In recent times, Europe has experienced a series of intense droughts. These droughts affect the health of our forests, and thereby threaten their ability to store carbon. This presents a pressing concern in global and national policies to offset anthropogenic carbon emissions with nature-based sequestration. Therefore, we need to understand and quantify the impacts of extended periods of drought on European forests.
A very intense drought occurred in Europe in the summer of 2022. We saw record-breaking temperatures over the South of Europe, with temperatures recorded over 40 oC in European countries such as France, the UK and the Netherlands. Such record temperatures are often paired with atmospheric blocking events, which are suggested to happen more often with progressing climate change . The European droughts of 2003 and 2018 both played out under such circumstances, with widespread impacts on vulnerable people, the economy, but also European carbon uptake [2, 3, 4].
The previous extreme drought occurred in 2018 and manifested mainly over the northern part of Europe, strongly affecting Scandinavia, the Netherlands and Germany. From previous research, we knew the drought had affected the carbon uptake of the forest in this area [5,6]. However, before we were able to study de 2018 drought, we had to undergo the time-consuming process of compiling the required data. Mainly because of this, it took us two years to analyse the impact of the drought on the European carbon cycle that we published in 2020. However, for urgent issues like climate change, waiting for such an extended data-gathering process can significantly delay the adoption of crucial policies. Therefore, over the past years, significant progression has been made in providing high-quality, near real-time data on European carbon exchange thanks to the Integrated Carbon Observing System, ICOS.
With this near real-time data, we were able to analyse the impact of the 2022 drought on the carbon exchange over Europe much more quickly. The data consisted of direct measurements of carbon exchange over 12 forest sites in Europe, atmospheric CO₂ measurements of 26 sites in Europe and a high-resolution satellite product that informs us on plant phenology. Moreover, we used a high-resolution model that estimates the carbon exchange over Europe, based on meteorological data . All the used data is available within 2 weeks afer real-time.
Using these data sets, we found that the biosphere in the drought-affected area took up 60TgC less during the summer of 2022 than normal. This represents as much carbon as emitted each year by humans in a country like Spain. This reduction in net uptake is due to a reduction in the gross primary productivity (GPP), which reduced by 20% at drought-affected sites. This reduction in the gross uptake of CO₂ was also visible in the canopy of trees itself either just in your backyard, or as observed by the Near-Infrared Reflectance of vegetation (NIRv); a satellite product . By correlating anomalies in NIRv to direct measurements of GPP at the 12 forest sites, we found that the GPP was reduced by about 450TgC; a factor of 1.8 higher than the total GPP reduction in 2018 over a similar-sized area.
Besides a decrease in the gross uptake of CO₂ , also the ecosystem respiration (the gross emissions of CO₂ by the ecosystem) decreased. Although respiration and gross primary production are separate processes, they are sensitive to similar drivers and therefore, reduced GPP almost always coincides with reduced ecosystem respiration. Nevertheless, some sites in the south of France became a source of CO₂ during the drought, even during summertime.
This reduction in the net uptake of CO₂ by the European biosphere resulted in higher-than-expected mole fractions at atmospheric stations over Europe, with stations in the drought-affected area showing measuring CO₂ mole fractions that were up to 2 ppm higher than the previous years. Similar enhancements in atmospheric mole fractions were found in the drought-affected area in 2018 .
Although in 2018 the drought reduced forest uptake of CO₂ in summer, the warm and sunny conditions in spring, paired with soil water present from a wet winter, enhanced uptake in spring. The enhanced spring uptake compensated about 75% of the reduced summer uptake in 2018 . In 2022, we found no such enhanced spring uptake, but rather enhanced autumn uptake, due to continuing warm temperatures combined with delayed leaf senescence. We find that the warm autumn compensated about 30% of the reduction in summer uptake.
Of course, we can only observe at the locations at which we have measurement stations. Generally, these measurement stations are located in the western part of Europe (with most of the stations being located in France and Germany). The 2022 drought however, also affected the eastern part of Europe, where we have very few measurement sites, and rely mostly on satellite retrievals. These satellite observations showed us that the eastern part of Europe has responded strongly to the drought, accounting for about 75% of the total reduction in summer uptake calculated with our biosphere model. Due to the lack of measurements however, we cannot evaluate our biosphere model in this region, even though in other parts of Europe we were able to positively evaluate the drought response in the model.
Finally, the hot and dry conditions of the summer of 2022 resulted in more widespread wildfires in the month of July, especially in the south of France. Nevertheless, despite the drought, the amount of European wildfires was below average in spring and autumn of 2022. Therefore, Europe-averaged wildfire emissions of carbon were similar to the 2003-2021 average, and much smaller (~8 TgC) than carbon loss by forests under stress.
With this first look into the carbon effects of the 2022 European drought, we were not yet able to assess legacy effects of the drought [2,9]. Nevertheless, the reduction in CO₂ uptake by forests indicates that plans to use forests for natural sequestration of CO₂ needs a very careful consideration of both the magnitude and persistence of such sinks, especially in a climate in which droughts are more frequent. Finally, we note that the 60TgC reduced uptake in 2022 added to the much larger 750TgC yearly emissions of CO₂ from anthropogenic activities in Europe. This clearly identifies rapid emission reductions as a much more urgent and safe path towards climate neutrality.