We might be seriously underestimating sea level rise caused by climate warming. That is the conclusion of a study recently published in Proceedings of the National Academy of Sciences. 

Previous estimations have suggested a maximum rise of 98 centimeters by 2100. Experts are now saying it could be more than double that.

Needless to say, this is bad news for all of us. "Such a rise in global sea level could result in land loss of 1.79 million square kilometers (690,000 square miles), including critical regions of food production, and potential displacement of up to 187 million people," Jonathan Bamber, lead author from the University of Bristol, said in a statement. "A [sea level rise] of this magnitude would clearly have profound consequences for humanity."

To put things into perspective, this is a land mass equivalent in size to seven Californias or 17 Floridas. And that is a population greater than Canada, Germany, and the UK – combined. It would put cities like New York, London, and Shanghai at risk.

Bamber and his colleagues collated the work of 22 researchers, leaders in their field, on the melting of the Greenland and Antarctic ice sheets. Together, the findings suggest that if we stick to the goals laid out in the Paris Agreement and global temperatures increase no more than 2°C above pre-industrial levels by the end of the century, sea levels will likely rise by 26 centimeters (the median estimate) – but could rise as much as 81 centimeters (the 95th percentile).

But if we fail the Paris Agreement and continue on the "business as usual" route, warming will likely increase to 5°C above pre-industrial levels and we can expect a rise between 51 and 178 centimeters. When you add in thermal expansion (the propensity of matter to change its shape and size in response to temperature) and glacier contributions, that figure exceeds 2 meters in the 95th percentile, the study authors say. (Even this is conservative by some estimates. Some studies have gone even higher, predicting a 2.5-meter rise.)

As Bamber and co. point out, the possible contributions of melting ice sheets (or continental glaciers) to sea level rise is the single greatest source of uncertainty we currently face. Hence, the fairly large range of sea level rise predicted in the study.

However, recent research has highlighted the effect various positive feedbacks (that accelerate melt) and negative feedbacks (that do the opposite) may have on overall ice melt in the future. What's more, we can see with our own eyes the acceleration of glacial melt, which is happening at a faster pace than expected. 

The good news is that a 2-meter rise is unlikely. That does not mean it is impossible.

"If I said to you that there was a one in 20 chance that if you crossed the road you would be squashed you wouldn't go near it," Bamber told the BBC. "Even a 1 percent probability means that a one in a 100-year flood is something that could happen in your lifetime. I think that a 5 percent probability, crikey – I think that's a serious risk."

Fortunately, there is still time to stop it.