Mapping ecological and human systems responses to land-atmosphere interactions altered by climate change

Land cover alteration due to anthropogenic activities modify land surface properties in absorbing, reflecting and emitting radiation as well as infiltrating, evaporating and storing water. This consequently modifies surface energy and water fluxes and, thus, climatic conditions. Progressive surface sealing results in higher runoff rates, less groundwater recharge, inhibited diurnal evaporative cooling and increased substrate heat storage, leading to augmented heat exchange by convection and, consequently, to an intensification of urban heat. We have identified a profound and robust relationship between the individual fluxes of the surface energy balance. From this, we derived an index including decisive aspects of land-atmosphere interactions and its feedbacks for assessment of the implication of surfaces to the climate system. The Surface Thermal Contribution Index (STCI) is intuitive to understand and can be calculated directly from Normalised Difference Vegetation Index (NDVI), from climate models or using data from on-site measurements. We provide a comprehensive framework to measure ecological and human systems responses to changes in land-atmosphere interactions and resulting feedbacks under global warming as well as critical malfunctions related to environmental and human well-being. Here, we use the index to map the partitioning of surface energy and water fluxes and assess surface thermal contribution at global to intra-urban microscale. Our results show that increasing global land evapotranspiration from 1999 to 2020, visible through a higher proportion of latent heat fluxes, is primarily observable in forested and irrigated regions and dominant on the northern hemisphere. Regional aridity, visible through a higher proportion of sensible and substrate heat fluxes, in combination with the 2019 European heatwave inhibited diurnal intra-urban evaporative cooling indicating that current urban adaptation measures cannot cope with decreasing water availability. Results confirm the hypotheses that land evapotranspiration should increase in a warming climate accompanied by increasing land aridity, amplified by land-atmosphere feedbacks, and thus reaffirm an intensification of the global water cycle. Although increasing latent heat fluxes favour surface cooling, land-atmosphere feedbacks lead to a decrease in surface water availability with increasing evapotranspiration, due to an acceleration in the transfer of water into the atmosphere. Global warming intensifies the global water cycle and increases the water holding capacity of the atmosphere as defined by the Clausius-Clapeyron relation. This further decreases surface water availability. The combination of increasing temperatures, land aridity and frequency of extreme heat events deteriorates urban vegetation health, diminishes the evaporative cooling effect and eventually leads to degradation of urban ecosystems. We conclude that green infrastructure interventions to reduce urban heat will not cope with future consequences, by means of regional water scarcity, if not irrigated extensively, which in turn will increase the pressure on local water resources and global water challenges. We stress the importance of restoring natural surface energy and water balances for climate-sensitive development. With global cities projected to shift to warmer and drier conditions, increasing resilience requires more comprehensive urban water management that sustainably provides sufficient water availability to avoid fatalities of ecological and human systems and maintain the evapotranspiration-driven cooling effect for successful urban heat mitigation.