What does 1.5C mean in a warming world?

Over the past three years, climate scientists have shifted the definition of what they believe is the "safe" limit of climate change.

For decades, researchers argued the global temperature rise must be kept below 2C by the end of this century to avoid the worst impacts.

But scientists now argue that keeping below 1.5C is a far safer limit for the world.

Everyone agrees that remaining below that target will not be easy.

This week in South Korea, researchers will report on the feasibility and costs of achieving this lower limit.

The scientists of the Intergovernmental on Climate Change (IPCC) are gathering in the city of Incheon to hammer out a plan in co-operation with government delegates, on the actions that would need to be taken to meet this new goal.

In a word - politics.

The idea of two degrees as the safe threshold for warming evolved over a number of years from the first recorded mention by economist William Nordhaus in 1975.

By the mid 1990s, European ministers were g up to the two-degree limit, and by 2010 it was official UN policy. Governments agreed in Cancun to "hold the increase in global average temperatures below two degrees".

However, small island states and low-lying countries were very unhappy with this perspective, because they believed it meant their territories would be inundated with sea water as higher temperatures caused more ice to melt and the seas to expand.

They commissioned research which showed that preventing temperatures from rising beyond 1.5C would give them a fighting chance.

Sign up for a weekly chat about climate change on Facebook Messenger

At the ill-fated Copenhagen climate summit in 2009, the climate-vulnerable nations pushed for the lower figure, but their efforts were lost in the blame-game that followed the collapse of the conference.

But the idea didn't go away completely - and by the time of the Paris negotiations in 2015, it emerged centre-stage as French diplomats sought to build a broad coalition of rich and poor nations who would a deal.

It worked.

More than you might think!

Leaked drafts of the IPCC Summary for Policymakers that will be published after a week of haggling with government delegates in South Korea point to some major differences in of the impacts on the world of 1.5 and 2C. We've summarised the main ones here:

"Two degrees is no longer the two degrees we thought it was," said Kaisa Kosonen from Greenpeace who is monitoring the progress of the IPCC 1.5C report.

"It's increasingly becoming meaningless as a climate goal, when you look at the risks that would come with it and what we are already witnessing with one degree - why would you have a goal that doesn't protect anything that we care about":[]}