Changes in net ecosystem exchange over Europe during the 2018 drought based on atmospheric observations

Abstract

The 2018 drought was one of the worst European droughts of the twenty-first century in terms of its severity, extent and duration. The effects of the drought could be seen in a reduction in harvest yields in parts of Europe, as well as an unprecedented browning of vegetation in summer. Here, we quantify the effect of the drought on net ecosystem exchange (NEE) using five independent regional atmospheric inversion frameworks. Using a network of atmospheric CO 2 mole fraction observations, we estimate NEE with at least monthly and 0.5° × 0.5° resolution for 2009–2018. We find that the annual NEE in 2018 was likely more positive (less CO 2 uptake) in the temperate region of Europe by 0.09 ± 0.06 Pg C yr −1 (mean ± s.d.) compared to the mean of the last 10 years of −0.08 ± 0.17 Pg C yr −1 , making the region close to carbon neutral in 2018. Similarly, we find a positive annual NEE anomaly for the northern region of Europe of 0.02 ± 0.02 Pg C yr −1 compared the 10-year mean of −0.04 ± 0.05 Pg C yr −1 . In both regions, this was largely owing to a reduction in the summer CO 2 uptake. The positive NEE anomalies coincided spatially and temporally with negative anomalies in soil water. These anomalies were exceptional for the 10-year period of our study. This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.

A historical, geographical and ecological perspective on the 2018 European summer drought

“…These measurements have large surface flux sensitivity footprints [39] and suggest an increased spring CO 2 sink, followed by a reduction of this sink in summer. Indeed, atmospheric transport model inversions support below-average net CO 2 uptake in Central Europe during summer 2018 [40][41][42]. Strong negative impacts on ecosystem productivity were observed in Germany and southern Sweden, supported by remote sensing datasets [42] and vegetation models [33].…”

The fingerprint of the summer 2018 drought in Europe on ground-based atmospheric CO2measurements

“…Except for Mace Head (MHD), where we used a selection scheme based on wind speed, direction, and hourly standard deviation of CO 2 [24], we have applied a simple selection to all stations. It consists of retaining mid-afternoon (12-17 h local winter time) data at tall tower and coastal stations, and retaining night-time (20-05 h local winter time) data at mountain stations, when the air is well-mixed, providing a large spatial representativeness with minimum influence from local sources [15,25,26]. In addition to this temporal filtering we also excluded hourly means with a standard deviation greater than 0.5 ppm from the selected time series.…”

“…Generally, they assign uncertainties to atmospheric observations that are much larger than the instrumental uncertainty, to account for transport model uncertainties [10,11]. This approach is developed in three publications of the special issue [12][13][14][15], using the atmospheric CO 2 dataset that we describe in this article.…”

The European carbon cycle response to heat and drought as seen from atmospheric CO 2 data for 1999–2018

“…We use NEE estimates from the Jena CarboScope atmospheric inversion (update of [11]). Though this is a global inversion with a resolution of fluxes and atmospheric transport considerably coarser than in the regional inversions presented by Thompson et al [12], it estimates the NEE history over a longer time frame. The analysis is mostly done for NEE at the spatial scale of European subregions (figure 1) similar to the regions used by the EUROCOM project [13].…”