Where Is All That Water Coming From?
By Ed Caryl
DirkH asked an interesting question in the comments to my last post: “It would be interesting to know how much of sea level rise is due to ancient aquifer depletion. Anyone got a number for that, in mm/yr?” The short answer (paper #1) is, about 0.77 mm/year for all anthropogenic causes.
Rusting ship hulks on what used to be the Aral Sea. Source: Wikipedia
But, this question deserves a longer post. The number above includes several anthropogenic sources of the extra water. Aquifer depletion is currently offset by dam impoundment,(see below), but there are several very visible, (but forgotten) sources of that extra water. Water diversion from rivers that flow into lakes and seas with no outlet to the ocean (endorheic lakes) are very important.
Examples include the Caspian Sea, Lake Aral or Aral Sea, Lake Balkhash, Lop Nur, Lake Chad, the Dead Sea, the Great Salt Lake, and many smaller lakes. As these lakes dry up, the diverted water eventually, through evaporation and rain, ends up in large part in the world’s oceans.
Figure 1 is the Aral Sea, 1989 on the left, 2008 on the right. Wikimedia Commons here.
As mentioned above, aquifer depletion is currently offset by dam impoundment. A paper in Geophysical Research Letters here (paper #2) describes the numbers. Quote:
Our results show that the contribution of groundwater depletion to sea-level increased from 0.035 (±0.009) mm yr−1 in 1900 to 0.57 (±0.09) mm yr−1 in 2000, and is projected to increase to 0.82 (±0.13) mm yr−1 by the year 2050. We estimate the net contribution of terrestrial sources to be negative of order −0.15 (±0.09) mm yr−1 over 1970–1990 as a result of dam impoundment. However, we estimate this to become positive of order +0.25 (±0.09) mm yr−1 over 1990–2000 due to increased groundwater depletion and decreased dam building. We project the net terrestrial contribution to increase to +0.87 (±0.14) mm yr−1 by 2050. As a result, the cumulative contribution will become positive by 2015, offsetting dam impoundment (maximum −31 ± 3.1 mm in 2010), and resulting in a total rise of +31 (±11) mm by 2050.”
Unfortunately, other papers don’t agree. This study (paper#3) says aquifer depletion results in 0.4 mm/year sea level rise, without mentioning dam impoundment. Another study (paper #4) says the aquifer depletion is 0.3 mm/year with impoundment at -0.4 mm/year leaving a net of -0.1 mm/year. Neither of these studies mention drying lakes and seas as a contributor.
They also don’t agree on the numbers for the various contributors.
Figure 2 is a chart using data from the four papers cited.
Papers two, three, and four don’t mention sea level rise from water diverted from land-locked lakes and seas. They also differ by more than their error bars. In the chart above, the “drying lakes” number is derived by subtracting paper one’s result from paper two’s total.
All four papers are behind pay-walls, so details not mentioned in the extracts, and any charts that might explain the ambiguities, are not available.
There is a lot of guess-work in the above papers. We really don’t know with any degree of certainty what the sources of the equally fuzzy sea level rise are. About all one can say is that sea level rise is coming roughly equally from man messing with the water, thermal expansion, and ice melting.