Holocene climates of the Iberian Peninsula: pollen-based reconstructions of changes in the west-east gradient of temperature and moisture

Liu, Mengmeng; Shen, Yicheng; González-Sampériz, Penelope; Gil-Romera, Graciela; ter Braak, Cajo J. F.; Prentice, Iain Colin; Harrison, Sandy P.

doi:https://doi.org/10.5194/cp-2021-174

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https://doi.org/10.5194/cp-2021-174

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20 Dec 2021

Review status: this preprint is currently under review for the journal CP.

Holocene climates of the Iberian Peninsula: pollen-based reconstructions of changes in the west-east gradient of temperature and moisture

Mengmeng Liu¹, Yicheng Shen², Penelope González-Sampériz³, Graciela Gil-Romera³, Cajo J. F. ter Braak⁴, Iain Colin Prentice¹, and Sandy P. Harrison² Mengmeng Liu et al.

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¹Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot SL5 7PY, UK
²Geography & Environmental Science, Reading University, Whiteknights, Reading, RG6 6AH, UK
³Instituto Pirenaico de Ecología-CSIC, Avda. Montañana 1005, 50059, Zaragoza, Spain
⁴Biometris (Applied Mathematics and Applied Statistics Centre), Wageningen University & Research, 6708 PB Wageningen, The Netherlands

Received: 14 Dec 2021 – Accepted for review: 20 Dec 2021 – Discussion started: 20 Dec 2021

Abstract. The Iberian Peninsula is characterised by a steep west-east moisture gradient today, reflecting the dominance of maritime influences along the Atlantic coast and more Mediterranean-type climate further east. Holocene pollen records from the Peninsula suggest that this gradient was less steep during the early to mid-Holocene, possibly reflecting the impact of orbital changes on circulation and thus regional patterns in climate. Here we use 7121 pollen samples from 117 sites covering part or all of the last 12,000 years to reconstruct changes in seasonal temperature and in moisture across the Iberian Peninsula quantitatively. We show that there is an increasing trend in winter temperature at a regional scale, consistent with known changes in winter insolation. However, summer temperatures do not show the decreasing trend through the Holocene that would be expected if they were a direct response to insolation forcing. We show that summer temperature is strongly correlated with plant-available moisture (α), as measured by the ratio of actual evapotranspiration to equilibrium evapotranspiration, which declines through the Holocene. The reconstructions also confirm that the west-east gradient in moisture was considerably less steep than today during the early to mid-Holocene, indicating that atmospheric circulation changes (possibly driven by orbital changes) have been important determinants of the Holocene climate of the region.

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Mengmeng Liu et al.

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RC1:
'Comment on cp-2021-174', Anonymous Referee #1, 30 Dec 2021 reply

Summary: The manuscript presents the analysis of a large paleolimnological network in the Iberian Peninsula with records spanning the Holocene. Bases on these records the authors reconstruct the temperature of the warmest and coldest month and a measure of humidity deficit. They conclude that whereas the winter temperatures follow the evolution of the orbital forcing through the Holocene, summer temperatures are fairly stable and do not follow the summer insolation. This finding roughly agrees with other climate reconstructions from these area and diverges from the results obtained with state-of-the art climate simulations. The explanation, according to the authors, lies in the reduced humidity zonal gradient across the peninsula, and the larger alleviate humidity in summertime for most areas, which subdues the effect of solar insolation on surface temperatures.

Recommendation: The manuscript is in my opinion well written and the results are informative. The dynamical explanation of these results lies a bit on the side of - permissible- speculation, as the scope of the study is not a detailed analysis of climate simulations. The conclusions are relevant for climate modelling, as it seems that climate models grapple with a correct simulations of Holocene climate, here and for instances also related to the so called Holocene conundrum . My recommendation, is therefore, of acceptance of the manuscript after a few minor revisions have been considered.

1. line 34

' Projections of future climate change suggest that the region will become both warmer and drier, but nevertheless show that this west-east differentiation is maintained. '

Reference(s) is needed here

2. line 16

'early to mid-Holocene'

Here, and in other instances in the text, the authors use terms like mid-Holocene, early Holocene, present, without having defined them explicitly. In particular, the reference to 'present' is relevant, ans the manuscript often assesses Holocene temperatures compared to 'present temperatures'. Is 'present' the 20^th average, pre-industrial, around 1950 , temperatures ?

3. line 49

'Although these records are extensive, they seem to indicate fairly complex spatial patterns of change#

I did not understand why the word 'although' needs to be used here. I do not see an implicit contradiction between being extensive and showing complex spatial patterns

4. line 53

Peninsular

line 53

'Furthermore, quantitative reconstructions of summer temperature made at individual sites using chironomid data (Muñoz Sobrino et al., 2013; Tarrats et al., 2018) are not consistent with reconstructed changes based on pollen for the same sites.'

with reconstructed temperatures. Otherwise the sentence is grammatically somewhat odd

5. line 60

'We analyse how these trends are related to external forcing a'

I think the authors refer here to external climate forcing, but the sentence could be misinterpreted as meaning remote forcing, e.g. from the North Atlantic

6. line 93

'We excluded individual pollen samples with large age uncertainties (standard error larger than 100 years)'

What is the typical time resolution of the reconstructions? I think it is no where stated.

7. line 105

'and assessed the significance of differences in these trends through time compared

to 0.5 ka based on p values'

compared to 0.5ka ? It is for me unclear

8. line 141

'Summer temperatures are strongly correlated with changes in α, bo'

spatially or temporally correlated ?

9. line 158

'their reconstructions show a cooling of 3°C in the early Holocene are comparable in magnitude#

I guess that 'are' should be deleted

10. line 165

'change at some of the individual sites is much larger (ca 10°C) and there is no assessment off the uncertainty on these reconstructions. '

of

11. line 193

'However, they show a persistent cooling of 1.5 °C compared to present between 4.5 and 2 ka, not seen in these reconstructions'

do the authors mean persistent cooling trend or persistent cool conditions ?

12. line 214

'Specifically, the increased advection of moisture into eastern Iberia

created wetter conditions leading to increased evapotranspiration, less allocation of available

net radiation to sensible heating, and resulting in cooler air temperatures.'

I think the authors' point is not necessarily that increased evapotranspiration leads to colder temperatures, but rather to less temperature variations through time and space. The study suggests that summer temperatures did not fall as experted from the solar insolation alone, so this sentence is a bit confusing. Please, clarify.

13. line 222

'Stronger moisture advection is not a feature of the transient climate model simulations, which may explain why these simulations do not show a strong modification of the insolation-driven changes in summer

temperature. The failure of the current generation of climate models to simulate the observed

strengthening of moisture transport into Europe and Eurasia during the mid-Holocene has been

noted by other studies (e.g. Bartlein et al., 2017; Mauri et al., 2014)'

The first sentence is confusing in view of Figure S9. The models do show temperature evolution through time. I think the authors mean that the models are not able to counteract or shield the insolation forcing.

Also, is this conclusion (moisture advection) derived by other studies (then please cite references) or did the authors look into the simulated moisture advection or is this conclusion reached by indirect reasoning ? Please, be here as clear as possible.

14. Conclusion sections

The usual conclusion section is missing. This is to some extent a matter of style (or editorial guideline), but I find useful that a manuscript finishes off with a few bullet-point style list of most important take-home messages

15. Fig S9.

The reader will benefit from a reference to the model runs. I guess that the authors are using the runs described in Braconnot et al. (2019; doi:10.5194/cp-15-997-2019) and Bader et al. (2020;doi:10.1038/s41467-020-18478-6), but please, spell the names of the models in full, e.g. MPI-ESM-P, and give references to the runs used and shown in this figure. Also, some additional information could be useful for the reader as well, such as the spatial resolution.

Also, the time axis is not clear enough : years BP ?

Reply

Citation: https://doi.org/10.5194/cp-2021-174-RC1
- AC1: 'Reply on RC1', Mengmeng Liu, 13 Jan 2022 reply
  
  Thank you very much for your comments! Please see attached our response.
  
  Reply
  
  Citation: https://doi.org/10.5194/cp-2021-174-AC1

Mengmeng Liu et al.

Supplement

https://doi.org/10.5194/cp-2021-174-supplement

Data sets

Pollen data and charcoal data of the Iberian Peninsula Sandy Harrison, Yicheng Shen, Luke Sweeney http://dx.doi.org/10.17864/1947.294

Model code and software

Iberia paper code Mengmeng Liu https://github.com/ml4418/Iberia-paper.git

Mengmeng Liu et al.

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Short summary

We reconstructed the Holocene climates in Iberian Peninsula using a large pollen dataset and found that the west-east moisture gradient was much flatter than today. We also found that the winter was much colder, which can be expected from the low winter insolation during the Holocene. However, summer temperature didn’t follow the trend of summer insolation, instead, it was strongly correlated with moisture.


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