Even if the nations of the world honor the Paris Climate Accord to cut carbon emissions, the planet may already be on an irreversible course to becoming "Hothouse Earth".

A new study published in the journal "Proceedings of the National Academy of Sciences" says the warming expected in the next few decades could turn some of the Earth's natural protectors into its enemies.

"Human emissions of greenhouse gas are not the sole determinant of temperature on Earth.  Our study suggests that human-induced global warming of 2 degrees Celsius may trigger other Earth system processes, often called 'feedbacks', that can drive further warming - even if we stop emitting greenhouse gases," says lead author Will Steffen from the Australian National University and Stockholm Resilience Centre.  "Avoiding this scenario requires a redirection of human actions from exploitation to stewardship of the Earth system."

Each year the oceans, forests, and land absorb some 4.5 billion tons of carbon that would otherwise end up in our atmosphere adding to temperatures.  But as temperatures continue to rise, this carbon will be released and make the problems of climate change significantly worse.  This would especially be true in the case of the northern permafrost thawing and releasing its carbon stores. 

The study argues that current carbon cutting plans as adopted by world government may not be enough to counter the natural releases.

"What we are saying is that when we reach 2 degrees of warming, we may be at a point where we hand over the control mechanism to Planet Earth herself," said co-author Prof Johan Rockstrom, also from the Stockholm Resilience Centre.  "We are the ones in control right now, but once we go past 2 degrees, we see that the Earth system tips over from being a friend to a foe. We totally hand over our fate to an Earth system that starts rolling out of equilibrium."

What would a "Hothouse Earth" look like?

The climate might stabilize with an average temperature that is four to five degrees warmer than the pre-industrial age.  As ice sheets melt, the seas could be ten to 60 meters higher than now.  That means that many current waterfront communities and cities as well as desert settlements will be uninhabitable.