French summer droughts since 1326 CE: a reconstruction based on tree ring cellulose <i>δ</i><sup>18</sup>O

Abstract

Abstract. The reconstruction of droughts is essential for the understanding of past drought dynamics and can help evaluate future drought scenarios in a changing climate. This article presents a reconstruction of summer droughts in France based on annually resolved, absolutely dated chronologies of oxygen isotope ratios (δ18O) in tree ring cellulose from Quercus spp. Samples were taken from living trees and timber wood from historic buildings at two sites: Fontainebleau (48°23′ N, 2°40′ E; 1326–2000 CE) and Angoulême (45°44′ N, 0°18′ E; 1360–2004 CE). Cellulose δ18O from these sites proved to be a good proxy of summer climate, as the trees were sensitive to temperature and moisture availability. However, offsets in average δ18O values between tree cohorts necessitated a correction before joining them to the final chronologies. Using the corrected δ18O chronologies, we developed models based on linear regression to reconstruct drought, expressed by the standardized precipitation evapotranspiration index (SPEI). The significant correlations between the SPEI and cellulose δ18O (r ≈ −0.70), as well as the verification of the models by independent data support the validity of these reconstructions. At both sites, recent decades are characterized by increasing drought. Fontainebleau displays dominantly wetter conditions during earlier centuries, whereas the current drought intensity is not unprecedented in the Angoulême record. While the δ18O chronologies at the two studied sites are highly correlated during the 19th and 20th centuries, there is a significant decrease in the correlation coefficient between 1600 and 1800 CE, which indicates either a weaker climate sensitivity of the tree ring proxies during this period, or a more heterogeneous climate in the north and the south of France. Future studies of tree ring isotope networks might reveal if the seasonality and spatial patterns of past droughts can explain this decoupling. A regional drought reconstruction based on a combination of the two sites shows good agreement with historical records of grape harvest dates in France, which provide another proxy of summer climate.

Absence of Age‐Related Trends in Stable Oxygen Isotope Ratios From Oak Tree Rings

“…The oxygen isotope ratios in the latewood (summer wood) of UK oak trees and timbers thus record past changes in the amount of May to August rainfall (Young et al, 2015). Oaks in France appear to record a similar signal (Daux et al, 2018;Labuhn et al, 2016).…”

Past Summer Temperatures Inferred From Dendrochronological Records of Fitzroya cupressoides on the Eastern Slope of the Northern Patagonian Andes

“…Several studies provided regional to hemispheric syntheses of the Earth's climate history over the past 2,000 years (e.g., Gennaretti et al, 2017;Labuhn et al, 2016;Neukom et al, 2014). Most of them have been focused on the Northern Hemisphere, which has a dense network of paleoclimatic records (Ahmed et al, 2013;Intergovernmental Panel on Climate Change, 2013;Wilson et al, 2016).…”

The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations

“…There are multiple ways to reconstruct past continental temperature and precipitation, for instance from pollen (Bartlein et al, 2010;Herbert and Harrison, 2016;Wahl et al, 2012) or tree ring data (Labuhn et al, 2016;Lavergne et al, 2017). However, there is a serious lack of proxies suitable for reconstructing, in a quantitative way, past variations in continental atmospheric RH.…”