An international team of scientists has published a study in Proceedings of the National Academy of Sciences (PNAS) showing that even if the carbon emission reductions called for in the Paris Agreement are met, there is a risk of Earth entering what the scientists call “Hothouse Earth” conditions. A “Hothouse Earth” climate will in the long-term stabilize at a global average of 4-5°C higher than pre-industrial temperatures with sea level 10-60 m higher than today, the paper says. The authors conclude it is now urgent to greatly accelerate the transition towards an emission-free world economy.
“Human emissions of greenhouse gas are not the sole determinant of temperature on Earth. Our study suggests that human-induced global warming of 2°C 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.”
Currently, global average temperatures are just over 1°C above pre-industrial and rising at 0.17°C per decade.
The authors of the study consider ten natural feedback processes, some of which are “tipping elements” that lead to abrupt change if a critical threshold is crossed. These feedbacks could turn from being a “friend” that stores carbon to a “foe” that emits it uncontrollably in a warmer world. These feedbacks are: permafrost thaw, loss of methane hydrates from the ocean floor, weakening land and ocean carbon sinks, increasing bacterial respiration in the oceans, Amazon rainforest dieback, boreal forest dieback, reduction of northern hemisphere snow cover, loss of Arctic summer sea ice, and reduction of Antarctic sea ice and polar ice sheets.