Rahmstorf et al 2015 Figure 5 shows a coral d15N series from offshore Nova Scotia (see left panel below). The corresponding plot from the source is shown on the right. Original captions for both follow. There’s enough information in the figures and captions to figure out Rahmstorf’s next trick. See if you can figure it out before looking at my explanation below the fold.
Figure 1. Left – Rahmstorf et al Figure 5. Original caption: Figure 5 A compilation of different indicators for Atlantic ocean circulation. The blue curve shows our temperature-based AMOC index also shown in Fig. 3b. The dark red curve shows the same index based on NASA GISS temperature data-48 (scale on left). The green curve with uncertainty range shows coral proxy data – 25 (scale on right). The data are decadally smoothed. Orange dots show the analyses of data from hydrographic sections across the Atlantic at 25 N, where a 1 K change in the AMOC index corresponds to a 2.3 Sv change in AMOC transport, as in Fig. 2 based on the model simulation. Other estimates from oceanographic data similarly suggest relatively strong AMOC in the 1950s and 1960s, weak AMOC in the 1970s and 1980s and stronger again in the 1990s (refs 41,51). Right – Sherwood et al 2011 Figure 3 excerpt. Original caption: time series … annual mean bulk d15N from six colonies of the deep-sea gorgonian P. resedaeformis. Shaded areas represent 95% confidence intervals around annual means. Dashed lines indicate long-term trends, where significant. Note the cold periods (blue bars) of the 1930s/1940s and 1960s and sustained warm period (red bar) since 1970. Bulk d15N is most strongly correlated with NAO at a lag of 4 years (r= -0.19) and with temperature at a lag of 3 years (r=-0.27, p<0.05). … Squares in bulk d15N plot show values of the eight individual samples used for d15N-AA analysis.
The Trick
Rahmstorf’s Figure 5 shows a positive correlation between temperature and coral d15N (both decline together), while Sherwood et al reported a negative correlation (r=-0.27) between temperature and coral d15N. How’d they do that?
Sherwood et al 2011
Sherwood et al 2011 stated that coral d15N was negatively correlated to gridcell temperature. Their figure 3 (a larger excerpt is shown on left) shows gridcell temperature increasing over the 20th century, while coral dN15 is going down. Sherwood et al 2011 attributed the negative correlation to higher d15N values in the cold Labrador Slope Water (LSW) relative to the warm Western Slope Water (WSW):
Differences in nutrient processing histories underlie the distinct end-member d15N_NO3 signatures between WSW and LSW. The d15N_NO3 of WSW ( 5.02+-0.27% ) is identical to the nominal mean ocean value of 5% (26), reflecting its origin in the nutrient-rich waters beneath the Gulf Stream (27,28). The d15N_NO3 of LSW (6.00+-0.28%) reflects the low nutrient surface currents from which it is formed in the northern Labrador Sea. Because LSW is depleted in nutrients and not replenished by upwelling, the residual nitrate is enriched over that of WSW by 1%. These end-member d15N_NO3 signatures reflect basin-scale circulation and likely did not vary significantly over the late Holocene
Sherwood et al also observed d15N values similar to the early 20th century in corals dated several centuries older.
On the right, I’ve digitized the data in the Sherwood figure and re-scaled and re-oriented the d15N data to fit to the temperature data: see inverted d15N scale on the right axis. This is opposite to the orientation shown in the Rahmstorf figure.
Figure 2. Left – excerpt from Sherwood et al 2011 Figure 3, showing gridcell temperature (as per Sherwood et al reference); right panel: digitized version of Sherwood 2011 Figure 3, with d15N shown in inverted scale.
Rahmstorf et al 2015
Rahmstorf et al orient d15N to temperature in an opposite orientation to Sherwood et al, justifying their opposite orientation as follows:
Figure 5 illustrates corroborating evidence in support of a twentieth-century AMOC weakening…The green curve denotes oceanic nitrogen-15 proxy data from corals of the US north-east coast from ref. 25. These annually resolved d15N data represent a tracer for water mass changes in the region, where high values are characteristic of the presence of Labrador Slope Water. The time evolution of the d15N tracer agrees well with that of our AMOC index (Fig. 5). Ref. 25 reports four more data points from ancient corals preceding the twentieth century, the oldest one from AD500. These lie all above 10.5, providing (albeit limited) evidence that the downward excursion to values below 10 between 1975 and 1995 and the corresponding watermass change may be unprecedented in several centuries.
Discussion
In their abstract, Rahmstorf says that offshore Nova Scotia coral d15N values are one of the “consistent” lines of evidence supporting a cooler subpolar gyre and, in turn, reduced Atlantic Meridional Overturning Circulation:
Here we present multiple lines of evidence suggesting that this cooling may be due to a reduction in the AMOC over the twentieth century and particularly after 1970. Since 1990 the AMOC seems to have partly recovered. This time evolution is consistently suggested by an AMOC index based on sea surface temperatures, by the hemispheric temperature difference, by coral-based proxies and by oceanic measurements.
[Apr 7 – The Nova Scotia coldwater coral series are, like the alkenone series offshore east coast North America discussed at CA last month, located near the front between the Labrador Current and Gulf Stream, as shown in the location map from Sachs et al 2007, updated below to show the location of the Nova Scotia coldwater corals.
In that earlier post, I reported that very large SST decreases had been estimated off the east coast during the Holocene, considerably larger than the North Atlantic as a whle. I noted the following explanation from Julian Sachs in terms of changing ocean currents:
Sachs observed that a relatively small coastward displacement of the Gulf Stream could account for the difference and plausibly speculated that the Gulf Stream hugged the East Coast much more closely in the mid-Holocene.
-Apr 7]
But all that the coral d15N series show is (at most) that there has been increased [Apr 7 – mixing proportion of lower d15N Gulf Stream waters, which could account for] increased temperatures offshore Nova Scotia) – but the significance of this relationship is very tenuous. On its face, the d15N series does not show that subpolar gyre temperatures have decreased [the definition of Rahmstorf’s AMOC index].
Rahmstorf chose the right-axis d15N scale and orientation so that smoothed versions of each trending series will have similar scales. But this could be done with any smoothed and trending series. This is tautological mathematics, not science of deep insight. One could equally use coral-14 values from the Red Sea or the Dow Jones Index. Rahmstorf’s third trick.
The idea that coldwater corals offshore Nova Scotia can be thermometers for ocean temperature in the subpolar gyre has little more plausibility than the belief that stripbark bristlecones in the distant Sierra Nevadas or contaminated Finnish sediments can be thermometers for the subpolar gyre.
It’s not even well established that coral d15N is a proxy for local ocean temperature [Apr 7 – or mixing of water masses]. Coral d15N is not a well-studied proxy, to say the least. Only a few examples are reported with results from only one article being archived at NOAA. In one of the original articles on the proxy, co-authored by the lead author of Rahmstorf’s citation, d15N values were thought to depend on distance from sewage source:
While coral reefs decline, scientists argue, and effective strategies to manage land-based pollution lag behind the extent of the problem. There is need for objective, cost-effective, assessment methods. The measurement of stable nitrogen isotope ratios, d15N, in tissues of reef organisms shows promise as an indicator of sewage stress.
Alkenone Proxies
To the extent that climate scientists believe that offshore East Coast d15N is relevant to the interpretation of the subpolar gyre, you’d think that they would have canvassed information from alkenones – an important SST proxy with a number of samples offshore East Coast. Curiously, on January 21, 2015, a week before acceptance of Rahmstorf et al, I’d looked at East Coast alkenone proxies, including some high-resolution data, in a CA post here. These comparisons seem incomparably more informative than Rahmstorf’s meager coral d15N data and I’ll discuss it in a forthcoming post.
Update: Coral d15N values offshore Hawaii also show a decline over the 19th and 20th centuries. On Rahmstorf’s reasoning, this is further evidence of cooling in the Atlantic subpolar gyre. Or maybe warming. Or maybe both.
