Sea levels are rising much faster than the Intergovernmental Panel on Climate Change's (IPCC) projections predict - 60 percent faster, in fact.
They are creeping upwards at a rate of 3.2mm a year, compared to the report's best estimate of 2mm a year.
The researchers, from the Potsdam Institute for Climate Impact Research, Tempo Analytics and Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, analyzed global temperatures and sea-level data over the past two decades.
After stripping out the three known phenomena that cause short-term variability in global temperatures - solar variations, volcanic aerosols and El Nino/Southern Oscillation - the researchers found that the overall warming trend at the moment is 0.16°C per decade, which closely follows the IPCC's projections.
However, satellite measurements of sea levels showed a different picture, with the data much worse thaan predicted. The study shows that it's very unlikely that the increased rate is down to internal variability in our climate system.
It also shows that non-climatic components of sea-level rise - such as water storage in reservoirs and groundwater extraction - don't appear to be involved.
"This study shows once again that the IPCC is far from alarmist, but in fact has under-estimated the problem of climate change," says lead author of the study, Stefan Rahmstorf.
"That applies not just for sea-level rise, but also to extreme events and the Arctic sea-ice loss."