If the past year of floods, fires, heat waves, and droughts are any indicator, climate change is going to be rough. But in case you weren’t already alarmed, a team of prominent climate scientists has penned a paper warning that just two degrees Celsius of human-caused warming could send us spiraling toward a “hothouse Earth”—one that’s up to five degrees Celsius (nine degrees Fahrenheit) warmer with sea levels tens of meters higher than present-day.
It’s an awful, apocalyptic scenario. But how likely is it to occur, and at what temperature threshold? That remains to be seen.
The new paper is a literature review that doesn’t try to pin down probabilities, and its suggestion that two degrees of warming—the goal set forth in the Paris Agreement just a few years ago—might be enough to send us careening toward much hotter world is just a hypothesis. Mostly, it serves as a reminder that catastrophic cascades are within the realm of possibility.
The authors, from the Stockholm Resilience Center, the Australian National University, and elsewhere, looked at dozens of studies to understand how components of the Earth system have behaved during the climate swings of the past 1.2 million years. They found that the planet has never really had a long period in which the climate remained “stable” at two degrees Celsius above pre-industrial temperatures.
“That raised alarm bells because all of our assumptions regarding the Paris Agreement and controlling climate change are based on the idea that if we ‘just’ stop emitting CO2 to the atmosphere, the processes causing climate change will stop and the climate will remain stable,” study co-author Katharine Richardson of the University of Copenhagen told Earther.
While the authors don’t go so far as to say two degrees is a point of no return, they argue that at some threshold, we’ll set off a cascading series of tipping points that prevent the climate from stabilizing.
For instance, a massive thaw-out of northern permafrost soils, or the dieback of a large portion of the Amazon due to climate-induced drought, could create a huge new source of planet-warming carbon emissions. More speculatively, accelerated melting of the Greenland ice sheet could disrupt a key Atlantic ocean conveyor belt, resulting in heat build-up in the Southern Ocean that ultimately causes East Antarctica’s ice to destabilize over many centuries.