Human contributes directly to the rise of the sea level

Changing groundwater use is the main reason for the effect

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Climate change is not the only cause of sea-level rise: human beings and their interventions in the global water cycle are also making a major contribution, a Japanese research team has calculated. The biggest factor here is the consumption of groundwater, which does not flow back into the original system after use, but ultimately reaches the oceans via various channels. Although a part of the resulting sea-level rise is absorbed by the effects of reservoirs and other artificial water reservoirs. However, about 42 percent of the increase between 1961 and 2003 is due to man-made factors, according to Yadu Pokhrel of the University of Tokyo and his colleagues in the journal Nature Geoscience.

Meltwater and thermal expansion

In the second half of the 20th century, sea levels rose by an average of 1.8 millimeters per year worldwide. The direct effects of global warming have so far been responsible for this. Because glaciers and ice caps are melting more and more like the ice masses on Greenland and the Antarctic, and the meltwater is flowing directly into the oceans. In addition, the density of seawater decreases with increasing temperature, which is accompanied by an increase in volume - so the water expands and requires more space.

However, in its latest report in 2007, the IPCC found that these factors could not be the sole cause of the observed increase: taken together, they account for just 1.1 of the 1.8 millimeters per year. The remainder must be due to changes in the global water cycle, which are probably mostly man-made, the climate experts suspected even then. However, there was hardly any data.

Reservoirs and groundwater consumption play a major role

Pokhrel and his colleagues have now developed a new, elaborate model that allows them to estimate such contributions. The main focus was on two factors: reservoirs and other artificial water reservoirs as well as the use of groundwater by humans. The former have a capacity of more than 8, 000 cubic kilometers worldwide and thus keep large amounts of water away from the oceans. So you lower the sea level, explains the team. If they had been completely filled, they would have dropped the level by a total of 22 millimeters between 1951 and 2000, and the average actual capacity would have been 15 millimeters.

So to speak, the counterpart is the consumption of groundwater: if it is taken from the ground, if only for irrigation, it then enters either the sewers or, through evaporation and rain, the rivers, from where it finally lands in the oceans. The total consumption world wide was according to calculations of researchers at about 359 cubic kilometers per year. Assuming that 97 percent of them arrive in the oceans, one would expect a rise of 48 millimeters. Another 8 millimeters contributed factors such as changes in soil moisture, a reduction in snowfall and thawing permafrost during the period under consideration. display

Best approach so far, but reliable data problematic

Overall, the 0.77 millimeter per year and thus 42 percent of the actually observed increase from, sums up the team. That corresponds approximately to the proportion, which the previous models could not explain. Nevertheless, the researchers emphasize that the calculations, and especially the data used as a basis, should be optimized to reflect the true situation. For example, in some parts of the world, there is hardly any reliable information about precipitation or the consumption of water. Only then can suitable measures be initiated.

(doi: 10.1038 / ngeo1476)

(Nature Geoscience, 21.05.2012 - ILB)