Global sea levels would be higher and rising faster, if not for reservoir water trapped behind dams around the world, a new study suggests.
But the conclusion does not fully account for other human-caused changes to the water cycle, another researcher cautions.
Sea level rise caused by global warming has the potential to severely impact coastal and island communities by encroaching on populations there and increasing storm damage.
Most of the sea level rise in recent decades has been attributed to the thermal expansion of the oceans (water expands as it heats up) and ice melt from the Antarctic and Greenland ice sheets.
Scientists have known for some time that changes in land water storage, including the amount of water locked behind dams, was an important element in the sea level rise equation, but just how important was uncertain, said study leader Ben Chao of the National Central University in Taiwan.
Chao and his colleagues investigated this question by doing a comprehensive tally of all the world's dams constructed since 1900 (about 29,484) and estimating the amount of water they hold. The data was taken from the International Commission on Large Dams' World Register of Dams.
Chao and his team found that altogether these dams hold about 2,600 cubic miles of water. This corresponds to a drop in global sea levels of about 1.2 inches, meaning ocean levels today would be that much higher if some river water wasn't trapped behind dams and prevented from flowing back into the ocean.
"If you look in just the past half century, the observed sea level rise is about 10 centimeters, and the negative effect of the reservoirs in total has been as much 3 centimeters (almost 2 inches), so in other words, the sea level could have risen 13 centimeters (about 5 inches)," Chao told LiveScience.
The study's findings are detailed in the March 14 issue of the journal Science.
Accounting for the drop from dams means that the average sea level rise over the 20th century estimated by the Intergovernmental Panel on Climate Change (IPCC) (about 1.7 millimeters per year over the last century) would actually be higher than thought, Chao said.
But other scientists caution against making the leap from the amount of sea level rise to the rate at which it has risen. "That may be one step too far," said Vivien Gornitz of Columbia University's Center for Climate Systems Research, who was not involved with Chao's study.
All of the ways in which humans manipulate water on land must be considered, Gornitz says, from groundwater pumping to the increased runoff from cities covered in concrete, which isn't as permeable as soil.
"You have to consider all of these aspects together," Gornitz said, and some scientists think these effects may cancel each other out in terms of their impact on sea level. This view was noted in the last IPCC report.