The world is at risk of entering "hothouse" conditions where global average temperatures will be four to five degrees Celcius higher even if emissions reduction targets under a global climate deal are met, according to a new report by top international scientists.

The new study was published in the US Proceedings of the National Academy of Sciences (PNAS) on Monday.

It said it is likely that if a critical threshold is crossed, several tipping points will lead to abrupt change.

Such processes include permafrost thaw; the loss of methane hydrates from the ocean floor; weaker land and ocean carbon sinks; the loss of Arctic summer sea ice and the reduction of Antarctic sea ice and polar ice sheets, said the report.

"These tipping elements can potentially act like a row of dominoes. Once one is pushed over, it pushes Earth towards another," said Johan Rockstrom, co-author of the report and executive director of the Stockholm Resilience Centre.

"It may be very difficult or impossible to stop the whole row of dominoes from tumbling over. Places on Earth will become uninhabitable if 'Hothouse Earth' becomes the reality," he added.

Scientists from the Stockholm Resilience Center, the University of Copenhagen, Australian National University and the Potsdam Institute for Climate Impact Research worked on the study.

They said that "Hothouse Earth" is likely to be dangerous to many and uncontrollable.

Rivers would flood, storms would wreak havoc on coastal communities and coral reefs would be eliminated - all by century's end or even earlier.

Global average temperatures would exceed those of any interglacial period - meaning warmer eras that come in between Ice Ages - of the past 1.2m years. Melting polar ice caps would lead to dramatically higher sea levels, flooding coastal land that is home to hundreds of millions of people.

'Wolves in sight'

A 2015 Paris climate agreement aimed to keep global warming below 2 degrees Celcius, compared with pre-industrial levels.

But according to the report, the risk of tipping cascades could be significant at a 2C temperature rise - and could increase sharply beyond that point.

"This cascade of events may tip the entire Earth system into a new mode of operation," warned co-author Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research.

The "carrying capacity" of a 4C or 5C degree world, he has said previously, could drop to a billion people.

Maximising the chances of avoiding such a "hothouse" state requires more than just reducing greenhouse gas emissions, the report said.

For example, improved forest, agricultural and soil management, as well as biodiversity conservation and technologies that remove carbon dioxide from the atmosphere and store it underground are all needed.

Commenting on the research, some experts said uncontrolled warming is still uncertain, but not implausible.

"In the context of the summer of 2018, this is definitely not a case of crying wolf, raising a false alarm: the wolves are now in sight," said Phil Williamson, climate researcher at the University of East Anglia.

SOURCE: Various News Agencies