 18 months will decide our ability to keep climate change to survivable levels and to restore nature to the equilibrium we need for our survival," said Prince Charles, speaking at a reception for Commonwealth foreign ministers recently.
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So why are the next 18 months so important?
The Prince was looking ahead to a series of critical UN meetings that are due to take place between now and the end of 2020.
Ever since a global climate agreement was signed in Paris in December 2015, negotiators have been consumed with arguing about the rulebook for the pact.
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