Terrestrial carbon-water relations driven by internal climate variability

Terrestrial ecosystems currently sequester ~25% of anthropogenic carbon (C) emissions and regulate atmospheric carbon dioxide concentrations [CO2] at timescales from seasons to centuries. Previous work has shown that interannual variability in land C uptake (NBP or net biosphere production) is controlled by terrestrial water storage (TWS) and has argued that the NBP – TWS sensitivity is underestimated in Earth system models (ESMs), calling into question ESM-utility for climate change projections. Further, observational analyses have argued that a heightened NBP – TWS sensitivity in recent decades has resulted from climate change, in contrast to ESMs which show no change or a decreasing NBP – TWS sensitivity. However, internal climate variability (ICV) can obscure forced climate trends derived from observational time series. We use four ESM large ensembles to show that observed increase in the NBP – TWS sensitivity falls within the 95% confidence interval of slope changes for all ESMs examined. Further, we show that the observed change in NBP – TWS sensitivity over the last 60 years cannot be confidently distinguished from ICV. The variance across ensemble members when looking at the global NBP – TWS relationship is greatly reduced by leveraging spatially explicit maps that are available in ESM output and increasingly accessible observationally with new remote sensing technologies.