Observed Global Photosynthesis Response to Changing Storm Frequency and Magnitude

Rain events are becoming less frequent, but stronger in many global locations under a changing climate. These intra-seasonal rainfall features have received less attention than changes in mean temperature and total annual rainfall in their influence on the global carbon cycle. Field rainfall manipulation experiments consistently show non-negligible changes to annual photosynthesis in response to rainfall frequency alterations while holding total annual rainfall constant. However, field and modeling experiments show little consensus on the sign and magnitude of change of annual photosynthesis due to changing storm frequency and magnitude. In this study, we ask: based on satellite observations, how is global photosynthesis changing due to shifts in storm frequency and magnitude? What are the soil-plant-atmosphere drivers of the response?Using several global satellite-based photosynthesis proxies, we find that the annual photosynthesis response to storm frequency is as high in magnitude and global spatial extent as its response to total annual rainfall. The satellite-based photosynthesis proxies and field tower sites indicate that years with fewer, stronger storms tend to show decreased photosynthesis in humid ecosystems and increased photosynthesis in drylands. The absolute magnitudes of annual photosynthesis trends show 10-20% per century changes due to rainfall frequency trends over nearly half of vegetated surfaces, which is consistent with the magnitude and extent of total annual rainfall trend effects. The contrasting responses observed in humid locations and drylands are shown to be driven by patterns of plant pulse response, soil texture, and mean atmospheric aridity response to rain frequency. Ultimately, our results indicate that intra-seasonal rainfall variability drives global photosynthesis interannual variability similarly to interannual rainfall variability.