The World Bank has warned the planet is on track to warm by four degrees Celsius this century - causing increasingly extreme heat waves, lower crop yields and rising sea levels - unless significant action is taken to cut greenhouse gas emissions.
In a major report released ahead of the year-end United Nations climate summit in Qatar, the bank says changes associated with four degrees of warming would have dramatic and devastating effects on all parts of the world, including Australia, but that the poor would be most vulnerable.
Scientists say global warming must be kept within two degrees of pre-industrial temperatures to give the world the best chance of avoiding the worst impacts of climate change.
The report – a snapshot of the most recent climate science prepared for the bank by the Potsdam Institute for Climate Impact Research and Climate Analytics – says global mean warming is now about 0.8 degrees above pre-industrial levels.
It says that if current promises by nations to curb emissions are met then it is most likely there will be more than three degrees warming. However, under that scenario it warns there is also a 20 per cent likelihood that four degrees of warming will occur by 2100.
If current promises are not met, then the world is "plausibly" on a path to warm by four degrees this century, possibly as early as 2060, the bank says.
The report, titled Turn Down the Heat, says if the world experiences four degrees of warming it would:
* See a 150 per cent increase in ocean acidity, leading to the extinction of some sensitive coral reef ecosystems.
* Result in sea-level rise of 0.5 to 1 metres by 2100, with more in following centuries, affecting low-lying islands and coastal communities.
* Lead to more extreme heatwaves, reduced run-off into major rivers and a significant decline in biodiversity, all risking the support systems of humans.
The report says the full impact on human development of a four-degree-hotter world is unknown, making it is unclear whether humanity would be able to adapt.
"A 4°C world is likely to be one in which communities, cities and countries would experience severe disruptions, damage, and dislocation, with many of these risks spread unequally," the report says.
"It is likely that the poor will suffer most and the global community could become more fractured, and unequal than today."
World Bank Group president Jim Yong Kim said: "A four-degree-warmer world can, and must be avoided – we need to hold warming below two degrees."
"Lack of action on climate change threatens to make the world our children inherit a completely different world than we are living in today. Climate change is one of the single biggest challenges facing development, and we need to assume the moral responsibility to take action on behalf of future generations, especially the poorest."
United Nations Secretary-General Ban Ki-moon said: "This new report from the World Bank reminds us that climate change is happening – now. The evidence is clear. No country is immune. If we mobilise today, we can make a difference tomorrow."
The World Bank is the second major international body this month to raise concerns about the rate of greenhouse gas emissions being released into the atmosphere.
The International Energy Agency last week warned in its latest World Energy Outlook that no more than a third of the world's proven fossil fuel reserves can be consumed to 2050, without carbon capture and storage technology, if the two degrees target is to be met.
The warnings come as nations meet in Doha, Qatar next week for the next major round of international climate change negotiations.