Burning all fossil fuels could lead to global warmth not seen in 65 million years

If every last crumb of fossil fuel on the planet was burned — all 5.5 trillion tons of the Earth's oil, coal and natural gas now underground — the world could heat up by as much as 18 degrees in the next three centuries, according to a scientific study out Monday that researchers say is a critical warning message on global warming.

This would be as warm as when dinosaurs roamed the Earth about 65 million years ago, one expert said.

"In that scenario, most of the planet would be unlivable for humans (simply too hot for human civilization to function), and all of the major coastal cities of the world and all of the low-lying regions of the world would disappear into the ocean," said Penn State meteorologist Michael Mann, who was not affiliated with the study.

"The unregulated exploitation of the fossil fuel resource could ultimately result in considerably more profound climate changes than previously suggested," according to the study.

While an extreme scenario, such warmth would render some parts of the world uninhabitable and damage the planet's economy, human health and food supply, the study said. The Arctic could warm by as much as 35 degrees.

The study appeared in the peer-reviewed British journal Nature Climate Change.

“Our study shows a profound climate change in the absence of further mitigation,” said study lead author Katarzyna B. Tokarska of the University of Victoria in British Columbia in a statement. How the Earth’s climate responds to such high levels of emissions adds a new dimension to scientific knowledge, she added.

The study used simulations from climate models to explore the relationship between warming and the total amount of potential carbon dioxide emissions, based on estimates of world fossil-fuel reserves.

Burning oil, coal and natural gas releases emissions of "greenhouse gases" such as carbon dioxide and methane that cause the Earth's atmosphere and oceans to rise to temperatures that cannot be explained by natural variability.

The emissions already in the atmosphere have contributed to record warm years in both 2014 and 2015, with another record likely in 2016.

A global climate accord, the Paris Agreement, was signed last December to limit the temperature rise from global warming to under 3.8 degrees compared to pre-industrial levels.

Speaking about the study, Mann said, "If Hollywood were looking for a new dystopian vision of our worst possible future, it probably couldn’t do much better than this scenario."