02.03.2023 Ι A group of researchers around Mandy B. Freund presents a climate field reconstruction spanning the entire European continent based on tree-ring stable isotopes.

In recent decades, Europe has experienced more frequent flood and drought events. However, little is known about the long-term, spatiotemporal hydroclimatic changes across Europe. A pronounced seasonal consistency in climate response across Europe leads to a unique, well-verified spatial field reconstruction of European summer hydroclimate back to AD 1600.

Unexpected results

The authors differentiate three distinct phases of European hydroclimate variability as possible fingerprints of solar activity (coinciding with the Maunder Minimum and the end of the Little Ice Age) and pronounced decadal variability superimposed by a long-term drying trend from the mid-20th century.

They show that the recent European summer drought (2015–2018) is highly unusual in a multi-century context and unprecedented for large parts of central and western Europe. The reconstruction provides further evidence of European summer droughts potentially being influenced by anthropogenic warming and draws attention to regional differences.

Effects of Droughts

Severe drought conditions in the summers of 2003, 2010, 2015 and 2018 demonstrated the damaging and expensive impacts of widespread European droughts. Flooding caused by extreme precipitation in 2021 showed the harmful nature and devastating impacts of hydroclimatic extremes on society. With anthropogenic warming projected to exacerbate the occurrence of hydrological extremes around the world, Europe faces large uncertainties in risks to its population, ecosystems and economies.

The most recent European summer drought (2015–2019) has sparked a debate as to whether it is within the range of natural variability or related to anthropogenic warming. Long-term observations suggest that there had not been two consecutive summer droughts in central Europe in 250 years until 2018–2019. Based on traditional tree-ring records and reanalysis products, the recent drought is well within the range of natural variability thus not unprecedented.

By contrast, a long tree-ring isotope record from the Czech Republic indicates that the recent consecutive drought was unprecedented in the last two millennia. It remains unclear whether this recent drought is part of natural climate variability or potentially a consequence of anthropogenic climate change.

Obtaining a precise picture of the natural variability of the European hydroclimate is a challenge due to the sparse spatial coverage of moisture-sensitive proxy data.

Isotope records from tree rings are used to obtain a reconstruction of the European hydroclimate

To understand the spatial complexity of the large-scale European hydroclimate, spatially explicit, long-term data sets are needed. Multi-century climate data are essential for the validation of climate models including comparisons to other paleoclimate, historical, and archaeological data sets. Tree-ring widths and maximum latewood density provide the longest and most replicated high-resolution records used in previous reconstructions of Holocene past climate.

However, tree-ring widths require statistical detrending, making it difficult to use them to reconstruct some aspects of low-frequency climate variability. Furthermore, tree-ring width chronologies from European low-lands can display weak and ambiguous climate signals. In contrast, tree-ring stable isotopes are considered to be a more powerful proxy as they potentially require less statistical data treatment, and often exhibit clearer climate signals particularly in the temperate European lowlands. In this study, we reconstruct European hydroclimate based on a network of tree-ring stable isotopes of oxygen and carbon ratios.

The network combines up to 400-year long, annually resolved records of deciduous oaks from European lowlands and conifers from boreal and mountainous sites. This is the first time that stable isotope records from tree rings, with their so far untapped high climate reconstruction potential, are used to obtain a gridded spatial reconstruction of the European summer hydroclimate.

European tree-ring isotope network

The tree-ring isotope network is composed of 26 individual, well-distributed sites from across Europe. Each location has a stable isotope record of carbon and oxygen derived from tree-ring cellulose obtained from old living trees using standard procedures of dendrochronological dating and cross-dating and stable isotope analyses.

The network is composed of eight deciduous broadleaf oak stands and 18 coniferous stands (Pinus, Juniperus, Larix, Cedrus) ranging from elevations of 10–2200 m. Most of the broadleaf stands are concentrated in central-western Europe while the coniferous stands are mainly located at high latitudes and high elevations. The common period of all carbon and oxygen isotope records is 1850–1998. Mass spectrometry of stable isotope ratios was conducted using pooled alpha-cellulose samples from at least four trees per site. Coniferous species were measured using whole tree-ring wood material, whereas cellulose extraction for most oak sites was achieved on latewood only .

Previous studies based on the isotope network and on single sites have revealed common climate signals across Europe, enabling a pooling of isotopic records derived from broadleaf trees and conifers into a single dataset.