Sea levels are rising faster than predicted as a result of climate change, scientists said.
Satellite measurements show sea levels rose 3.2mm a year between 1993 and 2011, 60% above the 2mm estimate in central projections made by the Intergovernmental Panel on Climate Change (IPCC) in its most recent review of climate science.
A study published in the Institute of Physics journal Environmental Research Letters said it was "very unlikely" the higher rate of sea level rise is due to natural variability such as temporary ice discharge from ice sheets.
Lead author Stefan Rahmstorf said: "This study shows once again that the IPCC is far from alarmist, but in fact has underestimated the problem of climate change. That applies not just for sea level rise but also to extreme events and the Arctic sea loss."
The researchers also warn that the rate of annual sea level rise may increase as global temperatures go up, a suggestion backed up by past sea level data and which leads to larger projections of future sea level rise than the IPCC predicts.
And they said the concern that the IPCC's estimates for future sea level rise are low is supported by the fact ice sheets in Greenland and Antarctica are increasingly losing mass, while the panel's projections assume that Antarctica will gain enough ice to compensate for losses from Greenland.
The study analysing global temperature and sea level data over the past two decades also found that temperature rises are consistent with the IPCC's projections in the fourth assessment report, published in 2007.
Once factors which cause natural variability in global temperatures, including solar and volcanic activity and the El Nino weather patterns in the Pacific, are removed there is an overall warming trend of 0.16C a decade.
This closely matches predictions by the IPCC, the researchers found.
The scientists, from the Potsdam Institute for Climate Impact Research, Tempo Analytics and Laboratoire d'Etudes en Geophysique et Oceanographie Spatiales, said it was important to assess how well past projections match what is happening as IPCC projections are increasingly used in decision-making.