Sensitivity of atmospheric CO2 growth rate to observed changes in terrestrial water storage

Abstract

Land ecosystems absorb on average 30 per cent of anthropogenic carbon dioxide (CO) emissions, thereby slowing the increase of CO concentration in the atmosphere. Year-to-year variations in the atmospheric CO growth rate are mostly due to fluctuating carbon uptake by land ecosystems. The sensitivity of these fluctuations to changes in tropical temperature has been well documented, but identifying the role of global water availability has proved to be elusive. So far, the only usable proxies for water availability have been time-lagged precipitation anomalies and drought indices, owing to a lack of direct observations. Here, we use recent observations of terrestrial water storage changes derived from satellite gravimetry to investigate terrestrial water effects on carbon cycle variability at global to regional scales. We show that the CO growth rate is strongly sensitive to observed changes in terrestrial water storage, drier years being associated with faster atmospheric CO growth. We demonstrate that this global relationship is independent of known temperature effects and is underestimated in current carbon cycle models. Our results indicate that interannual fluctuations in terrestrial water storage strongly affect the terrestrial carbon sink and highlight the importance of the interactions between the water and carbon cycles.

Sources of Uncertainty in Regional and Global Terrestrial CO2 Exchange Estimates

“…The sensitivity of soil moisture from DGVMs to ONI in the regions with significant effects (NSA, BRA, SAF, and SEAS, negative and CAS, MIDE, and SAS, positive) is generally consistent with that of TWS from the GRACE reconstruction (Humphrey et al, ), although slightly underestimated for NSA and BRA (Figure S4). In SAF and SEAS the sensitivity of SM from DGVMs to ONI is close to that of TWS, but in the former DGVMs show a large range.…”

“…Sensitivity of NBP to annual temperature (CRU‐JRA, top panel) and to soil moisture for inversions (GRACE reconstruction from (Humphrey et al, )) and DGVMs (with simulated soil moisture) (bottom panel). The bars indicate the range of sensitivities from each set of data, and the crosses show the ensemble mean.…”

“…Several studies have shown that variability in tropical CO fluxes (mainly driven by ENSO) is better explained by variations in soil water storage than by temperature alone (Bastos et al, ; Humphrey et al, ; Poulter et al, ). The regions with stronger negative coefficients for ONI (NSA, BRA, SAF, and SEAS, Figure ) are those where El Niño events impose strong warm and dry conditions, and those with positive coefficients (CAS, MIDE, and SAS) are associated with cool and wet conditions during El Niño (Bastos et al, ; Mason & Goddard, ).…”

Cropland Carbon Uptake Delayed and Reduced by 2019 Midwest Floods

“…Many studies have described the impacts of drought on the carbon cycle (Ciais et al, ; Humphrey et al, ; J. Liu et al, ; Sun et al, ; Wolf et al, ); however, the impacts of extreme wetness (i.e., floods) have been less well documented. Floods are among the major climate‐related disasters that are projected to increase in a warmer climate (Hirabayashi et al, ); quantifying their impact on the terrestrial carbon cycle is critical for the assessment of future climate change impacts.…”

Assessing the response of forest productivity to climate extremes in Switzerland using model–data fusion

“…Forests provide a wide range of ecosystem functions and services from global to local scale (Brockerhoff et al, ). It is therefore essential to understand how forest ecosystem productivity responds to climate extremes across environmental gradients (Ciais et al, ; Cramer et al, ; Reichstein et al, ) and how those responses feed back to the climate system (Humphrey et al, ). Climate change can affect forests on various levels, for example, by modifying the balance and interactions between direct abiotic constraints on tree growth (Cuny et al, ), shifting the timing of the growing season (Bigler & Bugmann, ), or altering disturbance regimes (Senf et al, ; Sommerfeld et al, ).…”