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Looking at the Cause of Global Seal Level Rises

Last week the science community was shocked by the claim that 42% of the sea-level rise of the past decades is due to groundwater pumping for irrigation purposes. What could this mean for the future – and is it true?

The causes of global sea level rise can be roughly split into three categories: (1) thermal expansion of sea water as it warms up, (2) melting of land ice and (3) changes in the amount of water stored on land. There are independent estimates for these contributions, and obviously an important question is whether their sum is consistent with the total sea level rise actually observed.

Sea Level Rise
foto (c) Stefan Rahmstorf 2012

In the last IPCC report (2007), the time period 1961-2003 was analysed in some detail, and a problem was found: the individual contributions summed up to less than the observed rise – albeit with rather large uncertainties in the estimates. In the years since then, much research effort has been devoted to better quantify all contributions. For the last decade there improved observation systems have improved, e.g. the GRACE satellite mission and thousands of autonomous ARGO floats monitoring globally the warming ocean.

Last year Church et al. (2011) provided a new sea-level budget analysis (see Fig. 1). For the period 1972-2008 the budget is closed, with a total rise of about 7 cm. A bit over half of that is due to melting land ice, and a bit less than half due to thermal expansion. Land water storage makes a small negative contribution, because the water stored in artificial reservoirs (which lowers sea level) is estimated to be larger than the amount of fossil groundwater pumped up for irrigation (which mostly ends up in the sea). Also for the shorter recent period 1993-2008 (for which we have satellite measurements of global sea level rise, found to be about 3 mm per year) John Church and colleagues successfully closed the sea level budget. Granted, the uncertainties in the estimates are still significant so the issue cannot be considered completely resolved. Nevertheless, the Church et al. paper defines the current state of the art against which all further studies need to measure up.

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  • Recon Dad on June 05 2012 said:
    Frank Lansner’s first graph surprised me. It’s well known and often quoted that sea levels have been rising by 2-3mm a year every year for the last 20 years. But it’s not well known that the original raw satellite data doesn’t show that at all.

    What astonished me was the sea levels first recorded by the Topex Poseidon satellite array showed virtually no rise at all from 1993-2001. Surely not, I thought. I asked sea-level expert Nils Axel-Morner, and he confirmed: “Yes, it is as bad as that.“ Now, given that Envisat (the European satellite) showed no rise from 2003-2011 (until it was adjusted) that means we have almost 20 years of raw satellite data showing very little rise.

    We thought satellites would finally give us a definitive answer on sea levels. Instead, like the tide gauges, and every other tool available to mankind, apparently satellites systematically underestimate the rising trends. And despite the speed of light being quite quick and all, it can take years for the data to finally arrive. Sometimes 4 or 5 (or 10 years) after the measurement was made scientists “discover” that it was wrong.

    Now of course, any one of these adjustments could be for very legitimate reasons and give us results closer to the truth. But the adjustments always bring data closer to the modeled trend. It’s decidedly non-random. Either there is a God who thinks teasing climate scientists is spiffy, or else there is something fishy going on, and some investigative journalists need to ask some investigative questions. Is that sea-level rise due to global warming or is it due to global adjustments?

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