Lagged effects regulate the inter-annual variability of the tropical carbon balance

Inter-annual variations in the tropical land carbon (C) balance are a dominant component of the global atmospheric CO2 growthrate. Currently, the lack of quantitative knowledge on processes controlling net tropical ecosystems C balance on inter-annual20 timescales inhibits accurate understanding and projections of land-atmosphere C exchanges. In particular, uncertainty on therelative contribution of ecosystem C fluxes attributable to concurrent forcing anomalies (concurrent effects) and thoseattributable to the continuing influence of past phenomena (lagged effects) stifles efforts to explicitly understand the integratedsensitivity of tropical ecosystem to climatic variability. Here we present a conceptual framework—applicable in principle toany land biosphere model—to explicitly quantify net biospheric exchange (NBE) as the sum of anomaly-induced concurrentchanges and climatology-induced lagged changes to terrestrial ecosystem C states (NBE = NBECON + NBELAG 25 ). We apply thisframework to an observation-constrained analysis of the 2001-2015 tropical C balance: we use a data-model integrationapproach (CARDAMOM) to merge satellite-retrieved land-surface C observations (leaf area, biomass, solar-inducedfluorescence), soil C inventory data and satellite-based atmospheric inversion estimates of CO2 and CO fluxes to produce adata-constrained analysis of the 2001-2015 tropical C cycle. We find that the inter-annual variability of both concurrent and30 lagged effects substantially contribute to the 2001-2015 NBE inter-annual variability throughout 2001-2015 across the tropics(NBECON IAV = 80% of total NBE IAV, r = 0.76; NBELAG IAV = 64% of NBE IAV, r = 0.61) and the prominence of NBELAGIAV persists across both wet and dry tropical ecosystems. The magnitude of lagged effect variations on NBE across the tropicsis largely attributable to lagged effects on net primary productivity (NPP; NPPLAG IAV 113% of NBELAG IAV, r = -0.93, pvalue<0.05), which emerge due to the dependence of NPP on inter-annual variations in foliar C and plant-available H2O states. 2We conclude that concurrent and lagged effects need to be explicitly and jointly resolved to retrieve an accurate understandingthe processes regulating the present-day and future trajectory of the terrestrial land C sink.