Earth heading towards irreversible 'hothouse' state: Study

Our planet is at the risk of entering an irreversible 'hothouse' condition - where the global temperatures will rise by four to five degrees and sea levels may surge by up to 60 metres higher than today - even if targets under the Paris climate deal are met, a study warns.

According to the researchers, keeping global warming to within 1.5-2 degrees Celsius may be more difficult than previously assessed.

"Human emissions of greenhouse gas are not the sole determinant of temperature on Earth," said Will Steffen from the Australian National University.

"Our study suggests that human-induced global warming of two degrees Celsius may trigger other Earth system processes, often called "feedbacks," that can drive further warming - even if we stop emitting greenhouse gases," said Steffen, lead author of the study published in the journal PNAS.

"Avoiding this scenario requires a redirection of human actions from exploitation to stewardship of the Earth system," he said.

A team of scientists showed 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 stabilise at a global average of 4-5 degrees Celsius higher than pre-industrial temperatures with sea level 10-60 metres higher than today, according to the study.

Currently, global average temperatures are just over 1 degree Celsius above pre-industrial and rising at 0.17 degree Celsius per decade.

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 include 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.

"These tipping elements can potentially act like a row of dominoes. Once one is pushed over, it pushes Earth towards another. It may be very difficult or impossible to stop the whole row of dominoes from tumbling over," said Johan Rockstrom, Executive Director of the Stockholm Resilience Centre in Sweden.

"Places on Earth will become uninhabitable if "Hothouse Earth" becomes the reality," Rockstrom said.

"We show how industrial-age greenhouse gas emissions force our climate, and ultimately the Earth system, out of balance," said Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research in Germany.

"In particular, we address tipping elements in the planetary machinery that might, once a certain stress level has been passed, one by one change fundamentally, rapidly, and perhaps irreversibly. This cascade of events may tip the entire Earth system into a new mode of operation," Joachim said.

Maximising the chances of avoiding a "Hothouse Earth" requires not only reduction of carbon dioxide and other greenhouse gas emissions but also enhancement and creation of new biological carbon stores through improved forest, agricultural and soil management, and technologies that remove carbon dioxide from the atmosphere and store it underground, researchers said.