The emergence of snow droughts as drivers of negative extremes in plant productivity over the past decades.

Abstract Hydroclimatic extremes such as droughts cause significant anomalies in the global carbon cycle, e.g., via reductions in ecosystem gross primary production (GPP), and hamper the storage capacity of the terrestrial carbon sink. At the same time, across snow covered ecosystems, plant-water availability is often dependent on the snowpack melt during spring and summer. Snow droughts, i.e., anomalous reductions in the amount of water stored in the snowpack, represent an emerging hydroclimatic extreme for these ecosystems. However, the role of snow droughts as drivers of carbon cycle anomalies remains largely unknown. In this study, we quantify the adverse impacts of snow droughts as drivers of negative GPP extreme events across the Northern Hemisphere ( 30-60°N) for the period 1982-2016. We attribute -4.48 PgC [-2.48|-7.09 PgC] (mean [min|max] value across GPP datasets) of cumulative negative GPP anomalies to preceding snow droughts. They subsequently drive 23.5 % [21.2|25.7 %] of the negative GPP anomalies in the study region which corresponds to 9.6 % [8.9|10.8 %] of the global negative GPP anomalies. The attributable GPP anomalies show marked increases of + 46.5 % [+ 34.2|+ 66.2 %] in the recent study period (2000-2016 compared to 1982-1998). Particularly, more severe snow droughts act as drivers of negative GPP anomalies and lead to the most pronounced increases in anomalies between the two periods. Snow droughts act as isolated drivers for 15.9 % [9.0|20.8 %] |of the cumulative negative GPP anomalies. Additionally, 30.7 % [26.5|35.3 %] |of the negative GPP anomalies that were attributed to droughts occurring during a later stage of the growing season also reveal preceding snow droughts. Our study demonstrates the prominent role of snow droughts as drivers of negative GPP anomalies and subsequently impacting the carbon cycle, highlighting the importance of snow droughts in the context of advancing climate change.