This is part of my critical review of the palaeoenvironmental evidence for the influence of solar activity on climate.
Stolze, S., Muscheler, R., Dörfler, W., Nelle, O. (2013) Solar influence on climate variability and human development during the Neolithic: evidence from a high-resolution multi-proxy record from Templevanny Lough, County Sligo, Ireland, Quaternary Science Reviews 67, 138—159
Stolze et al have impressively high-resolution (~decadal) pollen, non-pollen palynomorph and geochemistry records from Templevanny Lough, a small lake in western Ireland. Their record is reasonably well dated with 16 radiocarbon dates (5 rejected) and spans 4100-2900 BCE, a period that covers the elm decline and most of the Neolithic.
The pollen record is interpreted as showing the impact of the elm decline, Neolithic agriculture and climate change on the vegetation around the lake. So far so good.
I begin to part company with the authors when they synthesise some of their records from Templevanny Lough and two adjacent sites to generate a record of “periods of increased spring/summer temperature” and “periods of increased precipitation”. The derivation of these synthetic variables is opaque. The figure caption states “Alnus pollen, suberised basal cells of mucilaginous hairs of Nymphaeaceae and loss on ignition data are key proxies used in the reconstruction of the combined climate record.” Some justification is given in the text as to why these proxies might indicate changes in temperature or water depth, but no criteria are given as to what constitutes high or low temperature/precipitation. This is unfortunate given the subsequent importance of these synthetic variables for comparison with other European proxy climate records and with solar proxies.
The proportion of inorganic carbon estimated by loss-on-ignition (mass lost on heating to 950°C) appears to be the main indicator of spring/summer temperature. The inorganic carbon will mostly be calcium carbonate, which can be produced within the lake by snails, ostracods, or as a result of photosynthesis in the lake. Calcium carbonate can also be washed into the lake. The production of photosynthetically-derived calcium carbonate might vary with temperature, but will also be sensitive to other factors such as nutrient status (which will change as a result of the elm decline and Neolithic agriculture). The concentration of this calcium carbonate in the sediment will also depend on dilution by organic matter, diatom frustules and terrestrial detritus. It is at best a weak indicator of temperature over the small temperature range expected in western Ireland.
The main indicator of “increased precipitation” is an increase in suberised (corky) hair cells of water lilies. There is a long and fraught logical chain connecting the two. Water lilies are restricted to water less than 3 m deep. Stolze et al interpret an increase in the percent of palymorphs from water lilies as an indication that water levels fell and water lilies were able to grow nearer the middle of the lake. The lake is currently 6 m deep, and was probably nearer 9 m deep in the Neolithic because less sediment had accumulated. As the modern lake is steep-sided and flat-bottomed, fairly substantial lake level changes are needed to move 3 m deep water much closer to the coring site.
I have three concerns regarding the use of water-lily remains as a precipitation indicator: one statistical, one ecological and one on the relationship between lake depth and precipitation.
The relative abundance changes in suberised hair cells are mostly small. Some of the interpreted changes may be chance fluctuations due to counting uncertainty. There will also be changes in the relative abundance of hair cells due to variations in the pollen production rate, for example during the elm decline.
Water depth is not the only variable that water lilies will respond to. They will also respond to nutrient status, and nutrient status would have changed in response to Neolithic agriculture. The correlation between grass pollen and water-lily remains is interesting, and suggests a catchment process was controlling the long-term trends in water lilies.
Even if we accept the interpretation of water-lily remains as a pure lake-level signal, there are problems interpreting this as a precipitation signal. If precipitation decreased, lake level would decrease, but can the level of this hydrologically open lake change enough to materially influence the distribution of water lilies? I’m doubtful. A hydrological model is required to test whether the lake level can vary enough under feasible precipitation regimes (I’m always doubtful of reconstructions of lake-level from open lakes without an accompanying hydrological model). Changes in precipitation are not the only factor that could change lake level; there is sediment accumulation, and in the Neolithic, the outflow stream would be chock full of coarse woody debris that could form temporary dams or otherwise impede the drainage of the lake, affecting water level.
Given these three issues, suberised water lily cells are a dubious indicator of precipitation changes. Other proxies might be much more reliable, for example isotopic analysis of the carbonates or diatom frustules.
For the sake of argument, I’m going to ignore these doubts about the reliability of the synthetic temperature and precipitation variables, and examine the purported climate-solar relationship at Templevanny Lough.
“The timing of the recorded climate shifts in northern and western Europe and the Alps is in good agreement with changes in the production rates of the solar proxies 14C and 10Be, indicating that the climate was largely influenced by variations in solar irradiance. Periods of increased summer temperatures and lower rainfall overlap with declining and low recordings of the production rates of both 14C and 10Be, indicating an increasing and higher solar activity. Lower summer temperatures, increased precipitation and storminess coincide with intervals of higher radionuclide production rates. It is remarkable that the succession of wet and dry shifts across Europe mirrors the sequence of low and high solar activity during the studied time interval.”
This is a subjective correlation by eye. There is no attempt to quantify the relationship or test whether it is any stronger than that expected by chance alone. I am unaware of a physical effect that could cause a relationship between the rate of change (“declining”), rather than just the absolute level (“low”) of cosomogenic isotope production. This implies that there is a leading indicator of solar activity that climate responds to. I am sure that solar physicists, who currently struggle to predict the magnitude of the next 11 year sunspot cycle, would be very interested in this finding, if verified.
Given the uncertain relationship between the proxies and the climatic variables they are claimed to represent, and the eyeball correlation between the proxies and the solar record, I conclude that Stolze et al is not credible evidence for a solar-climate relationship.
Over wastes of blasted heather,
Where the pine-trees stand together,
Evermore my footsteps wander,
Evermore the shadows yonder
Deepen into gloom.
Where there lies a silent lake,
No song-bird there its thirst may slake,
No sunshine now to whiteness wake
The water-lily’s bloom.