the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Using data and model to infer climate and environmental changes during the Little Ice Age in tropical West Africa
- 1Laboratoire d’Océanographie et du Climat. Expérimentation et Approche numérique/IPSL. Sorbonne Université-CNRS-IRD-MNHN. 4 Place Jussieu. 75005. Paris. France
- 2Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain
- 1Laboratoire d’Océanographie et du Climat. Expérimentation et Approche numérique/IPSL. Sorbonne Université-CNRS-IRD-MNHN. 4 Place Jussieu. 75005. Paris. France
- 2Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain
Abstract. Here we present hydrological and vegetation paleo-data extracted from 28 sites in West Africa from 5° S to 19° N and the past1000/PMIP4 IPSL-CM6A-LR climate model simulations covering the 850–1850 CE period to document the environmental and climatic changes that occurred during the Little Ice Age (LIA). The comparison between paleo-data and model simulations shows a clear contrast between the area spanning the Sahel and the Savannah in the North, characterized by widespread drought, and the equatorial sites in the South, where humid conditions prevailed. Particular attention was paid to the Sahel, whose climatic evolution was characterized by a progressive drying trend between 1250 and 1850 CE. Three major features are highlighted: (1) the detection of two early warning signals around 1170 and 1240 CE preceding the onset of the LIA drying trend; (2) an irreversible tipping point at 1800–1850 CE characterized by a dramatic rainfall drop and a widespread environmental degradation in the Sahel; and (3) a succession of drying events punctuating the LIA, the major of which was dated around 1600 CE. The climatic long-term evolution of the Sahel is associated with a gradual southward displacement of the Inter-Tropical Convergence Zone induced by the radiative cooling impacts of major volcanic eruptions that have punctuated the last millennium.
Anne-Marie Lézine et al.
Status: open (until 13 Oct 2022)
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RC1: 'Comment on cp-2022-57', Anonymous Referee #1, 23 Sep 2022
reply
The manuscript presented by Lézine et al deal with the climate changes experienced in tropical West Africa during the last millennia. The manuscript first makes use of a number of paleo-records in the study area to define two new indices able to quantify the hydrological and vegetation context. The goodness of these paleo records is first evaluated by computing two multi-proxy indices which are validated against instrumental data for the period 1840-present with good results. Then, the authors make use of modelled data (from 850 to 1850) to characterise the precipitation in pre-instrumental period, subsequently discussing the relation between the modelled climate and the observed variability of the paleo-records, offering an interesting discussion and relevant results.
The paper is well written and in general it is clear. Personally, I find this work quite interesting, as it deals with a region still poorly characterised because of the sparsity of instrumental or even proxy records. Therefore, I recommend its publication in Climate of the Past.
I however have some concerns (general comments) that, if addressed, will probably improve the clarity of the manuscript:
The most important is related with the methodology used to homogenize the paleo data in section 2.1. As far as I understand, the method used is based on rescaling each individual paleo record (table 1) to a common 6-level scale. However, the details of this conversion are not explained in the manuscript making it impossible to know how this index is ultimately computed (this is essential at the time of evaluating the goodness of the original data or even to allow reproducibility). In my opinion, the authors should describe a little more the way this rescaling has been performed.
Another question is related to the reason why the authors have limited their study period to 850-1850. I’m not familiarised with the past1000 experiment data but ending in 1850 most probably indicates that the past1000 experiment was conceived to model the pre-industrial era. However, if possible, it will be extremely interesting that the modelled precipitation series were extended to present time. This would allow to compare the model results with the instrumental ASWI (figure 2 of the manuscript) and, providing the result is good, it would add a lot of confidence to the results. Anyway, I would like to stress that I find figure 2 very interesting as, beyond some indirect evidence, the humid period described by the ASWI between 1840-1890 had not be confirmed by independent data up to now.Apart of these questions, there are some minor aspects that could help to clarify the text (specific comments):
Lines 31-32. The west African monsoon is not only driven by land-sea contrast, but it is also a consequence of the migration of the ITCZ (see for example Gagdil et al. https://doi.org/10.1007/s12040-017-0916-x).
Line 50. I consider that this manuscript is not a “review” but a “research”.
Table 1: Maybe expressing the latitude and longitude in sexagesimal form will be clearer.
Line 97: In my view, the validation performed is not indicating that the methodology is “realistic” but instead, it is testing the similarities between the paleo-data and the instrumental ASWI.
Figure 1. The blue arrows are a little difficult to see where the underlying colour is also blue.
Line 180. I believe that the way the past1000 index is constructed should be more explained.
Figure 4. I’m sure that presenting such amount of series in a single figure is not easy, but it is quite difficult to interpret some of the y-axis scales in this figure. For example (not the only case) in figure 4A “Jikaryia” the axis is scaled by not consecutive values (3, 1.5, 2 , 0.5). Please clarify.
Line 326. Please indicate the methodology used to compute statistical significance.
Line 352. The local term “Heug” could be unknown by readers not familiarised with the climate of this region. Please explain.
Anne-Marie Lézine et al.
Anne-Marie Lézine et al.
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