One of the major rationales for palaeoecological analyses is to provide data that can be used to validate climate models — if the models can predict past climate from periods with different climate forcings our confidence in their projections of future climate change under increased greenhouse gas concentrations should be enhanced. The Last Glacial Maximum, 21 kBP (LGM), is often used as a target because the climate forcing and response are large. Another often used target is the mid-Holocene, 6±0.5 kBP, towards the end of the Holocene thermal maximum when orbital forcing gave warmer summers and cooler winters at high latitudes. The orbital forcing was stronger earlier in the Holocene but this is a less suitable target as the climate was complicated by the remnants of the Laurentide Ice Sheets and massive pulses of meltwater, such as at the 8.2 kBP event.
At the LGM, most proxies, especially at mid and high latitudes, give a coherent response (colder) that is larger than the uncertainty, and is in agreement with the sign and approximate magnitude of the modelled climate.
The mid-Holocene is a more difficult target as the climate change is much smaller than that of the LGM. A new paper by Hessler et al, under review at Climate of the Past Discussions, examines a global compilation of mid-Holocene sea-surface temperature (SST) reconstructions inferred from different proxies and finds significant discrepancies between proxies and that the reconstructed anomalies are often smaller than the uncertainty. They conclude that the SST reconstructions cannot serve as a target for testing climate models.
Hessler et al’s compilation includes reconstructions inferred from alkenones, calcium-magnesium ratios in foram tests, and dinocyst and foram assemblages. Their conclusion that the proxies (which they insist on calling sensors) cannot be used to validate the models is in contrast to a recent paper in CPD which initially claimed that the mid-Holocene climate models’ simulations of sea ice had no skill because they could not match dinocyst-inferred sea-ice cover (the replies to reviewers suggests that the final paper will be more balanced).
Hessler et al’s conclusions are depressing. If palaeoceanographic proxies cannot give consistent reconstructions for the mid-Holocene, are they of any value in the Holocene? If not, compilations such as the global Holocene temperature reconstruction from Marcott et al (2013) are questionable as they include many marine proxies.
I’ve written several papers questioning how good proxies are. Now I want to consider if Hessler et al might be throwing the baby out with the bathwater: what might be going wrong with their analysis that makes the proxies appear to be of doubtful utility?
Chronological error. Lack of a match between proxies from different cores can always be blamed on chronological error. Perhaps they would match if the chronology was better. The inclusion criteria from Hessler et al required at least two radiocarbon dates (or other chronological control points) within the last 10 kBP. This is rather minimal, but presumably many fewer records would be included with stricter criteria. No attempt was made (probably wisely given the grief that Marcott et al received) to remake the age-depth models. It is perfectly possible that some of the chronologies are in error by more that a thousand years. I don’t think that is a big problem – there is no particular reason to expect the climate at 5 kBP or 7 kBP to be very different from 6 kBP. This would be a problem is they were trying to find records of a short-duration event like the 8.2 kBP cooling.
Under-estimation of proxy error. The uncertainty estimates for both the dinocyst and foram assemblage-based reconstructions are underestimated. The most important cause of this is the spatial autocorrelation in the calibration set which violates the assumption of independent observations. The true uncertainty is probably 50-100% larger than that reported. This is both good and bad news. The good news is that the proxies are less likely to contradict each other as the uncertainties on the reconstructions are wider. The bad news is the larger uncertainties will mean that even fewer of the reconstructions would be deemed statistically significant.
The proxies’ sensitivities differ. Much as we would like to reconstruct SST, some of the proxies may not directly care about it. In at least part of the ocean, including the Norwegian Sea, forams are more sensitive to sub-surface conditions than those at the surface. Worse, surface and sub-surface temperature trends are not necessarily the same. The seasonal sensitivity of the proxies may also differ. Hessler et al reconstruct summer and winter SST with some proxies; these are largely fanciful as there is no evidence that the assemblages contain sufficient information to make independent seasonal reconstructions. In the Nordic Seas, most foram production occurs in summer, but the transfer functions imply that winter SST is more important than summer SST. What is happening is that the temperature at the depth at which the forams are growing is set during winter overturn, so it is the summer temperature at depth which is important. These issues mean that even in the (exceedingly) implausible event that all the reconstructions were perfect, they might still appear to contradict each other.
Bad apples. It takes one bad apple to spoil the whole barrel. Is one bad proxy enough to make everything appear inconsistent? The authors have taken the diplomatic strategy of treating all proxies equally, but they are probably not all equally reliable. Nor are all reconstructions equally good – some may have non-analogue communities or other issues. The dinocyst reconstructions from the Nordic Sea at the LGM are seriously strange (warmer than modern), I wouldn’t want to bet much on the Holocene reconstructions being much better. The dinocyst reconstructions in Hessler et al are more varied than the other reconstructions, but they are not entirely responsible for the inconsistencies.
It may be too much to hope for coherent responses on a core-by-core basis from noisy proxies. Perhaps the regional signals are valid for at least some proxies, seasons and depths, but determining which proxies, seasons and depths are valid will take some effort. I think there is utility in Holocene reconstructions, but they need care in interpretation. Any hope that the climate modellers had that they could take mid-Holocene reconstructions off the shelf and use them to validate their simulations is unlikely to be fulfilled.
Hessler, I., Harrison, S. P., Kucera, M., Waelbroeck, C., Chen, M.-T., Andersson, C., de Vernal, A., Fréchette, B., Cloke-Hayes, A., Leduc, G., and Londeix, L. 2014. Implication of methodological uncertainties for Mid-Holocene sea surface temperature reconstructions, Clim. Past Discuss., 10, 1747-1782, doi:10.5194/cpd-10-1747-2014