When the Land Surface Shifts Gears

Abstract

Northern Europe is typically characterized by wet conditions, where the total evaporation and transpiration (together, "evapotranspiration") largely depend on atmospheric energy supply. Dirmeyer et al. (2021) report that in the hot and dry summer of 2018 evapotranspiration became water-limited such that decreased evaporative cooling amplified the heatwave temperatures. This way, drought magnitude increased beyond a critical point with consequent disruptions in ecosystem services. The land surface provides essential ecosystem services for society. Soils and vegetation take up atmospheric CO 2 (Heimann & Reichstein, 2008), their moisture can attenuate wildfires (Forkel et al., 2019, O.S., Hou, et al., 2020), and their evaporative cooling can mitigate hot temperatures (Seneviratne et al., 2012). These services depend on the meteorological conditions; high radiation and low rainfall can induce water stress in ecosystems and consequently limit their services (Figure 1). In semiarid regions experiencing dry seasons such as Central North America and Southern Europe, the lack of water supply limits evapotranspiration, a condition known as water limitation (Mueller & Seneviratne, 2012). This is usually not the case for northern Europe, where normally wet and cloudy weather creates conditions where evaporation rates are limited by energy supply. However, the 2018 summer drought introduced water-limited conditions to Great Britain and large parts of Northern Europe (Dirmeyer et al., 2021). Thereby, this region became a hot spot of land-atmosphere interactions; in addition to the prevailing impact of the meteorological conditions on soils and vegetation (atmosphere → land), soil moisture availability affected vegetation functioning and therefore evapotranspi-ration and the (near-surface) atmospheric moisture and energy budgets (land → atmosphere). This process is reinforced through a positive feedback (Seneviratne et al., 2010; Teuling, 2018); decreasing evapotranspiration contributes to warmer temperatures which in turn increases the evaporative demand which further depletes the soil moisture. Also, the increased atmospheric dryness can hinder cloud formation (Teuling et al., 2017), leading to more incoming radiation, which also contributes to warmer temperatures. Moreover, this feedback loop can spread impacts in space by advection of drier air masses to neighboring regions (Schumacher et al., 2019), and in time through lagged recovery of the vegetation after severe water stress (Bastos et al., 2020). Soil moisture is a key variable controlling the land-atmosphere coupling regime and its strength (Seneviratne et al., 2010). Dirmeyer et al. (2021) determine soil moisture thresholds across Europe, below which evapora-tive cooling becomes water-limited, enhancing peak temperatures. These thresholds vary in space, highlighting the role of vegetation and soil types in modulating land-atmosphere interactions (Denissen et al., 2020). While potential threshold variations in time, for example in response to land cover change, are yet to be investigated , their estimates can inform (near-)future forecasts and anticipation of record-breaking temperatures. Estimating these thresholds from observation-based data sets (Denissen et al., 2020) is only possible in regions that have already entered a water-limited regime to reveal the underlying soil moisture. In other regions, we have to rely on (future) modeling experiments to provide such estimates. This requires an accurate representation of the land-atmosphere coupling, including the different processes shaping near-surface weather between energy-and water-limited conditions (Flach et al., 2018). Land surface models are imperfect in this respect (Best et al., 2015; Dirmeyer et al., 2018; O. S., Dutra, et al., 2020). Nevertheless, they are indispensable tools; particularly in view of ongoing climate change, which increases the occurrence probability of unprecedented conditions as in 2018. Modeling and forecasting should rely on physical laws rather than empirical relationships that might not be applicable under transient conditions (O. S., Dutra, et al., 2020). Continued model development that improves and expands the representation of relevant processes will further enhance their applicability (Balsamo et al., 2009; Lawrence et al., 2019; Thum et al., 2019). ORTH