Twenty days of lethal heat per year, collapsed eco-systems and more than one billion people displaced. Those are all probable scenarios that could devastate societies by 2050 if swift and dramatic action isn't taken to curb climate change, according to a think tank report backed by a former Australian military chief.

The paper, by the Melbourne-based Breakthrough National Center for Climate Restoration, is not a scientific study, but an attempt to model future scenarios based on existing research. It paints a bleak future in which more than a billion people are displaced, food production drops off and some of the world's most populous cities are left partially abandoned.

Its foreword is written by Chris Barrie, a retired admiral and former head of the Australian Defense Force, who said that "after nuclear war, human-induced global warming is the greatest threat to human life on the planet."

"A doomsday future is not inevitable," he notes. "But without immediate drastic action our prospects are poor."

Andrew King, a climate science lecturer at the University of Melbourne who was not involved in the report, said its findings were "plausible," although he did not expect human civilization to end in 2050.

King said that while he expected all of the issues mentioned in the paper to be occurring by 2050 remained to be seen how widespread they would be. He noted that there were many factors aside from climate change that could have an impact on global security and how humans react to altered conditions, such as population growth and inter-governmental action.

Authors David Spratt and Ian Dunlop, both longtime climate researchers, warn that climate change at present poses a "near-to-mid-term existential threat to human civilization." They drew on existing scientific research and "scenario planning" to forecast that if global temperatures rise 3 degrees Celsius or 37.4 Fahrenheit, by 2050, 55 percent of the world's population across 35 percent of its land area would experience more than 20 days of lethal heat per year, "beyond the threshold of human survivability." In that scenario, many ecosystems would collapse.

Across West Africa, tropical South America, the Middle East and Southeast Asia, there would be more than 100 days a year of deadly heat, leading to over 1 billion people being displaced. Food production would drop off due to the "catastrophic decline" in insect populations, weather too hot for humans to survive in significant food-growing areas and chronic water shortages. With not enough food for the world's population, prices would skyrocket, the paper's authors argue. Rising sea levels would cause people to abandon parts of Mumbai, Jakarta, Guangzhou, Hong Kong, Ho Chi Minh City, Shanghai, Bangkok and Manila, among other cities. Around 15 million people in Bangladesh would be displaced.

In May, IPBES, a UN-affiliated climate research group, released a damning report on global biodiversity, which found that 75 percent of the planet's land surface has been "significantly altered" and one million species already face extinction.