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FIGURE 3.8 Accumulated global reservoir water storage in dams from 1900 to 2025 (yellow bars), based on observations and projections. The rate of water storage in dams higher than 15 m or with a capacity of more than 3 million m3 has begun declining over the past decade because of sedimentation (blue and gray bars), potentially reducing the rate of sea-level rise. SOURCE: Lettenmaier and Milly (2009).

this report was nearing completion, a new evaluation by Wada et al. (2012b) found a net positive contribution to global sea-level rise of 0.25 ± 0.09 mm yr-1 during the 1990–2000 period as a result of a decrease in reservoir construction and an increase in groundwater depletion. If this result holds, terrestrial water storage could become a significant contributor to future sea-level rise.

CONCLUSIONS

The most comprehensive recent assessments of global sea-level rise is given in the IPCC Fourth Assessment Report, which evaluated data and research results published up to about mid-2006, and Church et al. (2011), which provided updated data on the components of sea-level rise. The IPCC (2007) found that the relative contributions to global sea-level rise varied over time, with thermal expansion contributing significantly more to sea-level rise for 1993–2003 than for 1961–2003. Since then, thermal expansion estimates have been corrected for instrument biases, which gave systematically warmer temperatures than the true value globally and cooler temperatures than the true value in a portion of the Atlantic Ocean. The corrected rates of thermosteric sea-level rise for the two IPCC (2007) periods are more similar, with a higher thermal expansion contribution for 1961–2003 and a lower thermal expansion contribution for the 1993–2003 period.

In addition, new types of measurements, notably the GRACE satellite system, and expanded data sets have become available since the IPCC Fourth Assessment Report was published. Estimates incorporating the new data suggest a faster growing contribution of land ice to sea-level change than was seen in IPCC (2007) for the two periods. Since 2006, ice loss rates have accelerated in the ice sheets and declined in glaciers and ice caps, likely reflecting interannual to multi-annual variability and possibly uncertainties in data processing or interpretation of short records. The most recent published estimate is that land ice melt accounted for about 65 percent of global sea-level rise for 1993–2008 (Church et al., 2011). The prospect of increased ice sheet melting is important to future sea-level rise because the Greenland and Antarctic ice sheets store the equivalent of at least 65 m of sea level.

New data and models also are available for estimating the contribution of terrestrial water (besides ice melt) to global sea-level rise. Although the contributions of the two largest terms—groundwater depletion, which transfers water to the ocean and raises sea level, and reservoir impoundment, which prevents water from reaching the ocean and lowers sea level—are significant, they are difficult to measure. As a result, most recent assessments have not assigned a rate to terrestrial storage or assigned a rate of zero, within the limits of uncertainty.



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