Experimental evidence of a biological soil crust degradation climate warming amplification feedback.

Biological soil crusts - photosynthetic communities of cyanobacteria, lichens, and/or mosses living on soil surfaces - represent about 12% of the global land surface, yet their role in climate mitigation remains uncertain. Here we address this knowledge gap by integrating multiscale remote sensing techniques within a unique long-term climate manipulation experiment. Two decades of in situ climate warming and changes in monsoonal rainfall drove a major shift in biocrust community composition, including a ~ 27% decrease in fractional coverage of late-successional mosses and a ~ 36% increase in fractional cover of early-successional, lightly-pigmented cyanobacteria. This community shift, in-turn, resulted in a ~ 6.3% reduction in photosynthetic potential, ~14.1% increase in surface brightness, ~15.2% decrease in surface moisture, and a ~ 1.5% or ~0.3 °C increase in surface temperature under clear sky daytime conditions for every 10% increase in the relative cover of lightly-pigmented cyanobacteria. Our findings are evidence of a biocrust degradation climate warming amplification feedback, whereby climate warming drives biocrust functional degradation and a net reduction in climate mitigation potential, which further drives warming. This apparent warming amplification feedback is currently missing from process-based models, and thus current climate projections might underestimate the rate of climate warming across global drylands.