Compensatory water effects link yearly global land CO2 sink changes to temperature

Abstract

Large interannual variations in the measured growth rate of atmospheric carbon dioxide (CO 2 ) originate primarily from fluctuations in carbon uptake by land ecosystems 1-3 . It remains uncertain, however, to what extent temperature and water availability control the carbon balance of land ecosystems across spatial and temporal scales 3-14 . Here we use empirical models based on eddy covariance data 15 and process-based models 16,17 to investigate the effect of changes in temperature and water availability on gross primary productivity (GPP), terrestrial ecosystem respiration (TER) and net ecosystem exchange (NEE) at local and global scales. We find that water availability is the dominant driver of the local interannual variability in GPP and TER. To a lesser extent this is true also for NEE at the local scale, but when integrated globally, temporal NEE variability is mostly driven by temperature fluctuations. We suggest that this apparent paradox can be explained by two compensatory water effects. Temporal waterdriven GPP and TER variations compensate locally, dampening water-driven NEE variability. Spatial water availability anomalies also compensate, leaving a dominant temperature signal in the yearto-year fluctuations of the land carbon sink. These findings help to reconcile seemingly contradictory reports regarding the importance of temperature and water in controlling the interannual variability of the terrestrial carbon balance 3-6,9,11,12,14 . Our study indicates that spatial climate covariation drives the global carbon cycle response.Large interannual variations in recently measured atmospheric CO 2 growth rates originate primarily from fluctuations in carbon uptake by land ecosystems, rather than from the oceans or variations in anthropogenic emissions [1][2][3] . There is a general consensus that the tropical regions contribute the most to terrestrial carbon variability 1,8,18,19 . The observed positive correlation between mean tropical land temperature and CO 2 growth rate 3,5,6,12,13 implies smaller land carbon uptake and enhanced atmospheric CO 2 growth during warmer years, with a sensitivity of about 5 gigatonnes of carbon per year per K. There is a tight relationship between this sensitivity on interannual timescales and long-term changes in terrestrial carbon per degree of warming across multiple climate carbon-cycle models 6 . Despite this strong emergent relationship with mean tropical land temperature, several studies suggest that variations in water availability have an important 8,10,11,14 , even a dominant role 4,9 , in shaping the interannual variability (IAV) of the carbon balance of extensive semi-arid and sub-tropical systems. Furthermore, the recent doubling of the tropical carbon cycle sensitivity to interannual temperature variability has been linked to interactions with changing moisture regimes 13 . A full understanding of the processes governing the climatic controls of terrestrial carbon cycling on interannual timescales and across spatial scales is therefore still lacking. H...

Vegetation greenness and land carbon-flux anomalies associated with climate variations: a focus on the year 2015

“…The need thus remains to reconcile the findings of temperature dominance on a large spatial scale and precipitation/moisture dominance on a fine scale. Recently, Jung et al (2017) suggested that the dominant role of soil moisture over land carbon-flux anomalies shifts to temperature when the scale of spatial aggregation increases, due to the compensatory water effects in the process of spatial upscaling. We also find that for all seasons except Q3, inversion-based land carbon-uptake anomalies in the tropics and southern extratropics are positively correlated with soil water content (data not shown), with 2015 having an extremely low soil water content anomaly in Q4, echoing the extreme high-temperature anomaly shown in Fig.…”

Understanding terrestrial water storage variations in northern latitudes across scales

“…Thus, we assessed the spatial coherence of simulated patterns of SWE and W by calculating the proportion of total positive and total negative covariances among grid cells (Eqs. 4 and 5 in Jung et al, 2017). If the sum of positive covariances outweighs the sum of negative covariances, it implies some degree of spatial coherence of the anomalies.…”

“…soil moisture anomalies, have much larger spatial variability than others. Due to this high smallscale heterogeneity, the effect on larger regional scale might be smaller than expected, as different local scale heterogeneities compensate for each other when the regional averages are calculated (Jung et al, 2017). Thus, we assessed the spatial coherence of simulated patterns of SWE and W by calculating the proportion of total positive and total negative covariances among grid cells (Eqs.…”

Reliability ensemble averaging of 21st century projections of terrestrial net primary productivity reduces global and regional uncertainties

“…We perform the REA procedure three times using different observationally constrained estimates of current NPP: retrievals of the terrestrial carbon cycle with the CARbon DAta Model fraMework (CARDAMOM; model-data fusion approach (Bloom and Williams, 2015a;, an approximation of NPP based on the up-scaled FLUXCOM gross primary productivity (GPP) datasets Jung et al, 2009Jung et al, , 2011Jung et al, , 2017Tramontana et al, 2016) and the MOD17A3 MODIS NPP product (Running et al, 2004;Zhao et al, 2005;Zhao and Running, 2010). Based on optimally weighted model averages, we evaluate the impact of the REA method on 21st century projections of NPP but also on the uncertainty in the future resilience of the CO 2 fertilization that exists among the models.…”