What if ...

With climate change accelerating at a pace totally unforeseen by researchers, the possibility of a world without ice could become reality, a condition last seen about 55 million years ago, the period known as the  Eocene.

The Paleocene–Eocene Thermal Maximum (PETM), alternatively "Eocene thermal maximum 1" (ETM1), and formerly known as the "Initial Eocene" or "Late Paleocene Thermal Maximum", was a time period with more than 8 °C warmer global average temperature than today.[citation needed] This climate event began at the time boundary of the Paleogene, between the Paleocene and Eocene geological epochs.[1] The exact age and duration of the event is uncertain but it is estimated to have occurred around 55.5 million years ago.[2]

The associated period of massive carbon injection into the atmosphere has been estimated to have lasted no longer than 20,000 years. The entire warm period lasted for about 200,000 years. Global temperatures increased by 5–8 °C.[3] The carbon dioxide was likely released in two pulses, the first lasting less than 2,000 years. Such a repeated carbon release is in line with current global warming.[2] A main difference is that during the Paleocene–Eocene Thermal Maximum, the planet was essentially ice-free.[4] However, the amount of released carbon, according to a recent study, suggests a modest 0.2 gigatonnes per year (at peaks 0.58 gigatonnes); humans today add about 10 gigatonnes per year.[5][6]

With intense effort, this might not happen but seeing how passive world governments are regarding this rather serious situation, the odds of preventing this catastrophic disaster is 50/50 at best.

Note: This just covers rising water levels, never mind the ecological, the weather or the level of CO2 we are injecting into the atmosphere at a rate thousands of time faster than the Eocene.

Note II: The amount of  CO2 during the Eocene was estimated to have been 1200+ parts per million, a level that eliminates clouds as a coolant mechanism, thus giving rise to the PETM.

Processes responsible for the cloud deck breaking up around 1,200 ppm CO2 in the model. Temperatures shown in units of kelvins.

Endgame: To tackle this, Schneider and his colleagues flipped things around. They utilized a model that can simulate these clouds in a small patch of atmosphere—given a simplified version of the world around them. Specifically, they simulated a patch of the subtropical ocean with stratocumulus clouds above and a neighboring patch of tropical ocean responding to global warming. They did this for varying concentrations of greenhouse gas equivalent to 400 parts per million of CO2 (similar to today) on up to 1,600 parts per million.

Up to about 1,000 parts per million, there were no major surprises. Things got around 4°C warmer and numbers changed for things like water vapor and cloud altitude. But the cloud deck generally looked familiar.

At about 1,200 parts per million, however, the simulated clouds suddenly dissipated. And without that shade reflecting sunlight, the world warmed another 8°C.

In other words, the PETM.