Articles | Volume 12, issue 5
Research article
03 May 2016
Research article |  | 03 May 2016

French summer droughts since 1326 CE: a reconstruction based on tree ring cellulose δ18O

Inga Labuhn, Valérie Daux, Olivier Girardclos, Michel Stievenard, Monique Pierre, and Valérie Masson-Delmotte

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.

Short summary
This article presents a reconstruction of summer droughts in France for the last 680 years, based on oxygen isotope ratios in tree ring cellulose from living trees and building timbers at two sites, Fontainebleau and Angoulême. Both sites show coherent drought patterns during the 19th and 20th century, and are characterized by increasing drought in recent decades. A decoupling between sites points to a more heterogeneous climate in France during earlier centuries.