James Hansen, former NASA director and well-known climate scientist, is out with another dire climate warning: The last time that the Earth was this hot, the oceans were about 20 feet higher than they are right now.

And while that doesn’t necessarily mean that we’re in for an unstoppable, 20-foot rise in sea level (although it ostensibly could get that bad), it does mean that the world is leaving a dangerous, and expensive, climate change problem for future generations.

“There’s a misconception that we’ve begun to address the climate problem,” Hansen told reporters on a press call Monday. “The misapprehension is based on the Paris climate summit where all the government leaders clapped each other on the back as if some great progress has been made, but you look at the science and it doesn’t compute. We are not doing what is needed.”

Hansen’s warning is based off a new, yet-to-be-peer-reviewed paper — submitted Tuesday to the Earth Systems Dynamics Journal — that he authored with 11 other climate scientists. In the paper, the authors argue that the Earth has warmed by about 1.3°C relative to pre-industrial levels, and that the atmospheric concentration of the most potent greenhouse gases — carbon dioxide, methane, and nitrous oxide — has been accelerating in recent years. The last time the Earth was this hot was during the last inter-glacial period, known as the Eemian, when sea level was about 20 to 30 feet higher than it is today.

In order to stay below the aspirational target of limiting the planet to 1.5°C of warming set in Paris, the paper argues that negative carbon dioxide emissions — that is, sucking carbon dioxide from the air — will be necessary. In a blog post accompanying the paper, Hansen warns that proposed techniques for carbon sequestration, like carbon capture and storage or air capture of carbon dioxide, could cost anywhere from $104 to $507 trillion this century, with “large risks and uncertain feasibility.”