A new analysis has examined the effects of melting polar ice sheets and the subsequent rise in sea levels over the last three million years. The paper summarizes 30 years of research into the relationships between melting ice sheets, fluctuating sea levels and the levels of carbon dioxide in the atmosphere. Its most startling conclusion: sea levels have risen by 6 meters (20 feet) multiple times in the past, and this increase was prompted by a rise in global mean temperatures of only 1–2^oC. 

It might put you on edge to discover that the current levels of carbon dioxide in the atmosphere are equivalent to those estimated to be present three million years ago, when sea levels increased by 6 meters.

Since 1880, our oceans have risen by roughly 20 centimeters (8 inches). This isn't the dramatic 6 meters described by the paper, but Anders Carlson, from Oregon State University (OSU) and coauthor of the study published in Science, explains why: "It takes time for the warming to whittle down the ice sheets.

"But it doesn't take forever. There is evidence that we are likely seeing that transformation begin to take place now."

Peter Clark, from OSU, and the other coauthor in the study, who noted that the levels of CO₂ today are the same as those 3 million years ago, concluded that "we are already committed to a certain amount of sea level rise," although currently we do not yet know how much this is going to be.

With the atmospheric levels of CO₂ still on the rise, we are steadily heading into uncharted territory. The predictions of the past may not be enough to accurately predict the knock-on effect of pouring more CO₂ into the atmosphere. It could take centuries or even millennia to see the full impact of sea level rises. 

The worry here isn't just the depletion of the ice caps to a point beyond what we've never before experienced, but also the downstream effect for the rest of the globe, in particular for coastal regions where millions of people live. Places that would be drastically affected by a rise in sea levels include low-lying locations of the U.S., such as Florida and parts of Louisiana, as well as Dhaka in Bangladesh, which has 14.4 million inhabitants, all living in low-lying areas. Tokyo and Singapore have also been identified as potentially being vulnerable to sea-level rises. 

"The influence of rising oceans is even greater than the overall amount of sea level rise because of storm surge, erosion and inundation," said Carlson. "The impact could be enormous."