In an article published in the journal Nature on November 27, 2019, the scientists call for urgent action to reduce greenhouse gas emissions to prevent key tipping points, warning of a worst-case scenario of a “hothouse,” less habitable planet.
“A decade ago we identified a suite of potential tipping points in the Earth system, now we see evidence that over half of them have been activated,” said lead author Professor Tim Lenton, director of the Global Systems Institute at the University of Exeter.

“The growing threat of rapid, irreversible changes means it is no longer responsible to wait and see. The situation is urgent and we need an emergency response.”

Co-author Johan Rockström, director of the Potsdam Institute for Climate Impact Research, said: “It is not only human pressures on Earth that continue rising to unprecedented levels.
“It is also that as science advances, we must admit that we have underestimated the risks of unleashing irreversible changes, where the planet self-amplifies global warming. This is what we now start seeing, already at 1°C global warming.
“Scientifically, this provides strong evidence for declaring a state of planetary emergency, to unleash world action that accelerates the path towards a world that can continue evolving on a stable planet.”

In the commentary, the authors propose a formal way to calculate a planetary emergency as risk multiplied by urgency.  Tipping point risks are now much higher than earlier estimates, while urgency relates to how fast it takes to act to reduce risk.

Exiting the fossil fuel economy is unlikely before 2050, but with temperature already at 1.1°C above pre-industrial temperature, it is likely Earth will cross the 1.5°C guardrail by 2040. The authors conclude this alone defines an emergency.

Nine active tipping points:

Arctic sea ice
Greenland ice sheet
Boreal forests
Permafrost
Atlantic Meridional Overturning Circulation
Amazon rainforest
Warm-water corals
West Antarctic Ice Sheet
Parts of East Antarctica
The collapse of major ice sheets on Greenland, West Antarctica and part of East Antarctica would commit the world to around 10 meters of irreversible sea-level rise.

Reducing emissions could slow this process, allowing more time for low-lying populations to move.

The rainforests, permafrost, and boreal forests are examples of biosphere tipping points that if crossed result in the release of additional greenhouse gases amplifying warming.