Warming Climate, Greening Earth, Carbon Sink

Greening and Carbon Sink: The agreement of satellite data and model results suggests that the leaf area index (green leaf area per unit ground area, LAI) setback seen in the satellite data during 1992-1993 represents the response of the biosphere to the Pinatubo cooling event. Further, while simulated net primary production was reduced by 21.5 gC/m2 , simulated heterotrophic (soil microbial) respiration was reduced by 32.2 gC/m2. When variability in natural biomass burning (a small but significant contribution to the overall carbon balance of boreal ecosystems) is also accounted for, the modeled effect of the Pinatubo anomaly is to increase the boreal-zone carbon sink (excluding agricultural lands) by 0.17 PgC/yr compared to the 17-year average. The modeled occurrence of an enhanced sink in the years immediately after the Pinatubo event agrees qualitatively with numerical inversion results based on spatial patterns of measured atmospheric CO2 concentrations in the remote troposphere (figure below). The climate-driven model results and the inversion results agree that boreal-zone carbon exchange anomalies prior to 1992 were neutral or positive, whereas an anomalous sink developed in 1992-1993. A difference in the magnitude of net ecosystem exchange and its anomalies between the model and the inversion is possibly caused in the former by remaining uncertainty in the temperature dependence of respiration while the latter is not fully robust for the boreal region due to the sparsity of the CO2 observation network at high northern latitudes, potentially leading to influence from adjacent temperate regions. The unbalanced effect of cooling on NPP and microbial respiration provides a simpler explanation for additional high-latitude CO2 uptake than a proposed mechanism of increased NPP due to an increase in diffuse sky light. Our findings support the suggestion that the effects of cooling in the boreal zone contributed to the reduction in the growth rate of the global atmospheric CO2 concentration during 1992-1993.

We conclude that there has been a greening trend in the North, associated with a gradual lengthening of the growing season (which has also been shown by additional evidence such as tree phenology trends and reduced snow cover extent), caused by a slight warming of boreal climate during the last two decades. The trend cannot easily be explained away as an artefact of the methods used to calibrate long satellite time series. The trend was dramatically interrupted by the Pinatubo eruption, which reduced LAI while producing an increased carbon sink which contributed to the observed (and temporary) slowdown in the global rate of atmospheric CO2 con-centration growth. Understanding the causes of such variability is an important step in distinguishing natural from human-induced perturbations of the Earth s eco-systems.

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