The impact of climate extremes on northern and southern hemisphere seasonal ecosystem productivity over four decades to 2018

Abstract. Climate extremes affect ecosystem performance across the globe, including net seasonal productivity (NSP), the difference between land surface carbon uptake in summer and release in winter. In this study we correlated estimates of NSP, obtained from atmospheric carbon (C) observations, with curated climate extreme indices that span from 1977 to 2018, split between the Northern (NH) and Southern hemispheres (SH). In the NH, wet climate extremes correlated with an increase in summer uptake of C, and hot climate extremes were correlated with reduced winter release of C. Together, hotter and wetter climate extremes in the NH correlated with increased NSP, a finding that supports a beneficial effect of warming climates in high latitude seasonal NH forests, where cold temperatures typically limit ecosystem productivity. In the SH, all 10 hot climate extremes examined correlated negatively with summer C uptake, and NSP in the SH was negatively correlated with hot extremes but positively correlated with wet and cold extremes. The results indicate that seasonality in the SH, which is predominately located in warm tropical and subtropical forests, seem to be more deleteriously impact by heat than the NH, presumably due to the ambient temperature of SH forests being already close to biological heat thresholds such as photosynthetic heat stability. Increases in the Southern Oscillation Index (SOI) in the NH was associated with wetter and hotter climates that drove down winter C loss and led to increases in NSP. In the SH an increase in the SOI was associated with wetter climates that correlated with increased summer C uptake and NSP. The findings highlight the contrast between NH and SH seasonal C flux responses to climate extremes, and the importance of climate drivers of temperature and rainfall extremes in determining global ecosystem performance.