Climate change behind uneven sea level rise: Study

The pattern of uneven sea level rise over the last 25 years has been driven in part by human-caused climate change, not just natural variability, according to a study.

The findings, published in the journal Proceedings of the National Academy of Sciences, suggest that regions of the world where seas have risen at higher than average rates can expect the trend to continue as the climate warms.

"By knowing that climate change is playing a role in creating these regional patterns, we can be more confident that these same patterns may linger or even intensify in the future if climate change continues unabated," said John Fasullo from the National Center for Atmospheric Research (NCAR) in the US.

"With sea levels projected to rise a couple of feet or more this century on average, information about expected regional differences could be critical for coastal communities as they prepare," Fasullo said.

The findings have implications for local officials, who are interested in improved forecasts of sea level rise for the areas they oversee, researchers said.

In the past, forecasters have had to rely on the global rate of change -- about 3 millimetres a year and accelerating -- and knowledge of the uneven regional impacts associated with continued melting of the ice sheets covering Greenland and Antarctica.

The researchers, including Steve Nerem at the University of Colorado Boulder in the US, analysed the satellite altimetry sea level record, which includes measurements of sea surface heights stretching back to 1993.

They mapped global average sea level rise as well as how particular regions deviated from the average.

For example, the oceans surrounding Antarctica and the US West Coast have had lower-than-average sea level rise, while the US East Coast and Southeast Asia have experienced the opposite, researchers said.

In some parts of the world, the rate of local sea level rise has been as much as twice the average, they said.

To investigate the role of climate change, the scientists turned to two sets of climate model runs, known as "large ensembles."

These large ensembles -- many simulations by the same model, describing the same time period -- allow researchers to disentangle natural variability from the impacts of climate change.

With enough runs, these impacts can be isolated even when they are relatively small compared to the impacts from natural variability.

The climate models suggest that in regions that have seen more or less sea level rise than average, as much as half of that variation may be attributed to climate change.

The scientists also found that the impacts from climate change on regional sea level rise sometimes mimic the impacts from natural cycles.