Extreme events in gross primary production: a characterization across continents

Climate extremes can affect the functioning of ter- restrial ecosystems, for instance via a reduction of the photo- synthetic capacity or alterations of respiratory processes. Yet the dominant regional and seasonal effects of hydrometeoro- logical extremes are still not well documented and in the fo- cus of this paper. Specifically, we quantify and characterize the role of large spatiotemporal extreme events in gross pri- mary production (GPP) as triggers of continental anomalies. We also investigate seasonal dynamics of extreme impacts on continental GPP anomalies. We find that the 50 largest pos- itive extremes (i.e., statistically unusual increases in carbon uptake rates) and negative extremes (i.e., statistically unusual decreases in carbon uptake rates) on each continent can ex- plain most of the continental variation in GPP, which is in line with previous results obtained at the global scale. We show that negative extremes are larger than positive ones and demonstrate that this asymmetry is particularly strong in South America and Europe. Our analysis indicates that the overall impacts and the spatial extents of GPP extremes are power-law distributed with exponents that vary little across continents. Moreover, we show that on all continents and for all data sets the spatial extents play a more important role for the overall impact of GPP extremes compared to the dura- tions or maximal GPP. An analysis of possible causes across continents indicates that most negative extremes in GPP can be attributed clearly to water scarcity, whereas extreme tem- peratures play a secondary role. However, for Europe, South America and Oceania we also identify fire as an important driver. Our findings are consistent with remote sensing prod- ucts. An independent validation against a literature survey on specific extreme events supports our results to a large extent.