Reconciling different approaches to quantifying land surface temperature impacts of afforestation using satellite observations

Satellite observations have been widely used to examine afforestation effects on local surface temperature at large spatial scales. Different approaches, which potentially lead to differing definitions of the afforestation effect, have been used in previous studies. Despite their large differences, the results of these studies have been used in climate model validation and cited in climate synthesis reports. Such differences have been simply treated as observational uncertainty, which can be an order of magnitude bigger than the signal itself. Although the fraction of the satellite pixel actually afforested has been noted to influence the magnitude of the afforestation effect, it remains unknown whether it is a key factor which can reconcile the different approaches. Here, we provide a synthesis of three influential approaches (one estimates the actual effect and the other two the potential effect) and use large-scale afforestation over China as a test case to examine whether the different approaches can be reconciled. We found that the actual effect (ΔTa) often relates to incomplete afforestation over a medium-resolution satellite pixel (1 km). ΔTa increased with the afforestation fraction, which explained 89 % of its variation. One potential effect approach quantifies the impact of quasi-full afforestation (ΔTp1), whereas the other quantifies the potential impact of full afforestation (ΔTp2) by assuming a shift from 100 % openland to 100 % forest coverage. An initial paired-sample t test shows that ΔTa&lt;ΔTp1&lt;ΔTp2 for the cooling effect of afforestation ranging from 0.07 to 1.16 K. But when all three methods are normalized for full afforestation, the observed range in surface cooling becomes much smaller (0.79 to 1.16 K). Potential cooling effects have a value in academic studies where they can be used to establish an envelope of effects, but their realization at large scales is challenging given its nature of scale dependency. The reconciliation of the different approaches demonstrated in this study highlights the fact that the afforestation fraction should be accounted for in order to bridge different estimates of surface cooling effects in policy evaluation.</p

A compact photonic horizontal spot-size converter realized in silicon-on-insulator

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Spoken Code-Switching in Written Form? Manifestation of Code-Switching in Computer Mediated Communication

Contains fulltext : 159840.pdf (publisher's version ) (Open Access)22 p

Bio-energy retains its mitigation potential under elevated CO2

Background If biofuels are to be a viable substitute for fossil fuels, it is essential that they retain their potential to mitigate climate change under future atmospheric conditions. Elevated atmospheric CO2 concentration [CO2] stimulates plant biomass production; however, the beneficial effects of increased production may be offset by higher energy costs in crop management. Methodology/Main findings We maintained full size poplar short rotation coppice (SRC) systems under both current ambient and future elevated [CO2] (550 ppm) and estimated their net energy and greenhouse gas balance. We show that a poplar SRC system is energy efficient and produces more energy than required for coppice management. Even more, elevated [CO2] will increase the net energy production and greenhouse gas balance of a SRC system with 18%. Managing the trees in shorter rotation cycles (i.e. 2 year cycles instead of 3 year cycles) will further enhance the benefits from elevated [CO2] on both the net energy and greenhouse gas balance. Conclusions/significance Adapting coppice management to the future atmospheric [CO2] is necessary to fully benefit from the climate mitigation potential of bio-energy systems. Further, a future increase in potential biomass production due to elevated [CO2] outweighs the increased production costs resulting in a northward extension of the area where SRC is greenhouse gas neutral. Currently, the main part of the European terrestrial carbon sink is found in forest biomass and attributed to harvesting less than the annual growth in wood. Because SRC is intensively managed, with a higher turnover in wood production than conventional forest, northward expansion of SRC is likely to erode the European terrestrial carbon sink

Drought effects on leaf fall, leaf flushing and stem growth in the Amazon forest: reconciling remote sensing data and field observations

Large amounts of carbon flow through tropical ecosystems every year, from which a part is sequestered in biomass through tree growth. However, the effects of ongoing warming and drying on tree growth and carbon sequestration in tropical forest is still highly uncertain. Field observations are sparse and limited to a few sites, while remote sensing analysis shows diverging growth responses to past droughts that cannot be interpreted with confidence. To reconcile data from field observations and remote sensing, we collated in situ measurements of stem growth and leaf litterfall from inventory plots across the Amazon region and other neotropical ecosystems. These data were used to train two machine-learning models and to evaluate model performance on reproducing stem growth and litterfall rates. The models utilized multiple climatological variables and other geospatial datasets (terrain, soil and vegetation properties) as explanatory variables. The output consisted of monthly estimates of leaf litterfall (R2= 0.71, NRMSE = 9.4 %) and stem growth (R2= 0.54, NRMSE = 10.6 %) across the neotropics from 1982 to 2019 at a high spatial resolution (0.1∘). Modelled time series allow us to assess the impacts of the 2005 and 2015 droughts in the Amazon basin on regional scales. The more severe 2015 drought was estimated to have caused widespread declines in stem growth (−1.8σ), coinciding with enhanced leaf fall (+1.4σ), which were only locally apparent in 2005. Regions in the Amazon basin that flushed leaves at the onset of both droughts (+0.9σ∼+2.0σ) showed positive anomalies in remotely sensed enhanced vegetation index, while sun-induced fluorescence and vegetation optical depth were reduced. The previously observed counterintuitive response of canopy green-up during drought in the Amazon basin detected by many remote sensing analyses can therefore be a result of enhanced leaf flushing at the onset of a drought. The long-term estimates of leaf litterfall and stem growth point to a decline in stem growth and a simultaneous increase in leaf litterfall in the Amazon basin since 1982. These trends are associated with increased warming and drying of the Amazonian climate and could point to a further decline in the Amazon carbon sink strength

Drought effects on leaf fall, leaf flushing and stem growth in Neotropical forest; reconciling remote sensing data and field observations

Large amounts of carbon flow through tropical ecosystems every year, from which a part is sequestered in biomass through tree growth. However, the effects of ongoing warming and drying on tree growth and carbon sequestration in tropical forest is still highly uncertain. Field observations are sparse and limited to a few sites while remote sensing analysis shows diverging growth responses to past droughts that cannot be interpreted with confidence. To reconcile data from field observations and remote sensing, we collated in situ measurements of stem growth and leaf litterfall from inventory plots across the Neotropics. This data was used to train two machine learning models and to evaluate model performance on reproducing stem growth and litterfall rates. The models utilized multiple climatological variables and other geospatial datasets as explanatory variables. The output consisted of monthly estimates of leaf litterfall (R2 = 0.67, NRMSE = 9.5 %) and stem growth (R2 = 0.51, NRMSE = 11.2 %) across the neotropics from 1982 to 2019 at a high spatial resolution (0.1°). Modelled time series allowed to assess the impacts of the 2005 and 2015 droughts in the Amazon basin on regional scales. Both droughts were estimated to have caused widespread declines in stem growth (−0.6σ ~ −1.8σ), coinciding with enhanced leaf fall (+0.7σ ~ +0.9σ). Regions in the Amazon basin that flushed leaves at the onset of both droughts (+1.1σ ~ +1.9σ), showed positive anomalies in remotely sensed enhanced vegetation index, while sun-induced fluorescence and vegetation optical depth were reduced. The previously observed counterintuitive response of canopy green-up during drought in the Amazon basin detected by many remote sensing analyses can therefore be explained by enhanced leaf flushing at the onset of a drought. The long-term estimates of leaf litterfall and stem growth point to a decline of stem growth and a simultaneous but weaker increase in leaf litterfall in the Amazon basin since 1982 that is not observed in long-term inventory plots. These trends are associated with increased warming and drying of the Amazonian climate

Silicon-on-insulator platform for integrated wavelength-selective components (invited paper)

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