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The terrestrial biosphere is currently a strong carbon (C) sink but may switch to a source in the 21st century as climate-driven losses exceed CO 2 -driven C gains, thereby accelerating global warming. Although it has long been recognized that tropical climate plays a critical role in regulating interannual climate variability, the causal link between changes in temperature and precipitation and terrestrial processes remains uncertain. Here, we combine atmospheric mass balance, remote sensing-modeled datasets of vegetation C uptake, and climate datasets to characterize the temporal variability of the terrestrial C sink and determine the dominant climate drivers of this variability. We show that the interannual variability of global land C sink has grown by 50-100% over the past 50 y. We further find that interannual land C sink variability is most strongly linked to tropical nighttime warming, likely through respiration. This apparent sensitivity of respiration to nighttime temperatures, which are projected to increase faster than global average temperatures, suggests that C stored in tropical forests may be vulnerable to future warming.climate change | climate feedback | asymmetrical warming | carbon budget | inversion model T errestrial ecosystems have been a substantial net sink of anthropogenic carbon (C) emissions since the 1960s (1-4), but the terrestrial C sink could switch to a C source in the 21st century, resulting in a positive C cycle-climate feedback that would accelerate global surface warming with potentially major consequences for the biosphere (5-7). The interannual variability of the terrestrial C sink can help constrain our understanding of C/climate feedbacks and identify regions and mechanisms of the terrestrial C cycle that are most sensitive to climate parameters, shedding light on the future of the sink and its possible transition to a source (8). Currently, several major drivers have been shown to be correlated with the interannual variability of the terrestrial C sink, including (i) tropical temperature, which is tightly coupled to interannual variability in the atmospheric growth rate (AGR) of CO 2 (8, 9); (ii) tropical drought stress, including major droughts in the Amazon (10-12), which has been suggested to underlie increasing sensitivity of the AGR to tropical temperature over the period from 1959-2010 (13); (iii) temperature and precipitation variability in semiarid regions (14, 15); and (iv) average minimum daily (hereafter "nighttime") temperatures, which studies of several local field sites in the tropics have found play a major role in interannual productivity (16)(17)(18).Determining the mechanism underlying the interannual variability of the terrestrial C sink, including the relative roles of precipitation vs. temperature stress and their effects on gross primary productivity (GPP) vs. total respiration (both autotrophic and heterotrophic; R), is critical to predict the sink's future and to improve Earth system models. Here, we quantify changes in the interannual variability...

The Southern Ocean is critically important to the oceanic uptake of anthropogenic CO 2 . Up to half of the excess CO 2 currently in the ocean entered through the Southern Ocean. That uptake helps to maintain the global carbon balance and buffers transient climate change from fossil fuel emissions. However, the future evolution of the uptake is uncertain, because our understanding of the dynamics that govern the Southern Ocean CO 2 uptake is incomplete. Sparse observations and incomplete model formulations limit our ability to constrain the monthly and annual uptake, interannual variability and long-term trends. Float-based sampling of ocean biogeochemistry provides an opportunity for transforming our understanding of the Southern Ocean CO 2 flux. In this work, we review current estimates of the CO 2 uptake in the Southern Ocean and projections of its response to climate change. We then show, via an observational system simulation experiment, that float-based sampling provides a significant opportunity for measuring the mean fluxes and monitoring the mean uptake over decadal scales.

Concerted community efforts have been devoted to producing an authoritative climatology of air-sea CO 2 fluxes, but identifying decadal trends in CO 2 fluxes has proven to be more challenging. The available surface pCO 2 estimates are too sparse to separate long-term trends from decadal and seasonal variability using simple linear models. We introduce Markov Chain Monte Carlo sampling as a novel technique for estimating the historical pCO 2 at the ocean surface. The result is a plausible history of surface pCO 2 based on available measurements and variability inferred from model simulations. Applying the method to a modern database of pCO 2 data, we find that two thirds of the ocean surface is trending toward increasing uptake of CO 2 , with a mean (year 2000) uptake of 2.3 ± 0.5 PgC yr −1 of anthropogenic carbon and an increase in the global annual uptake over the 30 year time period of 0.4 ± 0.1 PgC yr −1 decade −1 . The results are particularly interesting in the Southern Ocean, where we find increasing uptake of carbon over this time period, in contrast to previous studies. We find evidence for increased ventilation of deep ocean carbon, in response to increased winds, which is more than offset by an associated surface cooling.

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