Global warming might have a larger than expected impact on sea level rise, putting at risk the life and livelihood of millions of people worldwide. That's according to an international team of scientists, who found that global sea levels in the last century grew by about 14 centimeters (5.5 inches), more than double what it would have been without the contribution from global warming. 

To estimate the global sea level, the team had to develop a new statistical database using geological sea-level indicators like coral atolls, archeological sites, and marshes from 24 locations around the world. The database also used records of tide height from the last 300 years. Taken all together, they had data for more than 3,000 years.

"No local record measures global sea level," Robert Kopp, the lead author and an associate professor in Rutgers' Department of Earth and Planetary Sciences, said in a statement. "Each measures sea level at a particular location, where it is buffeted by a variety of processes that cause it to differ from the global mean. The statistical challenge is to pull out the global signal. That's what our statistical approach allows us to do."

The results, published in Proceedings of the National Academy of Sciences, found that the sea level rose and fell in the last three millennia. For example, from the year 1000 to 1400, sea level declined by about 8 centimeters (3 inches). This coincides with a period of global cooling known as the Little Ice Age. During that time, Earth cooled by about 0.2°C (0.4°F).

"It is striking that we see this sea-level change associated with this slight global cooling," Kopp said. So it is perhaps not surprising that we are seeing a more accelerated rise now; the global average temperature today is about 1 °C (1.8 °F) higher than it was in the late 1800s.

The new model was also used to predict the global sea level rise by the year 2100. The increase depends significantly on the amount of greenhouse gasses released in the next 84 years. These findings are in good agreement with another study published in the Proceedings of the National Academy of Science this week.

According to Kopp’s study, if the concentration of greenhouse gasses peaks this decade, the rise will have a mean value of 38 centimeters (15 inches). If the amount of gasses continues to increase throughout the 21st century, the average rise would be 76 centimeters (30 inches) globally.