Uncertainty in Projected Critical Soil Moisture Values in CMIP6 Affects the Interpretation of a More Moisture‐Limited World

Abstract

Evaporation is controlled by soil moisture (SM) availability when conditions are not extremely wet. In such a moisture‐limited regime, land‐atmosphere coupling is active, and a chain of linked processes allow land surface anomalies to affect weather and climate. How frequently any location is in a moisture‐limited regime largely determines the intensity of land feedbacks on climate. Conventionally this has been quantified by shifting probability distributions of SM, but the boundary between moisture‐limited and energy‐limited regimes, called the critical soil moisture (CSM) value, can also change. CSM is an emergent property of the land‐atmosphere system, determined by the balance of radiative, thermal and kinetic energy factors. We propose a novel framework to separate the contributions of these separate effects on the likelihood that SM lies in the moisture‐limited regime. We confirm that global warming leads to a more moisture‐limited world. This is attributed to reduced SM in most regions: the moisture effect. CSM changes mainly due to shifts in the surface energy budget, significantly affecting 27.7% of the globe in analyzed climate change simulations. However, consistency among Earth system models regarding CSM change is low. The poor agreement hints that variability of CSM in models and the factors that determine CSM are not well represented. The fidelity of CSM in Earth system models has been overlooked as a factor in water cycle projections. Careful assessment of CSM in nature and for model development should be a priority, with potential benefits for multiple research fields including meteorology, hydrology, and ecology.

Plain Language Summary

In the water cycle, moisture‐limited conditions exist when evaporation is limited by a lack of soil moisture. This occurs when soil moisture lies below a threshold called the critical soil moisture (CSM). As evaporation affects atmospheric temperature and humidity, the value of CSM is important for weather and climate, as it determines when land states can affect the atmosphere. Climate change simulations agree the world will become more moisture‐limited, mainly attributed to drying soils, but the value of CSM can also change because it is determined in part by local meteorology as part of a land‐atmosphere feedback. This study shows that simulations from different climate change models consistently agree on an overall drying of the soil in the future. Changes in CSM are also simulated, but Earth system models do not agree on the magnitude or direction of CSM change in most places. This disagreement introduces uncertainty in the places and times when soil moisture controls evaporation and its impact on the atmosphere. Models have not historically been calibrated or validated for CSM simulation; we advocate for more attention to be paid to observing and modeling CSM due to its importance for meteorology, hydrology, and ecology in a changing climate.

Key Points

• An increasingly moisture‐limited world under global warming depends on more than just reduced soil moisture

• Earth system models inconsistently simulate the critical soil moisture value that separates moisture‐limited and energy‐limited regimes

• Poor agreement among models on projected changes in critical soil moisture calls for greater focus on its observation and validation