Land-atmospheric feedbacks during droughts and heatwaves: state of the science and current challenges

Abstract

Droughts and heatwaves cause agricultural loss, forest mortality, and drinking water scarcity, especially when they occur simultaneously as combined events. Their predicted increase in recurrence and intensity poses serious threats to future food security. Still today, the knowledge of how droughts and heatwaves start and evolve remains limited, and so does our understanding of how climate change may affect them. Droughts and heatwaves have been suggested to intensify and propagate via land-atmosphere feedbacks. However, a global capacity to observe these processes is still lacking, and climate and forecast models are immature when it comes to representing the influences of land on temperature and rainfall. Key open questions remain in our goal to uncover the real importance of these feedbacks: What is the impact of the extreme meteorological conditions on ecosystem evaporation? How do these anomalies regulate the atmospheric boundary layer state (event self-intensification) and contribute to the inflow of heat and moisture to other regions (event self-propagation)? Can this knowledge on the role of land feedbacks, when available, be exploited to develop geo-engineering mitigation strategies that prevent these events from aggravating during their early stages? The goal of our perspective is not to present a convincing answer to these questions, but to assess the scientific progress to date, while highlighting new and innovative avenues to keep advancing our understanding in the future.

Keywords: drought; heatwave; land feedback; land-atmospheric interactions.

Figure 1

Land feedbacks as local intensifiers of hydro‐meteorological extremes. The large‐scale response of ecosystems to the high atmospheric demand, heat, and (surface and atmospheric) water stress is thought to be critical to set the magnitude of these events. Here, red color means positive relation (e.g., land desiccation enhances evaporation decrease), while blue color means negative relation. We highlight that the higher vapor pressure deficit (VPD) will typically reduce stomatal conductance and transpiration under conditions of surface water stress. We also note that the selection of relevant processes is boldly simplified in this conceptual diagram.

Figure 2

Evaporation during the 2012–13 Midwest drought. (A) Evolution of the event according to the U.S. Drought Monitor87 (top). Measurements of evaporation (mm month⁻¹) at three eddy covariance cropland sites in Nebraska: US‐Ne1, US‐Ne2, and US‐Ne3 (bottom). The satellite data come from GLEAM v3.2a.66, 88 The blue circles in the maps mark the location of the eddy covariance sites. (B) Anomalies in precipitation (P, mm month⁻¹), vapor pressure deficit (VPD, kPa), net radiation (R _n, W m⁻²), air temperature (T _a, K), and land evaporation (mm month⁻¹) during the drought.

Figure 3

CO₂ effect on plant physiology and impact on the 2003 heatwave. Differences in evaporation (top), sensible heat flux (middle), and maximum daily air temperature (bottom) between considering the effects of CO₂ on stomatal conductance and water use efficiency versus ignoring these effects (yellow lines and areas). The fraction of that effect that is purely physiological and does not relate to the cumulative impact of soil moisture savings due to the higher water use efficiency is depicted in green (lines and areas). For full details, see Lemordant et al.100

Figure 4

Self‐intensification and self‐propagation of droughts. Local and teleconnected land feedbacks may aid the intensification and propagation (expansion and concatenation) of meteorological droughts.

Figure 5

Synoptic conditions and land surface state during the 2010 Russian heatwave. For a 10‐day period prior to the heatwave (July 1–10), the onset (July 16–25), and the peak of the event (August 1–10): (top) average afternoon near‐surface air temperature (K) and mean sea‐level pressure (hPa) from ERA5,145 (middle) average anomalies in root‐zone soil moisture expressed in the number of standard deviations (σ) from GLEAM v3.2a,66, 88 (bottom) average surface sensible heat flux (W m⁻²) from GLEAM v3.2a.66, 88 Black contours mark the region affected by the heatwave, defined by the seasonal anomalies in average afternoon near‐surface air temperature exceeding a threshold of three standard deviations.146