A new climate report co-authored by a former fossil fuel executive has painted a bleak outlook for the future of human civilisation - including nuclear war.

Existential climate-related security risk: A scenario approach, published by Australian thinktank Breakthrough National Centre for Climate Restoration, suggests climate warnings from the likes of the UN are downplaying the sheer chaos that could erupt by 2050 as the world warms.

If emissions don't start to fall until 2030 - behind the goal set by the 2015 Paris Agreement - the authors say the world could warm 3C by 2050 "due to the activation of a number of carbon-cycle feedbacks". This could result in sea levels rising 0.5m by 2050, 3m by 2100 as ice sheets and permafrost melt, and up to 25m in the longer-term.

"Thirty-five percent of the global land area, and 55 percent of the global population, are subject to more than 20 days a year of lethal heat conditions, beyond the threshold of human survivability," by 2050, the report says.

"North America suffers from devastating weather extremes including wildfires, heatwaves, drought and inundation," as the Gulf stream weather system weakens.

"The summer monsoons in China have failed, and water flows into the great rivers of Asia are severely reduced by the loss of more than one-third of the Himalayan ice sheet... Aridification emerges over more than 30 percent of the world's land surface. Desertification is severe in southern Africa, the southern Mediterranean, west Asia, the Middle East, inland Australia and across the south-western United States."

Ecosystems in the Amazon, Arctic and coral reefs collapse and crop yields drop as food-growing regions become too hot to live in.

"Armed conflict between nations over resources, such as the Nile and its tributaries, is likely and nuclear war is possible."

At 25m of sea level rise, much of Auckland's west and south would be underwater, according to floodmap.net.