A few years ago Andrew Kemp and coauthors published a sea level reconstruction for the last two millennia from two salt marshes in North Carolina. Reconstructed sea-level was relatively stable until the start of the 20th Century, when it started to rise rapidly, consistent with the instrumental record. Kemp et al compare their reconstruction with sea-levels estimated from a 1500-year long global temperature reconstruction using a semi-empirical model and find that the two reconstructions are consistent. This agreement suggests that the recent warm period is anomalous in the last two millennia.
Climate sceptics didn’t much like the paper, perhaps because it showed that recent sea-level rise is anomalous and perhaps because it confirmed that the 1500-year long temperature reconstruction from Mann et al (2008) is substantially correct. But Kemp et al is a single study, perhaps local factors somehow made this site insensitive to sea-level changes, or chronological uncertainties created artefacts in the reconstruction.
A new paper by Lambeck et al reconstructs sea level over the last 35000 years from sea-level indicators from across the tropics. The sites used are remote from the complicating influences of ice sheets which simplified corrections required for land uplift or subsidence.I want to focus on the mid to late Holocene part of the sea-level curve, about which Lambeck et al write:
A progressive decrease in rate of rise from 6.7 ka to recent time. This interval comprises nearly 60% of the database. The total global rise for the past 6.7 ka was ∼4 m (∼1.2 × 106 km3 of grounded ice), of which ∼3 m occurred in the interval 6.7–4.2 ka BP with a further rise of ≤1 m up to the time of onset of recent sea-level rise ∼100–150 y ago (91, 92). In this interval of 4.2 ka to ∼0.15 ka, there is no evidence for oscillations in global-mean sea level of amplitudes exceeding 15–20 cm on time scales of ∼200 y (about equal to the accuracy of radiocarbon ages for this period, taking into consideration reservoir uncertainties; also, bins of 200 y contain an average of ∼15 observations/bin). This absence of oscillations in sea level for this period is consistent with the most complete record of microatoll data from Kiritimati (23). The record for the past 1,000 y is sparse compared with that from 1 to 6.7 ka BP, but there is no evidence in this data set to indicate that regional climate fluctuations, such as the Medieval warm period followed by the Little Ice Age, are associated with significant global sea-level oscillations.
This new sea level reconstruction is in substantial agreement with the reconstruction of Kemp et al, and hence confirm that the temperature reconstruction of Mann et al (2008) is substantially correct. It is not possible to have large multi-century scale changes in mean global temperature as this would cause sea level changes that are inconsistent with the reconstructions.
Rahmstorf’s (2007) semi-empirical model suggests that sea-level should rise after a global temperature increase at the rate of 3.4 mm/yr/°C. This rate will eventually decrease to zero as sea level reached equilibrium with the new global temperature, but can be assumed to be linear for a few centuries. A 0.3°C temperature anomaly over 200 years would cause a sea level change larger than reconstructed. Hence, any temperature anomalies must either be of short duration or of small magnitude.
The progressive rise in sea levels throughout the Holocene gives us some evidence towards resolving the Marcott et al vs Liu et al Holocene climate conundrum. The rise is consistent with Liu et al model-based estimates of increasing Holocene temperatures because of increasing greenhouse forcing (at least in terms of trend, I haven’t checked the magnitude). The progressive rise in sea levels is more difficult, but not impossible, to reconcile with Marcott et al. Sea level could rise despite global cooling if a) contributions from continued ice melt (probably in Antarctica) or b) slow adjustment of the deep ocean to Holocene temperatures after the cold glaciation can overwhelms the effect of cooling surface water and the (relatively small) glacier and ice sheet regrowth in the late Holocene. I don’t know the Antarctic melt history well enough to comment on the first possibility, and am doubtful about the second as much of the ocean overturns relatively quickly so should reach equilibrium within a few thousand years.
Predictably, this paper has caught the attention of WUWT. Just as predictably, they have little sensible to write about it.
First, Eric Worrall writes
The abstract notes that on longer timescales, SLR[sea level rise] up to at least 40mm / year has been observed – so in this context “a few” mm per year does not seem particularly alarming, and is well within the range of natural variation.
This is foolish. Only when the great ice sheet of the last glaciation were melting at their fastest rate did the sea level rise approach 40mm/yr. The last few thousand years without major ice melt is a much more appropriate background for comparison. When the sea level rose at 40mm/yr, there were no towns, cities, agricultural land, nuclear power stations or railway lines that needed protecting. There are now.
Worrall’s second claim is not stupid
The fluctuation claim – the claim that sea level change in the last 150 years is faster than any change over the last 6000 years – is very much dependent on accurate dating of each of the proxy series. As we saw with the Hockey Stick controversies, any uncertainty about dating proxies tends to impose a strong hidden averaging effect on the data series, smoothing away peaks and troughs.
Chronological uncertainty will tend to artificially smooth the reconstruction. This is probably a minor problem. First the 200 yr bins used in the Lambeck et al calculation are large relative to the typical chronological error on a date in the late Holocene. Second, the data would still show the sea-level anomalies and these would propagate into the uncertainty. Woodroffe et al (2012) finds no evidence of substantial sea-level fluctuations in the last 5000 years.
Not to be out done, Anthony Watts adds
So, if we had sea levels of 16-31 feet higher than the present 100,000 years ago, well before the dawn of the industrial revolution, what caused that? Inquiring minds want to know.
Inquiring minds found out long ago: insolation. From the IPCC AR5 5.3.4
LIG WMGHG concentrations were similar to the pre-industrial Holocene values, orbital conditions were
very different with larger latitudinal and seasonal insolation variations. Large eccentricity and the phasing of precession and obliquity (Figure 5.3a–c) during the LIG resulted in July 65°N insolation peaking at ~126 ka and staying above the Holocene maximum values from ~129 to 123 ka. The high obliquity (Figure 5.3b) contributed to small, but positive annual insolation anomalies at high latitudes in both hemispheres and negative anomalies at low latitudes