Global cycling and climate effects of aeolian dust controlled by biological soil crusts

The study addresses how biological soil crusts (biocrusts), which stabilize dryland soils and cover about 12% of Earth’s land surface, influence global aeolian dust cycling and associated climate effects. While biocrusts are known to bind soil particles and reduce erosion, their quantitative impact on global dust emissions, transport, deposition, and radiative forcing had not been explicitly assessed. Given dust’s roles in atmospheric radiation, cloud processes, nutrient transport, cryosphere albedo, and human health, the authors aim to quantify biocrust controls on the dust cycle under present-day conditions and to project changes under future climate and land-use scenarios.

Prior work shows biocrusts contribute to nutrient cycles, water balance, plant growth, and emit reactive nitrogen species affecting atmospheric chemistry. Dust aerosols influence radiation (short-wave scattering and long-wave absorption), serve as ice and cloud condensation nuclei, undergo atmospheric processing, and deposit nutrients that can fertilize ecosystems (e.g., Amazon P replenishment from African dust) but can also harm marine systems and human health. Experimental wind tunnel studies demonstrated biocrusts reduce dust emissions at plot scales, yet their global-scale impact remained unresolved. Global dust models (e.g., ECHAM-HAM, AeroCom intercomparisons) have wide emission ranges and often indirectly account for surface stabilization without explicitly representing biocrusts.

The authors compiled experimental data quantifying changes in threshold friction velocity (TFV) due to biocrust cover, including measurements for biocrust-covered and underlying bare soils. They derived exponential relationships between biocrust coverage and TFV increase: TFV increase ratio (%) = e^(a × biocrust cover), with parameter a set to match minimum, mean (geometric mean), and maximum observed TFV increases at 100% coverage: 114% (a = 0.0013), 479% (a = 0.0157), and 2,350% (a = 0.0316). Global biocrust cover maps (resampled to T63, ~210 km) were combined with these functions to produce spatially explicit TFV modifications. These were implemented in a modified aerosol–climate model, ECHAM6-HAM2-BIOCRUST, based on ECHAM6-HAM2.1 with online land coupling (JSBACH) and a dust source scheme resolving 192 particle size classes (0.2–1,300 μm), aggregated to insoluble accumulation and coarse modes. Five 30-year simulations (1990–2020) at T63L31 were performed: (1) standard ECHAM6-HAM2.1 (implicit stabilization via a global TFV correction factor 0.9), (2) three ECHAM6-HAM2-BIOCRUST runs using mean, minimum, and maximum TFV-increase parameterizations (with correction factor 0.80), and (3) ECHAM6-HAM2-NO BIOCRUST (biocrust TFV effect removed). The authors compared emissions, deposition (sedimentation, dry, wet), atmospheric dust burden, aerosol optical depth (AOD), and top-of-atmosphere (TOA) net radiative effects between BIOCRUST and NO BIOCRUST runs. Significance was assessed using one-way ANOVA with Tukey’s post hoc tests, including pixel-wise significance screening. Future impacts were simulated for 2055–2085 using RCP2.6, RCP4.5, and RCP8.5 climate and vegetation, with predicted biocrust cover reductions by 2070 (−25 to −40%). Differences between future simulations with current versus reduced biocrust cover isolated the biocrust-loss effect on dust cycling and radiative forcing. Data layers describing current and future TFV effects are publicly available.

- Biocrust enhancement of TFV: 100% biocrust cover increases TFV by a mean 480% (SE 176%), with a range from ~110% to ~2,350%. Globally, current biocrusts increase TFV by ~30% on average, highest in Australia (~60%) and Africa (~49%), lowest in Europe (~16%) and South America (~13%). - Present-day dust emissions: With mean or maximum biocrust TFV effects, global dust emissions are ~1,200 Tg yr−1; with minimum effect, ~1,550 Tg yr−1. These are comparable to ECHAM6-HAM2.1 (931–945 Tg yr−1) and within AeroCom model ranges (514–4,313 Tg yr−1). - Effect of removing biocrusts (1990–2020 mean): Additional dust emissions of ~700 Tg yr−1 (uncertainty ~350 to ~700 Tg yr−1), a ~60% increase (~30% to ~60%). Continental dust deposition increases by ~450 Tg yr−1 (~200 to ~450; ~40% increase) and ocean/water-body deposition by ~250 Tg yr−1 (~150 to ~250; ~55% increase), especially over the Indian, Pacific, and Atlantic Oceans. - Atmospheric load and optics: Biocrusts reduce the mean global atmospheric dust burden by ~8.5 Tg (~5.0 to ~8.5), corresponding to about a 55% decrease, and reduce dust AOD by ~20% (~5% to ~20%). - Radiative impacts: Removing biocrusts would decrease the net aerosol radiative effect at the TOA by up to 0.48 W m−2 (0.02 to 0.48). Short-wave contribution: −0.58 W m−2 (0.04 to 0.58 decrease); long-wave: +0.10 W m−2 (0.02 to 0.10 increase). The magnitude is comparable to the total direct forcing of anthropogenic aerosols (−0.35 ± 0.5 W m−2). - Biogeochemical implications: Without biocrusts, dust-borne P deposition over the Amazon would increase by ~7 g ha−1 yr−1, about 100% of the estimated current dust-borne P influx, potentially affecting rainforest productivity. - Future projections (to ~2070): Anthropogenic climate change and land-use intensification are projected to reduce global biocrust cover by ~25–40%. This loss increases global dust emissions and deposition by ~5–15% (scenario dependent), raises dust burden by up to ~16%, and reduces the global TOA radiative budget by ~0.12 to ~0.22 W m−2. Biocrust loss may counteract expected decreases in dust from Sahel greening and altered transport/residence times. - Regional hotspots: Strong emission increases projected for parts of North Africa and the Middle East; localized decreases in some Sahel, Asian, and Australian regions due to vegetation and wind changes.

The results directly demonstrate that biocrusts substantially suppress dust emission by increasing surface threshold friction velocities, thereby lowering atmospheric dust loads and altering radiation balance. The modeled removal of biocrusts produces large increases in emissions, deposition, and dust burden along major source regions and transport pathways, underscoring biocrusts’ regulatory role in the dust cycle. The associated radiative effect change (order 0.5 W m−2) is comparable to anthropogenic aerosol direct forcing, indicating that biocrust dynamics are climatically significant. Biogeochemically, altering dust fluxes affects nutrient delivery to terrestrial and marine ecosystems, with potential consequences for productivity (e.g., Amazon phosphorus budgets) and for human health through increased exposure to dust and associated microorganisms. Future reductions in biocrust cover due to climate change and land-use intensification are projected to intensify dust cycling and offset anticipated decreases from vegetation changes, with implications for regional climate, circulation patterns, and radiative forcing. These findings argue for explicitly incorporating biocrust processes into Earth system models and climate impact assessments.

Biocrusts currently prevent the emission and redistribution of roughly 700 Tg of dust per year, reducing atmospheric dust burden by about 55% and significantly influencing the global radiative balance. Projected biocrust cover losses by 2070 (−25 to −40%) are likely to increase dust emissions, deposition, and burden, and reduce the TOA radiative budget by ~0.12–0.22 W m−2, potentially overriding expected decreases in dust production from Sahel greening. Incorporating biocrust effects into dust and climate models will improve predictions of dust–climate–biogeochemistry interactions and associated health impacts. Future research should prioritize: expanding high-quality, spatially resolved data on biocrust cover and mechanical stabilization (TFV) across biomes; resolving community composition shifts (e.g., towards cyanobacteria-dominated crusts) and their effects on stabilization and albedo; improving dust size distribution representation in models; and integrating biocrust dynamics into land-use and climate mitigation strategies.

- Experimental TFV data are limited and biased towards plots with 100% biocrust coverage; missing paired bare-soil TFV measurements likely make the adopted TFV increases conservative. - Model spatial resolution (T63, ~210 km) cannot resolve small-scale atmospheric and geomorphological features that influence dust emission and transport. - Dust size distributions in many global models may underrepresent coarse particles, potentially biasing radiative effect estimates (cooling vs warming over bright surfaces). - The super-coarse dust mode was neglected due to short lifetime, which may affect near-source deposition estimates. - Biocrust maps and projected changes carry uncertainties from environmental predictors and resampling. - Use of global TFV correction factors and parameterizations introduces additional uncertainty; statistical significance was assessed, but structural model uncertainties remain.