Lucas R. Vargas Zeppetello

Due to its location at the interface between land surface and atmosphere, soil moisture (SM) plays an important role in modulating energy, water and carbon fluxes. During periods of decreasing SM, SM loss is dependent on evapotranspiration (ET), drainage and changes in plant water storage. Investigating SM loss can give important insights into these processes. Here we use 25 years of global remote sensing data to investigate how SM loss is controlled by vegetation and temperature. We find that positive vegetation anomalies lead to slower SM loss in most areas, except for cold boreal forests. We hypothesize that these effects arise from competing effects of soil shading, transpiration and root water uptake by the vegetation. The effect whereby positive vegetation anomalies increase SM loss is limited to high SM conditions and disappears at lower SM, likely due to water stress limiting transpiration. By analyzing temperature and vegetation anomalies jointly we find that the relationship between SM loss and temperature varies between regions, but vegetation cover effects persist across the full range of temperature anomalies. Using a simple energy and moisture budget model, we can reproduce observed vegetation and temperature effects, supporting the interpretation that vegetation controls topsoil SM loss through shading and transpiration. We also find widespread positive SM loss trends which indicates accelerated topsoil water cycling, likely due to higher atmospheric water demand driven by increasing temperatures.

Due to its location at the interface between land surface and atmosphere, soil moisture (SM) plays an important role in modulating energy, water and carbon fluxes. During periods of decreasing SM, SM loss is dependent on evapotranspiration (ET), drainage and changes in plant water storage. Investigating SM loss can give important insights into these processes. Here we use 24 years of global remote sensing data to investigate how SM loss is controlled by vegetation and temperature. We find that positive vegetation anomalies lead to slower SM loss in most areas, except for cold boreal forests. We hypothesize that these effects arise through competing effects of soil shading, transpiration and root water uptake by the vegetation. The effect that positive vegetation anomalies increase SM loss is limited to high SM conditions and disappears at lower SM, likely due to water stress limiting transpiration. By analyzing temperature and vegetation anomalies jointly we find that the relationship between SM loss and temperature varies between regions, but vegetation cover effects persist across the full range of temperature anomalies. Using a simple energy and moisture budget model we can reproduce observed vegetation and temperature effects, supporting the interpretation that vegetation controls topsoil SM loss through shading and transpiration. We also find widespread positive SM loss trends which indicates accelerated topsoil water cycling, likely due to higher atmospheric water demand driven by increasing temperatures.

Year-to-year variance of agricultural productivity is an important determinant of food security. Previous global analyses described increased yield volatility from warming, but it has become increasingly clear that changes in water availability are also a key determinant of yields. Here, we provide the first global quantification of climate change impacts on maize, soybean, and sorghum yield variance due to changes in temperature and soil moisture. Pairing an empirical crop model with CMIP6 simulations indicates that changes in temperature and soil moisture increase interannual weather-induced yield variance by 7   to   19 % ° C across crops. This increase is driven roughly equally by previously quantified increased temperature stress variance, as well as by increased covariance between temperature and soil moisture stresses. Results using a simple land surface model are consistent with those using CMIP6 simulations in indicating that this increased covariance is driven by warmer air more quickly drying soils and by dry soils promoting greater warming.

Observational analyses consistently find that yields of major rainfed crops increase with temperature up to a threshold of approximately 32C, above which they reduce sharply. Two damage pathways have been suggested to explain this relationship: that high temperatures directly stress crops and drive yield loss, or that high temperatures induce water stress in crops, which in turn drives yield loss. Here we explore a third pathway: that soil water stress limits both agricultural productivity and evaporative cooling, giving rise to the nonlinear relationship between temperature and yield. Determining which of these pathways underpins the yield‐temperature relationship is important for predicting future crop productivity because climate change is expected to alter the co‐variability between temperature and water availability. To examine this third pathway, we use cumulative growing‐season transpiration from an idealized land surface model as a proxy for yield. This approach reproduces the observed yield‐temperature relationship, even though the model includes no mechanisms that limit productivity at high temperatures. In experiments where the influence of temperature on soil moisture is suppressed, yields still decline during hot, dry periods in a manner consistent with the observations. We conclude that water stress, and its influence on evaporative cooling, temperature, and agricultural productivity, drives the yield‐temperature relationship found in crops that experience episodic water stress. This framework explains the muted sensitivity of irrigated yields to high atmospheric temperatures, and suggests that future yield outcomes depend more critically on changes in rainfall than suggested by estimates that attribute yield losses primarily to temperature variations.

Interannual fluctuations in average summertime temperatures across the western United States are captured by a leading empirical orthogonal function that explains over 50% of the total observed variance. In this paper, we explain the origins of this pattern of interannual temperature variability by examining soil moisture–temperature coupling that acts across seasons in observations and climate models. We find that a characteristic pattern of coupled temperature–soil moisture climate variability accounts for 34% of the total observed variance in summertime temperature across the region. This pattern is reproduced in state-of-the-art global climate models, where experiments that eliminate soil moisture variability reduce summertime average temperature variance by a factor of 3 on average. We use an idealized model of the coupled atmospheric boundary layer and underlying land surface to demonstrate that feedbacks between soil moisture, boundary layer relative humidity, and precipitation can explain the observed relations between springtime soil moisture and summertime temperature. Our results suggest that antecedent soil moisture conditions and subsequent land–atmosphere interactions play an important role in interannual summertime temperature variability in the western United States; soil moisture variations cause distal temperature anomalies and impart predictability at time scales longer than one season. Our results indicate that 40% of the observed warming trend across the western United States since 1981 has been driven by wintertime precipitation trends in the U.S. southwest. Significance Statement: Year-to-year fluctuations in summertime temperatures have a large impact on drought, wildfire, and extreme heat across the Western United States. We find strong evidence that soil moisture deficits in the preceding spring may be the primary driver of higher-than-average summer temperatures in this region. Our results suggest that memory in the water cycle may lead to greater predictability in the climate system from season to season and that trends in southwest precipitation have exerted a considerable influence on observed warming over the past 40 years.

Heat waves are broadly expected to increase in severity and frequency under climate change. Case studies highlight a number of physical mechanisms that play a role in present-day heat waves, which typically occur during a coalescence of anomalous atmospheric and land surface conditions. However, a unified model of heat wave physics is lacking, primarily owing to difficulty in disentangling the forcing versus feedback roles of soil moisture and atmospheric variability. Here, we provide observational evidence that soil moisture modulation of heat waves is a generic feature of midlatitude continental climates, and develop a theoretical framework to understand this modulation. Using separation of timescales we derive a diagnostic equation for the nonlinear response of temperature to soil moisture variations, and a dynamical Hasselmann-like model for the soil moisture variations themselves. We find that soil moisture fluctuations control the frequency and intensity of temperature extremes by slowly altering the background state on which rapid atmospheric variability acts, rather than by altering atmospheric variability itself. We also find the slow soil moisture variations are well-approximated as being primarily driven by stochastic precipitation variability. Our framework provides a first-principles understanding of soil moisture’s role in midlatitude temperature extremes.

The Heat Index is a metric that quantifies heat exposure in human beings. Here, using probabilistic emission projections, we show that changes in the Heat Index driven by anthropogenic CO₂ emissions will increase global exposure to dangerous environments in the coming decades. Even if the Paris Agreement goal of limiting global warming to 2 °C is met, the exposure to dangerous Heat Index levels will likely increase by 50–100% across much of the tropics and increase by a factor of 3–10 in many regions throughout the midlatitudes. Without emissions reductions more aggressive than those considered possible by our statistical projection, it is likely that by 2100, many people living in tropical regions will be exposed to dangerously high Heat Index values during most days of each typical year, and that the kinds of deadly heat waves that have been rarities in the midlatitudes will become annual occurrences.