The fingerprint of the summer 2018 drought in Europe on ground-based atmospheric CO2 measurements

Abstract

During the summer of 2018, a widespread drought developed over Northern and Central Europe. The increase in temperature and the reduction of soil moisture have influenced carbon dioxide (CO₂) exchange between the atmosphere and terrestrial ecosystems in various ways, such as a reduction of photosynthesis, changes in ecosystem respiration, or allowing more frequent fires. In this study, we characterize the resulting perturbation of the atmospheric CO₂ seasonal cycles. 2018 has a good coverage of European regions affected by drought, allowing the investigation of how ecosystem flux anomalies impacted spatial CO₂ gradients between stations. This density of stations is unprecedented compared to previous drought events in 2003 and 2015, particularly thanks to the deployment of the Integrated Carbon Observation System (ICOS) network of atmospheric greenhouse gas monitoring stations in recent years. Seasonal CO₂ cycles from 48 European stations were available for 2017 and 2018. Earlier data were retrieved for comparison from international databases or national networks. Here, we show that the usual summer minimum in CO₂ due to the surface carbon uptake was reduced by 1.4 ppm in 2018 for the 10 stations located in the area most affected by the temperature anomaly, mostly in Northern Europe. Notwithstanding, the CO₂ transition phases before and after July were slower in 2018 compared to 2017, suggesting an extension of the growing season, with either continued CO₂ uptake by photosynthesis and/or a reduction in respiration driven by the depletion of substrate for respiration inherited from the previous months due to the drought. For stations with sufficiently long time series, the CO₂ anomaly observed in 2018 was compared to previous European droughts in 2003 and 2015. Considering the areas most affected by the temperature anomalies, we found a higher CO₂ anomaly in 2003 (+3 ppm averaged over 4 sites), and a smaller anomaly in 2015 (+1 ppm averaged over 11 sites) compared to 2018. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.

Keywords: ICOS; atmospheric CO2 measurements; drought; net ecosystem exchange.

Figure 1.

Location of the CO₂ monitoring sites in Europe. The symbols represent the different type of stations: tall towers (red circles), coastal sites (blue diamonds), mountain sites (green triangles), other surface stations (crosses) and total carbon column observing network (TCCON) site (square). (Online version in colour.)

Figure 2.

Time series of CO₂ mole fractions (ppm) at 48 sites in Western Europe over the period 2009–2018. Each vertical coloured line represents a CO₂ daily average. The second column indicates the sampling height above ground level, and the third column indicates whether the station is a tall tower (Tt), coastal (Cs), mountain (Mt) or other surface site (Ot). The colour code indicates the CO₂ mole fraction in ppm. (Online version in colour.)

Figure 3.

CO₂ time series observed at Hyltemossa (HTM, Sweden), Gartow (GAT, Germany), Tacolneston (TAC, UK) and Observatoire Pérenne de l'Environnement (OPE, France). Each blue dot corresponds to a daily average, after data selection as described in the main text. The black curve shows the smoothed curve of these points, and the dash-dot line the long-term trends. The red curve represents the smooth curve of the CO₂ time series at Mace Head, Ireland, in the marine sector. (Online version in colour.)

Figure 4.

CO₂ seasonal cycles observed at Hyltemossa (HTM, Sweden), Gartow (GAT, Germany), Tacolneston (TAC, UK) and Observatoire Pérenne de l'Environnement (OPE, France). The 2018 cycle is shown in red, 2017 in blue, and a statistical summary of the full measurement period as box-and-whisker plots showing the median, first and third quartiles over the entire measurement period of each station, indicated in the bottom left corners of the plots. Corresponding figures for all other stations are shown in electronic supplementary material, figure S2. (Online version in colour.)

Figure 5.

Monthly mean CO₂ differences anomalies for each month during May to October 2018, compared to the same month during previous years, the data being available from 2009 to 2018 with variable coverage between stations, i.e. CO₂ (2018) minus CO₂ (previous years). Upper panel shows the vertical profile of tall towers, whereas the bottom panel shows separately coastal, mountain and other surface sites. (Online version in colour.)

Figure 6.

The top panel shows, at four tall towers (highest levels), the amplitude of the CO₂ daily cycles averaged every month for all available years (black line and grey area), and for 2018 (red line). The bottom panel shows, for the same sites, the differences of CO₂ daily cycles between 2018 and the average over all available years. The colours represent the different sampling levels at the towers (red curves correspond to the top panel figures). (Online version in colour.)

Figure 7.

Same as figure 4 but for the column-averaged dry air mole fraction of CO₂ (XCO₂) measured at the TCCON site of Sodankylä, Finland. (Online version in colour.)

Figure 8.

Map of the monthly mean CO₂ differences (2018 minus 2017) from May (left) to September (right). Circles represent surface stations in lowlands. Triangles indicate the mountain sites, and the square indicates a total column measurement station (TCCON). Top panels show the differences for 2018 minus 2017. Bottom panels show differences for 2018 minus 2010–2017 mean. (Online version in colour.)

Figure 9.

Same as figure 4 for the 2003 (green) and 2018 (red) seasonal cycles at Shauinsland (SSL, Germany), Heidelberg (HEI, Germany), Monte Cimone (CMN, Italy) and Hegyhatsall (HUN, Hungary). (Online version in colour.)

Figure 10.

Same as figure 4 for the 2015 (green) and 2018 (red) seasonal cycles at Puy de Dôme (PUY, France), Observatoire Pérenne de l'Environnement (OPE, France), Monte Cimone (CMN, Italy), Kasprowy (KAS, Poland). (Online version in colour.)