The year 2022 witnessed extreme temperatures across Europe, with summer highs exceeding 40 °C in several mid-latitude countries and Mediterranean Sea temperatures surpassing 30 °C. Autumn temperatures also remained elevated. These extreme heat events are often linked to blocking events, stationary Rossby wave trains that maintain high-pressure areas over the continent, diverting moisture from the Atlantic. These blocking patterns are more frequent during positive phases of the North Atlantic Oscillation (NAO) and are predicted to increase with climate warming. Intense droughts in 2003, 2015, 2018, and 2022 occurred under such conditions, with studies confirming the role of human-induced climate warming. While portrayed as exceptional, these events are becoming increasingly frequent, with return times reduced to 10-15 years under current warming trends, signifying a new normal for the coming decades. The impact of these extremes on the carbon exchange between European forests and the atmosphere is a significant area of concern, particularly for carbon sequestration strategies crucial to the Paris Agreement.

Previous research has established the link between extreme weather events like heatwaves and droughts and the North Atlantic Oscillation. Studies on past droughts (2003, 2015, 2018) have highlighted the significant impact on European ecosystems and confirmed the role of human-induced climate change. The 2018 drought, in particular, was studied extensively, revealing a reduction in carbon uptake and examining the effects of compensating factors like a warm spring. However, the 2022 drought, with its unique geographic distribution and intensity, demanded a specific investigation into its impact on the European carbon cycle.

This study employed a multi-faceted approach using data from ground-based and space-based monitoring platforms. The Integrated Carbon Observing System (ICOS) network provided near real-time atmospheric CO2 mole fraction data across Europe, allowing for rapid assessment of carbon exchange anomalies. The analysis encompassed four drought-affected regions ('North,' 'Centre,' 'South,' and 'East') defined based on the Standardized Precipitation Evapotranspiration Index (SPEI). Eddy covariance (EC) data from 14 forest sites were used to analyze carbon fluxes at a finer scale. Satellite observations, specifically the Moderate Resolution Imaging Spectroradiometer (MODIS) near-infrared reflectance of vegetation (NIRv), were utilized to assess vegetation productivity and its correlation with Gross Primary Productivity (GPP). The SiB4 biosphere model, driven by ERA5 meteorology, provided a larger-scale perspective on carbon fluxes, offering independent verification of the observations. The methodology also included a separate quantification of carbon loss due to wildfires, using data from the Global Fire Assimilation System (GFAS) and Sentinel-2 observations.

The 2022 summer drought resulted in a significant reduction in net carbon uptake (56-62 TgC) across the affected regions. This reduction was comparable to the 2018 drought but with a different geographic focus. Specific sites in southern France experienced widespread summertime carbon release from forests, in addition to wildfire impacts. Atmospheric CO2 anomalies from the ICOS network confirmed the reduced uptake, with sites in southern France showing a substantial increase in CO2 mole fractions. Biomass burning, while contributing to higher CO2 at certain sites, had a limited overall impact at the European scale. A warm autumn partially compensated for the decreased summer carbon uptake (approximately 32% in the 'Centre' and 'South' regions), but this compensation was less significant than that observed following the 2018 drought. The SiB4 model simulations showed a good correlation with both the atmospheric observations and the NIRv-derived estimates of GPP, validating its capability to capture the drought's impacts. The analysis showed that while soil moisture was the primary limiting factor in 2018, high vapour pressure deficit (VPD) played a larger role in the 2022 drought, especially in the southern regions. EC observations independently confirmed the effect of VPD on GPP reduction.

The findings demonstrate the vulnerability of European forests to recurring severe droughts, with the 2022 event mirroring the impacts of the 2003 drought in terms of reduced net carbon uptake. The similarity in magnitude of the carbon uptake reduction between 2018 and 2022 highlights the increasing frequency of such extreme events and their potential for long-term ecological and climatic consequences. The limited compensation effect from the warm autumn in 2022, compared to 2018, emphasizes the need for further investigations into the mechanisms behind seasonal variations in carbon fluxes. The study also highlights the importance of high VPD as a stressor for photosynthesis, particularly in southern regions. The study stresses the need for improved monitoring, especially in eastern Europe, to accurately assess the carbon sequestration potential of these forests. More frequent droughts and warming temperatures will significantly affect the carbon sink capacity of European forests, impacting climate change mitigation strategies.

The 2022 European drought significantly reduced forest carbon uptake, comparable to previous events, underscoring the vulnerability of European forests to extreme weather. While a warm autumn offered some compensation, it was less effective than in previous years. High VPD emerged as a critical stressor, particularly in southern Europe. Improved monitoring, especially in less-studied regions like eastern Europe, is crucial for accurate carbon budget assessments. These findings highlight the need to account for the increasingly frequent occurrence of such events in climate change mitigation plans. Future research should focus on better understanding the interplay of soil moisture, VPD, and legacy effects on forest carbon dynamics.

The study acknowledges limitations in observational data, particularly in eastern Europe, where model-based estimates played a more significant role. The analysis of legacy effects was limited, potentially underestimating the long-term impacts of the drought. While the SiB4 model showed good agreement with observed data, model uncertainties should be considered. The lack of comprehensive, continuous monitoring across all of Europe limits the ability to completely verify the findings.