Enhanced soil moisture-temperature coupling could exacerbate drought under net-negative emissions

Droughts are devastating natural hazards, causing significant economic losses and fatalities. With global warming, drought frequency and severity are projected to increase. While the Paris Agreement aims to mitigate climate change by reducing CO2 emissions, the impact of CO2 removal pathways on drought remains unclear. This study addresses this knowledge gap by examining drought risk under net-zero and net-negative emission scenarios. Land-atmosphere (LA) feedback, particularly soil moisture (SM) dynamics, plays a critical role in drought evolution. Persistent SM deficits lead to higher temperatures, impacting evapotranspiration and atmospheric aridity, further reducing precipitation likelihood. Conversely, in regions with high moisture, amplified atmospheric aridity may not prevent precipitation. The soil moisture-temperature (SM-T) coupling metric is key to understanding LA interactions and extreme climate events. Enhanced SM-T coupling, intensified by climate change, strengthens the negative relationship between SM and temperature, leading to more frequent and intense droughts. Plant physiological responses to CO2 concentrations also influence LA water and energy fluxes. While CO2 fertilization can drive cooling and greening, reduced transpiration may lead to surface warming. The complex interplay between CO2 dynamics, SM, and temperature requires investigation, especially concerning net-negative emissions techniques. This study aims to examine global drought evolution under net-zero and net-negative emissions, focusing on the LA process's influence on drought onset. The CESM2 model, along with comparisons to CESM2-LE and CMIP6 CDRMIP data, are utilized to achieve a robust understanding.

Existing literature extensively documents the detrimental impacts of droughts, highlighting their increasing frequency, severity, and spatial extent under climate change. Studies project escalating economic losses and population exposure to droughts under future climate scenarios. However, our understanding of how droughts respond to CO2 removal pathways remains limited. Research emphasizes the importance of land-atmosphere feedback, large-scale circulation changes, sea surface temperature anomalies, and teleconnections in shaping extreme events. Soil moisture (SM) is central to LA feedback, influencing water and energy fluxes through changes in air temperature and humidity. SM deficits amplify air temperatures, impacting evapotranspiration and increasing atmospheric aridity, thus reducing precipitation probability. The literature also explores the role of SM-T coupling in interpreting extreme events. A negative correlation between SM and temperature is observed in dry areas, with this coupling intensifying under warming climates due to increased surface energy flux sensitivity to SM dynamics. Studies have examined the influence of plant physiological responses to CO2, especially stomatal conductance and leaf area expansion, on LA water and energy fluxes. Rising CO2 levels increase atmospheric moisture content due to higher surface temperatures, though the effect of CO2 fertilization on surface temperature is complex, involving trade-offs between increased plant growth, transpiration, and changes in albedo. Existing literature also highlights the need for understanding how net-negative emission strategies affect the LA feedback process and drought occurrence.

This study employs the Community Earth System Model version 2 (CESM2) to simulate idealized CO2 emission scenarios, encompassing three phases: a linear increase, a decrease toward net-zero and net-negative emission targets, and a return to the initial CO2 concentration. The simulation covers a period from 2000 to 2400, with net-zero and net-negative targets achieved after 2123. A 30-year period (2000-2030) is defined as the reference period for comparison. The Standardized Precipitation Evapotranspiration Index (SPEI) with a 12-month timescale is used to assess global droughts. The modified Penman-Monteith method is employed for potential evapotranspiration (PET) estimation, incorporating the impact of atmospheric CO2 changes on surface resistance. Drought characteristics—duration, frequency, intensity, and area coverage—are analyzed using the run theory approach. The SM-T coupling metric, calculated using standardized temperature anomalies and an energy term representing the influence of SM scarcity on sensible heat flux, is used to quantify LA interactions. Results from CESM2 are compared with those from CESM2-LE and CMIP6 CDRMIP experiments to assess the robustness of the findings. The analysis focuses on the 60-year periods of net-zero and net-negative scenarios (2131-2197) to contrast with the reference period.

The study reveals a significant trend towards increased global dryness under both net-zero and net-negative emission scenarios. However, dryness is considerably more pronounced in terms of magnitude and area coverage under net-negative forcing. While net-zero emissions result in significant dryness covering approximately 17% of global land, net-negative emissions show significant dryness across about 38%. Regional variations are observed, with certain regions experiencing benefits from either net-zero or net-negative emissions. South America and North America benefit more from net-negative emissions, while Central America, Africa, and Asia benefit more from net-zero. Analysis of drought characteristics (duration, frequency, intensity) shows a substantial increase under both scenarios compared to the reference period, but with a greater increase under net-negative emissions—over 66% in duration, 68% in frequency, and 74% in intensity globally. Spatial patterns indicate intensified drought conditions across numerous regions under both scenarios, but more severe conditions under net-negative emissions. Analysis of the SM-T coupling metric reveals a significant increase under both scenarios, most notably under net-negative emissions, particularly in regions experiencing increased dryness. The coupling strength is demonstrably higher during drought years compared to non-drought years, and intensifies with drought severity, emphasizing the LA feedback's role in drought formation. Examination of climate drivers reveals a general decrease in precipitation and soil moisture under both scenarios, coupled with increased PET, exacerbating SM deficits. This is especially true under net-negative emissions, where increased PET surpasses the decrease in precipitation. The CMIP6 CDRMIP experiment largely supports the CESM2 findings, though regional differences exist.

The findings highlight the complex relationship between CO2 mitigation strategies and drought risk. Contrary to expectations, net-negative emissions exacerbate drought in several regions due to stronger SM-T coupling and increased evaporative demand outweighing reduced precipitation. This is partly attributable to plant physiological responses to reduced CO2, altering transpiration and SM content. The regional disparities in drought response emphasize the need for tailored adaptation strategies, as some regions benefit more from net-zero, while others benefit from net-negative emissions. The study's results emphasize the limitations of CO2 mitigation alone and highlight the importance of integrating advanced water management strategies to effectively mitigate future drought risks. The persistence of SM deficits even under net-negative emissions underscores the complexity of climate system responses and the need for sustained efforts in drought mitigation and adaptation.

This study demonstrates that achieving net-zero CO2 emissions is more effective for drought mitigation compared to net-negative emissions. While both scenarios lead to increased drought severity in certain regions, net-negative emissions result in more significant drought intensification due to stronger soil moisture-temperature coupling and increased evaporative demand. The regional disparities in drought response highlight the need for regionally tailored adaptation strategies, and the findings underscore that CO2 mitigation alone is insufficient. Advanced water management strategies are crucial for effective drought management even under ambitious climate mitigation targets. Future research should explore more comprehensive drought indices, investigate the impacts of different CO2 removal methods and assess the interaction between drought and other extreme climate events.

This study utilizes an idealized CO2 emission scenario and a single climate model (CESM2). While comparisons with CESM2-LE and CMIP6 CDRMIP data enhance robustness, variations in model structure and forcing scenarios could lead to differences in results. The modified Penman-Monteith method used for PET estimation might not perfectly capture the complex responses of vegetation to increased CO2. The analysis doesn't explicitly account for autocorrelation in drought time series, potentially affecting the statistical significance of some findings. These limitations should be considered when interpreting the results.