Abstract
We analysed gross primary productivity (GPP), total ecosystem respiration (TER) and the resulting net ecosystem exchange (NEE) of carbon dioxide (CO2) by the terrestrial biosphere during the summer of 2018 through observed changes across the Integrated Carbon Observation System (ICOS) network, through biosphere and inverse modelling, and through remote sensing. Highly correlated yet independently-derived reductions in productivity from sun-induced fluorescence, vegetative near-infrared reflectance, and GPP simulated by the Simple Biosphere model version 4 (SiB4) suggest a 130–340 TgC GPP reduction in July–August–September (JAS) of 2018. This occurs over an area of 1.6 × 106 km2 with anomalously low precipitation in northwestern and central Europe. In this drought-affected area, reduced GPP, TER, NEE and soil moisture at ICOS ecosystem sites are reproduced satisfactorily by the SiB4 model. We found that, in contrast to the preceding 5 years, low soil moisture is the main stress factor across the affected area. SiB4’s NEE reduction by 57 TgC for JAS coincides with anomalously high atmospheric CO2 observations in 2018, and this is closely matched by the NEE anomaly derived by CarbonTracker Europe (52 to 83 TgC). Increased NEE during the spring (May–June) of 2018 (SiB4 −52 TgC; CTE −46 to −55 TgC) largely offset this loss, as ecosystems took advantage of favourable growth conditions.This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.
Highlights
The summer of 2018 saw widespread and severe drought in north western Europe
Using the sixmonthly Standardized Precipitation Evapotranspiration Index (SPEI) from the SPEI global drought monitor after Bastos et al [27], the same criterion would exclude much of the UK and France, which we know to have been severely affected and which demonstrate strong anomalies in the remote sensing products shown in figure 1
There is an important role for vegetation stress due to high temperatures, high vapour pressure deficit, and low soil moisture, we find mixed signals of changes in water-use efficiency across sites and only evergreen needleleaf forests (ENF) exhibit the clear increase that was demonstrated in Ciais et al [5] and Peters et al [21] during the 2003 drought
Summary
Decreased precipitation in combination with record-high temperatures led to strong reductions in soil moisture availability and decreased atmospheric humidity [1,2]. The combination of high temperatures, low relative humidity, and reductions in soil moisture availability caused plants to close their stomata to conserve water. This led to reductions in evapotranspiration and photosynthetic activity, thereby reducing carbon uptake from the atmosphere [3,4]. The net impact of these reductions in uptake and release during widespread droughts in Europe, such as in 2003 and 2018, can have a significant impact on the carbon balance of the region
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