Reduced carbon uptake by European forests during the summer drought of 2022

Recent years have seen repeated record breaking temperatures and high impact events such as floods, storms, and droughts
across Europe, in line with the expected consequences of its +1.5 oC of warming over the past thirty years. In many metrics
the 2018 summer drought has ranked top of the list for severity and impacts, including in reductions of carbon exchange by
forests. Drought conditions thought unprecedented over the past 500 years trigger strong responses in forest, suffering
from leaf-level vapor-pressure deficits and root-level moisture deficits simultaneously. We report here that in the summer of
2022 close to 30% of the European continent was again under severe or exceptional drought, with temperatures exceeding
those even of 2018 and a similarly large size of area affected (3.4 million km2). Although a stationary blocking atmospheric
pressure pattern over the Atlantic was responsible for both droughts, the 2018 event mostly affected northwestern Europe while
the 2022 drought was centered over France. The more southerly centering exposed more drought resilient semi-arid vegetation
which dampened the peak loss of carbon uptake by forests relative to 2018. Observations and models suggest that vapor
pressure deficits rather than lack of soil moisture played a dominant role in reducing photosynthesis in 2022. Nevertheless we
find a similar cumulative reduction of net ecosystem carbon exchange (~50 TgC less uptake) in 2022, with specifically high
impacts in southern France where widespread summertime carbon release by forests, as well as extensive wildfires (emitting
close to 5 TgC) occurred. Our analysis demonstrates a much improved capacity in our community to rapidly quantify drought
impacts from the atmospheric and ecosystem monitoring network. However, strong impacts on eastern European broadleaf
forests suggested from observed near-infrared reflection by vegetation and simulated by terrestrial carbon cycle models can
not be confirmed currently through in-situ observations, signaling an important gap in our capacity to track carbon exchange in
the European terrestrial biosphere.