Serious underestimation of reduced carbon uptake due to vegetation compound droughts

Anthropogenic climate change intensifies climate extremes like droughts and heatwaves, jeopardizing ecological and societal sustainability. Drought, through low SM and/or high VPD, directly limits terrestrial water availability and carbon uptake, substantially reducing agricultural production and causing vegetation mortality. Compound droughts (CDs), characterized by concurrent low SM and high VPD, cause greater GPP reductions than individual drought types. Low SM and high VPD frequently co-occur, creating a feedback loop: SM deficits reduce evapotranspiration, leading to higher VPD, which further reduces SM. Projections indicate more frequent and intense CDs, potentially reducing land carbon sinks. However, existing CD assessments mainly focus on statistical extremes without considering environmental impacts. The lack of a universally accepted CD definition leads to uncertainty in global assessments. Quantile-based definitions, while common, are problematic; they may overestimate CD frequency in high-latitude regions and underestimate them in drylands. In drylands, while plants have adaptations, productivity is primarily water-limited. The inherent uncertainties associated with the quantile-based definition necessitate a more impact-centric approach.

Numerous studies highlight the non-monotonic relationship between GPP and SM/VPD. GPP generally increases with SM under water-limited conditions, but excessive SM can decrease GPP. Similarly, GPP may initially increase with VPD at low temperatures, but extremely high VPD inhibits photosynthesis. Existing studies utilizing a quantile-based approach to define CDs have limitations. In high-latitude regions, this approach may show positive GPP anomalies associated with CDs because increased VPD can enhance carbon uptake where water availability isn’t limiting. Conversely, in drylands, it underestimates the frequency and severity of CDs because it uses fixed thresholds, ignoring the variable responses of vegetation to water stress across different regions and biomes. These inconsistencies necessitate an improved, impact-based definition of CDs.

This study proposes an impact-based framework defining CDs based on GPP responses to low SM and high VPD. The framework identifies soil droughts and atmospheric aridity based on the assumption that low SM and high VPD limit GPP. CDs are defined as months when soil droughts and atmospheric aridity co-occur during the warm season. This approach was applied using observational datasets (GLEAM v3.5a for SM, MERRA-2 for VPD, FLUXCOM for GPP) for 1981-2017 and CMIP6 simulations (historical 1930-2014, future SSP1-2.6 and SSP5-8.5, 2016-2100). The segmented linear regression method was employed to identify SM and VPD thresholds significantly impacting GPP for each grid cell. The change-point in SM/VPD above which GPP no longer increases/decreases was compared with the SM/VPD associated with zero GPP anomaly; the lower/higher value was chosen as the threshold. Vegetation compound droughts (VCDs) were identified when SM fell below and VPD exceeded their respective thresholds. These VCDs were compared to statistical compound droughts (SCDs) defined using a quantile-based approach (SM below 10th percentile, VPD above 90th percentile). CD characteristics (frequency, duration, intensity, severity, GPP anomalies) were analyzed. Future projections considered two scenarios: using historical thresholds to isolate climate change impacts and using scenario-specific thresholds to account for potential CO₂ fertilization effects on GPP responses to SM and VPD. Dryland areas were defined based on the UNCCD definition.

The impact-based approach revealed significantly more frequent and severe VCDs than the quantile-based approach. VCDs were detected over 66% of the land area (observations) and 91% (CMIP6 simulations). Negative GPP anomalies were observed during VCDs, atmospheric aridity, and soil droughts; in contrast to the quantile-based approach, there were no positive GPP anomalies in high-latitude regions. Global area-weighted mean GPP anomalies during VCDs were -0.61±0.42 gC·m⁻²·day⁻¹ (CMIP6) and -0.15±0.10 gC·m⁻²·day⁻¹ (observations). The additional negative impact of low SM during VCDs was substantial, particularly in mid-latitude dry regions. SCDs frequency was much lower (11% of VCDs) and captured only 26% of global GPP anomalies. The intensity of VCDs was 50% higher and duration 35% longer than SCDs, resulting in approximately double the severity. Globally, total GPP anomalies due to VCDs were -1.44 PgC·yr⁻¹ (CMIP6) and -0.31 PgC·yr⁻¹ (observations), compared to -0.24 PgC·yr⁻¹ and -0.08 PgC·yr⁻¹ for SCDs, respectively. The quantile-based approach underestimated VCD impacts, particularly in drylands, where VCD frequency was higher due to strong SM-VPD coupling. Future projections show increased VCD frequency and intensity across most land areas under both SSP scenarios. Accounting for CO₂ fertilization effects slightly reduced the projected increase in frequency but maintained significant increases in intensity and negative GPP anomalies. SCDs also showed increased intensity and negative GPP anomalies, but less significantly than VCDs.

This study demonstrates the limitations of the quantile-based approach in assessing the impacts of CDs, particularly in drylands. The impact-based approach, considering regional variations in GPP responses to SM and VPD, more accurately captures the frequency, intensity, and severity of CDs and their carbon uptake impacts. The substantial underestimation of CD impacts in drylands highlights their vulnerability. The strong coupling of low SM and high VPD in drylands exacerbates water stress, limiting the resilience of dryland ecosystems to drought. The study also shows that Earth system models may underestimate the sensitivity of vegetation carbon uptake to drought, particularly extreme events. Future research should focus on improving model representation of extreme climate events and plant hydraulic processes. The projected increases in VCD frequency and intensity underscore the need for mitigation and adaptation strategies.

This research highlights a significant underestimation of reduced carbon uptake due to vegetation compound droughts (VCDs). The impact-based approach provides a more accurate assessment of VCD frequency and severity, revealing greater risks to land carbon sinks, especially in drylands. Future climate change is projected to exacerbate VCDs, leading to substantial carbon loss. Mitigating strategies, including reduced fossil fuel use, improved land management, and forest restoration, are crucial, with a particular focus on vulnerable dryland ecosystems. Adaptation measures, like cross-basin water transfers and drought-resistant crops, are also necessary to ensure water resource sustainability and agricultural production.

The study relies on the assumption that GPP responses to SM and VPD are concurrent within the same month, potentially ignoring ‘drought legacies’ where impacts persist beyond the immediate drought event. Earth system models may not accurately capture drought legacies, introducing uncertainty into the projections. While the study uses state-of-the-art models and observations, there is inherent uncertainty in models, particularly regarding extreme climate events and plant hydraulic processes. This could influence the accuracy of projected terrestrial carbon loss induced by VCDs.