One of the predicted consequences of the rise in global temperatures is a rise in ocean levels, one that may cause our infrastructure a some additional strain. Higher temperatures cause water to expand, and have caused the retreat of mountain glaciers and loss of volume in some of the major ice caps. A small but accelerating rise in ocean levels has been detected using tide records, but many questions about the variables that contribute to that rise remain unanswered.

A study that will be released online by Science attempts to fill one of these gaps by looking at how much water humanity has locked up in reservoirs over the last century. The authors of the study, both from Taiwan's National Central University, started with information from the World Register of Dams, maintained by the International Commission on Large Dams, and corrected and augmented their data using additional sources. They were able to track the year of completion and nominal capacity of nearly 30,000 dams. A statistical analysis of the data suggested that the total of small, unregistered reservoirs would alter their figure by less than one percent.

Their data showed a surge in construction following 1950, which peaked around 1980, and has rapidly tailed off; total retained water lagged this, as reservoirs take time to fill, but should be leveling off now. To compensate for water seeping into the surrounding soil and rock (estimates suggest this seepage may roughly equal the reservoir capacity), the authors created a dynamic, time-dependent formula. This additional factor means that the total water retention from the earlier building boom has yet to peak. A multiplier of 0.85 was used to compensate for the fact that reservoirs are not always at capacity.

With all of that accounted for, the final estimate is that roughly 10,800 cubic kilometers of water have been kept out of the oceans by reservoirs. That would account for roughly 30mm, or just over an inch, of additional sea level rises. At current rates of sea level rise, that accounts for roughly a decade's accumulation. The authors argue that this suggests we are underestimating the contributions of the changes in water volume and glacial melting.

Because the author's water retention model is dynamic with time, they were able to overlay the annual amount of water held in reservoirs on top of the measured change in sea level. Doing so essentially eliminates the acceleration in the rates of water rise that appears in the historical records. If dams weren't in the equation, the study suggests, the oceans would have been rising at a constant rate since the 1930s, just after the modern rise in temperatures started.

I expect this won't be the final word on the topic; the authors' model requires a few assumptions that may not hold up to criticism. They recognize that water displacement by silt is a dynamic process that isn't accounted for here. Nearly 10 percent of the total figure could also be accounted for the loss of volume in the landlocked Aral Sea, although it's not clear how much of that wound up diverted to reservoirs. Regardless, the paper is a good attempt at nailing down one of the open questions about how the planet is responding to climate change, and any criticisms should get us closer to a more accurate picture of what we can expect in the future.

Science, 2008. DOI: 10.1126/science.1154580