By Rud Istvan – Re-Blogged From http://www.WattsUpWithThat.com
There is no doubt that interglacials change sea level (SL). And that sea level rise (SLR) can be dramatic on millennial interglacial time scales. That’s what happens when the vast Laurentide ice sheet (among others) melts. But sea level has changed relatively little in the past 7 millennia. We know from archeology that it rose somewhat in the Medieval Warm Period, dropped some during the Little Ice Age, and has been rising slowly since based on tide gauge records. This mostly natural variation is, from 1950-2000, about +1.8mm/year to 2.2mm/year (discrepancy explanation and references follow below). That rate is no cause for alarm. We coped with it for the past century, and can cope with it for the next.
The anthropogenic global warming (AGW) question is whether SLR will accelerate into catastrophic AGW (CAGW) requiring urgent mitigation? Warmunists argue yes, with many alarming images such as National Geographic’s photoshopped Statue of Liberty half submerged (which would require that all of Greenland and Antarctica melts before the next
glaciation sets in). We know that in the last interglacial, the Eemian, the sea level highstand was about 6.6 meters above present sea level, so the ice sheets did melt more with global temperature about 2C higher (perhaps 8C higher in Greenland). This took about 3000 years to achieve. That is (660cm/30 centuries) ~22 cm/century, or roughly 2.2mm/year. That rate is no different than the best estimate of present ‘true’ SLR found in what follows.
There are two parts to the SLR alarm meme. 1. SLR is accelerating because of AGW. 2. SLR will therefore become a big problem even if not one today. Ergo, future CAGW requires urgent mitigation. Hansen says up to 1 meter SLR by 2060 (and maybe more with tipping), so goodbye Miami Beach. The Obama administration uses occasional recent high tide flooding there to support his climate urgency– without revealing that Miami Beach is 2/3 fill dredged from Biscayne Bay, and is subsiding as the fill compacts and as the underlying Biscayne Aquifer subsides as it is drained to provide Miami’s fresh water.
There are many graphics floating about that support the SLR acceleration alarm.
This seemingly authoritative graphic has been constructed out of two deceptions.
(1) Uses tide gauge records uncorrected for land motion. (C&W 2005 is actually doubly misrepresented: C &W data before 1950 is W,C,&G GRL 2006 [C&W 2005 is only 1950-2000], and there is no 1925-1992 reference period in the 2006 paper). C&W 2005 SLR 1950-2000 was 1.8 mm/year rather than 1.9 and was not accelerating. C&W used approximately 350 long record tide gauges for their ‘accelerating’ reconstruction. There are only 146 PSMSL tide gauges (TGs) within 1000 meters of a differential GPS to correct for vertical land motion of whatever cause, and only about 70 of those are long record TGs. So vertical land motion is NOT excluded in either C&W paper.
(2) Splice on satellite measurements since 1993 that fail the closure test below: observed SLR should roughly equal the sum of observed sea level contributions from thermosteric rise (OHC causes the estimated water column to expand as it warms) plus ocean mass increase (water volume from ice sheet losses).
Side comment: in the SLR literature, there are also two lesser possible SLR contributors: mountain glaciers, and groundwater extraction. Both are sufficiently uncertain and small that they are ignored in this post. There is little doubt that glaciers have been receding most places since 1900, for example in the Himalayas. But IPCC AR4 issued a retraction concerning its alarming conclusion that they would disappear by 2030. Most terrestrial groundwater is annually replenished; else water tables would fall and wells fail. That which is not replenished (e.g. the world’s largest aquifer, the Ogallala) is de minimus in the context of global SLR. (Ice density at 0C is 0.9187, water is 0.9998, so if 365 Gt ice (roughly 365 cubic km) equals 1mm of SLR [reference below] then 1mm of SLR requires adding about 335 cubic kilometers of water. Ogallala withdrawals are about 26-30 cubic km/year and in some places will be exhausted by 2030.)
Tide Gauge SLR
The actual rate of sea level rise is uncertain, let alone possibly detectable AGW acceleration. There are two basic uncertainties.
(1) SLR is inconstant because of multiannual changes in lunar tides (~18 year periodicy) and ocean currents/oscillations (~30-60 year periodicy). Tide gauges show this natural decadal variability almost everywhere. So fluctuations over just a decade or so should be discounted.
(2) SLR is locational because of geographic inhomogeneity. Land does not stand still thanks to plate tectonics, glacial isostatic adjustment (GIA), ground resource extraction (Miami water), and river delta sediment compaction (as in Bangkok). The NH is much