Pumping sea-level up

Global sea-level has risen at a pace of about 1.8 mm/year in the period 1960 to 2003. Several factors are contributing to this rise: ocean thermal expansion, glacier melting, and ground water depletion. Which factor has been the strongest contributor ?

According to a recent paper, the strongest contribution stems from pumping out ground water for irrigation, which subsequently either evaporates and precipitates over the ocean or each channelled to the oceans by rivers. The other side of the coin is construction of artificial reservoirs, which capture water that otherwise would have ended up in the oceans as precipitation or river flow. Thus groundwater depletion causes sea-level rise, whereas water storage in reservoirs depletes sea-level. These two anthropogenic contributions, in particular ground water depletion (GWD), are found to be one of the largest factors. GWD has attained a value of 1.05 mm/year, compared to about 0.5 mm/year caused by glacier and ice-cap melting in this period.

Estimates of various contributions to the observed SLC. The estimates of the IPCC AR4 and that of this study are for the period 1961–2003, whereas the estimates of the other studies are for the later half of the twentieth century, except for Wada et al.19 (1960–2000). Error bar indicates the uncertainty range in the residual of the sea-level budget estimated by the AR4. From Pokhrel et al. Nature Geosciences 

 

This most recent study provides estimations of GWD that are larger than previous studies so far. But it has to be borne in mind that these estimations are the results of model simulations and not of direct observations. Previous studies were also based in some way or another based on model estimations, but Pokhrel et al claim that their integrated model is superior to previous estimations. As usual, I would interpret that these estimations may be better than previous ones, but I would put more confidence on these figures once they have been confirmed by other groups in the next years. This paper will be likely discussed in the next IPCC report, but any follow-up studies, confirming or rebutting these numbers, will not, as the dead line to submit articles to be considered by the IPCC is due in 2 months.

The high contribution to sea-level rise by GWD indicated in this study doe not imply that the contribution of ocean expansion or glacier melting have to be readjusted. This is because the 'sea-level budget' is still not really closed for the period up to year 2000, and there is still a margin of uncertainty that is not accounted for by any of the known potential contributors. This is not the case for the last decade, for which the nearly global and more accurate measurements from satellite altimetry, satellite gravimetry and the Argo monitoring system, do allow to disentangle the individual contributions to the observed sea-level rise.

The new estimations of GWD may have, however, consequences for the so called 'semi-empirical' estimations of future sea-level rise. These methods are based on statistical relationships between global mean temperature and the rate of sea-level rise. If a larger part than previously thought of the observed sea-level rise is not climate-related, these statistical estimation may be then biased high.