Deadly climate change could threaten most of the world's human population by the end of this century without efforts well beyond those captured in the Paris Agreement.

That's the finding of a pair of related reports released yesterday by an international group of climate science and policy luminaries who warned that the window is closing to avert dangerous warming. They say carbon dioxide might have to be removed from the atmosphere.

Scientists Yangyang Xu and Veerabhadran Ramanathan found in a paper published in the Proceedings of the National Academy of Sciences (PNAS) that there already exists a 1 in 20 chance that the 2.2 trillion tons of carbon dioxide in the Earth's atmosphere could cause an existential warming threat. This "fat tail" scenario would mean the world experiences "existential/unknown" warming by 2100 — defined in the report as more than 5 degrees Celsius above preindustrial levels.

Temperatures haven't been that high since the Miocene warming period. That low-probability but very extreme scenario could expose most of the world's people to deadly heat stress, with 2.5 billion facing viruses linked to warming and 20 percent of the world's species becoming extinct.

"To put in perspective, how many of us would choose to buckle our grandchildren to an airplane seat if we knew there was as much as a 1 in 20 chance of the plane crashing?" said Ramanathan in a statement. "With climate change that can pose existential threats, we have already put them in that plane."

The report also found a 50 percent chance that temperatures would rise to 4 C under a business-as-usual scenario, a less extreme but still highly dangerous level. The long-term goal of the Paris accord was to maintain warming well below 2 C.

To avoid this fate, Xu and Ramanathan recommend that nations pull three mitigation "levers" in the very near future. The world must achieve carbon neutrality by 2050, they write, with greenhouse gas emissions peaking by 2020 — a rate that is not in line with the voluntary commitments made by countries in Paris. For contrast, the United States under President Obama pledged to cut emissions 80 percent below 2005 levels by 2050 — a promise that the Trump administration has said it will cancel.

The researchers say that countries must also tackle short-lived climate pollutants like hydrofluorocarbons that accelerate warming greatly in the near term, and take some of the carbon that is currently in the atmosphere out. If the turnaround is sufficiently swift on CO2 and other greenhouse gas reductions, fewer carbon sinks will be needed, they write. But the more carbon that is emitted, the more carbon extraction will be needed in the form of reforestation, sequestration and technologies.

Xu and Ramanathan handed their findings off to a cadre of 33 policy and science experts, who compiled a related report considering some of the steps countries could take to contain warming. These ranged from greater reliance on subnational government action to a sharp pivot to wind and solar energy and electric cars.

"We are quickly running out of time to prevent hugely dangerous, expensive, and perhaps unmanageable climate change," wrote the report's authors, who include former U.N. Environment Programme chief Achim Steiner and Mexican chemist Mario Molina, who won the Nobel Prize for his role in discovering the threat that chlorofluorocarbon gases pose to the Earth's ozone layer.

Paul Bledsoe, a co-author of the policy report, described the findings as "pretty disturbing."

"These studies are a wake-up call ahead of U.N. Climate Week — we must not only zero out CO2 emissions by 2050, but also rapidly limit superpollutants like HFCs and methane, and even undertake atmospheric carbon removal," said Bledsoe, a former Clinton White House climate adviser.
