A chilling Australian policy paper outlining a doomsday scenario for humans if we don’t start dealing with climate change suggests that by 2050, we could see irreversible damage to global climate systems resulting in a world of chaos where political panic is the norm and we are on a path facing the end of civilization.

The worst thing about it, experts say, is that it’s actually a fairly calm and rational look at just how bad things could get — and how quickly — if humans don’t stop emitting greenhouse gases into the environment.

The scenarios "don't seem that far-fetched to me. I don't think there's anything too crazy about them," said Adam Sobel, a professor of applied physics and mathematics at Columbia University in New York City who studies atmospheric and climate dynamics.

The paper was written by an independent think tank in Australia called Breakthrough National Centre for Climate Restoration. It offers a scenario for 2050 in a world where humans didn't lower carbon emissions enough to keep the global temperature from rising.

Last year's United Nation's Intergovernmental Panel on Climate Change report said the world’s nations must quickly reduce fossil fuel use to keep the rise in global temperatures below 1.5 degrees Celsius. The transitions, the report said, must start now and be well underway in the next 20 years.

The Australian report imagines a world where that didn't happen and global temperatures warmed by 3 degrees Celsius or even more. That's a rise of 5.4 degrees Fahrenheit. While that may not seem like a lot, on a worldwide scale it is expected to result in massive, catastrophic shifts to the weather, agriculture and even the habitability of some areas.

"Three degrees Celsius by 2100 is a pretty middle-of-the-road estimate. It's not extreme and it's totally believable," if serious action isn't taken, Sobel said.

The writers say their scenario offers a "glimpse into a world of 'outright chaos' on a path to the end of human civilization and modern society as we have known it, in which the challenges of global security are simply overwhelming and political panic becomes the norm."

Their scenario follows this outline:

2050

In the years leading up to 2050, policy makers fail to cut greenhouse gas emissions. The case for the global climate-emergency mobilization necessary to keep temperatures from rising is "politely ignored." Global greenhouse gas emissions peak in 2030 and begin to fall due to a drop in fossil fuel use, but damage has been done and warming reaches 3 degrees Celsius.

By 2050, sea levels have risen 1.6 feet and are projected to increase by as much as 10 feet by 2100.

Globally, 55% of the population lives in areas subject to more than 20 days of lethal heat a year, beyond the human threshold of survivability.

North America suffers from devastating weather extremes, including wildfires, heatwaves, droughts and flooding. China's summer monsoons fail and water in Asia's great rivers are severely reduced from the loss of more than one-third of the Himalayan ice sheet.

A billion people displaced

Within 30 years from today, ecosystems in coral reefs and the Amazon rainforest collapse, affecting fishing yields and rainfalls.

Deadly heat conditions turn many areas unlivable, resulting in more than a billion people being displaced in West Africa, tropical South America, the Middle East and South-East Asia.

Two billion people globally are affected by lack of water. Food production falls by one-fifth as droughts, heat waves, flooding and storms affect crops.

Rising ocean levels make some of the world's most populous cities uninhabitable, including Mumbai, Jakarta, Canton, Hong Kong, Shanghai, Lagos, Bangkok and Manila. Billions of people must be relocated.

This leads to fights over land, resources and water and potentially to war and occupations.

All too possible

The scenarios given in the paper are all too likely, experts say.

Jonathan Patz is a physician and director of the Global Health Institute at the University of Wisconsin-Madison. He's been studying the health effects of global warming for two decades.

"There are studies showing a doubling of the number of people at risk for hunger by mid-century because of droughts," he said. "And a wider prevalence of infectious diseases like malaria, dengue and the Zika virus. It could result in forced migrations and massive refugee problems."

He noted that just before the Syrian civil war began in 2011, one of the area's most severe droughts on record pushed rural to urban migration rates to four times normal and resulted in food riots.

We’re already getting a taste of what’s to come, said David Doniger, who directs the climate program at the Natural Resources Defense Council, an environmental nonprofit based in New York City. He cited this year’s extreme weather that’s resulting in historic flooding in the Midwest, as well as last year’s giant wildfires and severe storms nationwide.

Imagine that on a global scale, he says.

This past December, a record-shattering heat wave in Australia caused temperatures to soar above 120 degrees in some spots.

“All of these things are going to compound," he said. "People are going to be forced to migrate or die. All of this is going to get worse and combine in ways that worsen political tensions and create instability."

The United States is not immune to any of this, said Solomon Hsiang, who studies climate change economics and directs the Global Policy Laboratory at the University of California-Berkeley. His research has found that colder countries such as Canada and Russia may benefit from warming because they'll have more arable land. But not the United States, which "is already too warm to be a big winner," he said.

The Southeast and the Midwest will fast see bigger, stronger storms and wilder weather, causing flooding, damaging businesses and homes and disrupting farming. The West will see more droughts and wildfires.

Hsiang's research shows a roughly 20% chance that conditions not unlike the Dust Bowl could be almost continuous, he said. That was a four-year period from 1935 to 1938 in which a severe drought and dust storms swept from Texas to Nebraska, killing livestock and destroying crops. Dust from the storms reached as far as New York City.

We have the technology

The good news, scientists say, is that we have the technology to shift to a carbon-neutral energy system today.

"We’re not waiting for solutions," Patz said. "We’re simply waiting for the political will to understand that the solutions are here. Clean energy is not a matter of waiting, it’s a matter of implementing."

Such enormous undertakings are not unprecedented. Hsiang cites the tremendous economic shifts that helped fight World War II.

"When we've faced real threats, we've been willing to make these kinds of large-scale changes," he said.

The decisions we make will be ones future generations will remember us for, Hsiang said.

"The same way we look back today and have pride in the things our grandparents did to defend democracy — our grandchildren are going to look back and have feelings about what we did today," he said.

"What those feelings are will depend on what we decide to do."