Scientists warn "we are in a state of planetary emergency" and that we may already have crossed a series of climate change tipping points, creating an "existential threat to civilization".

In an urgent commentary article in the journal Nature, warn that one tipping point - such as the release of methane from thawing permafrost - may increase global warming's rate even faster.  That would in turn fuel others, leading to a cascade of events that occur faster than humanity's ability to cope with them.

Tipping points are when the impacts of global heating become unstoppable.  That would be seen as the runaway loss of forests, or of glaciers and ice sheets.  In the past, it was believed that extreme heating of 5 C Degrees over today's averages was necessary to reach these tipping points.

But now they say that the latest evidence shows that a global temperature increase of just 1 to 2 C degrees could make these cataclysmic events happen.  But they say that the time to act is short and the potential damage will be big, and thus "to err on the side of danger is not a responsible option".

"A saving grace is that the rate at which damage accumulates from tipping could still be under our control to some extent," the scientists write in the report.  "The stability and resilience of our planet is in peril.  International action - not just words - must reflect this."