Published eddy covariance measurements of carbon dioxide (CO2) exchange between vegetation and the atmosphere from a global network are distilled, synthesised and reviewed according to time scale, climate and plant functional types, disturbance and land use. Other topics discussed include history of the network, errors and issues associated with the eddy covariance method, and a synopsis of how these data are being used by ecosystem and climate modellers and the remote-sensing community. Spatial and temporal differences in net annual exchange, FN, result from imbalances in canopy photosynthesis (FA) and ecosystem respiration (FR), which scale closely with one another on annual time scales. Key findings reported include the following: (1) ecosystems with the greatest net carbon uptake have the longest growing season, not the greatest FA; (2) ecosystems losing carbon were recently disturbed; (3) many old-growth forests act as carbon sinks; and (4) year-to-year decreases in FN are attributed to a suite of stresses that decrease FA and FR in tandem. Short-term flux measurements revealed emergent-scale processes including (1) the enhancement of light use efficiency by diffuse light, (2) dynamic pulses in FR following rain and (3) the acclimation FA and FR to temperature. They also quantify how FA and FR respond to droughts and heat spells.

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‘Breathing’ of the terrestrial biosphere: lessons learned from a global network of carbon dioxide flux measurement systems

Global review and synthesis of trends in observed terrestrial near-surface wind speeds; implications for evaporation

Surface winds have declined in China, the Netherlands, the Czech Republic, the United States and Australia over the past few decades. Mesoscale model simulations suggest that an increase in surface roughness is contributing to the stilling trend in the mid-latitude Northern Hemisphere.

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Northern Hemisphere atmospheric stilling partly attributed to an increase in surface roughness

We synthesize estimates of the contemporary net air-sea CO2 flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models (Mikaloff Fletcher et al., 2006, 2007) and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO2 (pCO2) (Takahashi et al., 2008). These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO2 for the decade of the 1990s and the early 2000s with differences at the regional level of generally less than 0.1 Pg C a−1. This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in thehigh latitudes. Both estimates point toward a small (∼ −0.3 Pg C a−1) contemporary CO2 sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO2 and a strong uptake of anthropogenic CO2. A notable exception in the generally good agreement between the two estimates exists within the Southern Ocean: the ocean inversion suggests a relatively uniform uptake, while the pCO2-based estimate suggests strong uptake in the region between 58°S and 44°S, and a source in the region south of 58°S. Globally and for a nominal period between 1995 and 2000, the contemporary net air-sea flux of CO2 is estimated to be −1.7 ± 0.4 Pg C a−1 (inversion) and −1.4 ± 0.7 Pg C a−1 (pCO2-climatology), respectively, consisting of an outgassing flux of river-derived carbon of ∼+0.5 Pg C a−1, and an uptake flux of anthropogenic carbon of −2.2 ± 0.3 Pg C a−1 (inversion) and −1.9 ± 0.7 Pg C a−1 (pCO2-climatology). The two flux estimates also imply a consistent description of the contemporary meridional transport of carbon with southward ocean transport throughout most of the Atlantic basin, and strong equatorward convergence in the Indo-Pacific basins. Both transport estimates suggest a small hemispheric asymmetry with a southward transport of between −0.2 and −0.3 Pg C a−1 across the equator. While the convergence of these two independent estimates is encouraging and suggests that it is now possible to provide relatively tight constraints for the net air-sea CO2 fluxes at the regional basis, both studies are limited by their lack of consideration of long-term changes in the ocean carbon cycle, such as the recent possible stalling in the expected growth of the Southern Ocean carbon sink.

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Oceanic sources, sinks, and transport of atmospheric CO2

Patterns and drivers of post-socialist farmland abandonment in Western Ukraine

The end of the Soviet Union and the collapse of its agricultural structures in the early 1990s has induced the abandonment of a large croplands area, which have been recovered by herbaceous plants. This widespread unintended and abrupt land use change took place over 200,000 km(2), a large enough scale to impact the continental and global carbon budgets. The goal of this study is to estimate the net biome productivity (NBP) of the abandoned croplands and to assess the soil C storage dynamics due to recent land conversion. The soil C balance and its input (net primary productivity) and output (heterotrophic respiration) fluxes is simulated in a spatially explicit manner with the process-driven natural vegetation/crop model Organizing Carbon and Hydrology in Dynamic Ecosystems-Supra-Thermal Ion Composion Spectrometer prescribed with successive area changes of abandoned croplands during the 1990s. We estimate that regional agricultural abandonment is responsible of a cumulated carbon sink over 1991 2000 of 373 gC m(-2), or 64 TgC over the domain considered, which defines a mean annual C sink of 46.7 g C m(-2) a(-1). Agricultural practices during the former cultivation phase determine a legacy on the C sink following abandonment, which impacts by +37% to -25% according to the practice considered (no tillage, no fertilization, and export of some crop residues). We conclude that futures studies of this regional change in the C cycle should better consider management information in order to refine the NBP estimate.

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Carbon sequestration due to the abandonment of agriculture in the former USSR since 1990

http://forest.akadem.ru/Articles/02/tchebakova_2.pdf

Luca Belelli

Papers

Carbon sequestration due to the abandonment of agriculture in the former USSR since 1990

Carbon and water exchanges of regenerating forests in central Siberia