| 1. |
- Pacifico, F., et al.
(författare)
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Evaluation of a photosynthesis-based biogenic isoprene emission scheme in JULES and simulation of isoprene emissions under present-day climate conditions
- 2011
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7324. ; 11:9, s. 4371-4389
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Tidskriftsartikel (refereegranskat)abstract
- We have incorporated a semi-mechanistic isoprene emission module into the JULES land-surface scheme, as a first step towards a modelling tool that can be applied for studies of vegetation - atmospheric chemistry interactions, including chemistry-climate feedbacks. Here, we evaluate the coupled model against local above-canopy isoprene emission flux measurements from six flux tower sites as well as satellite-derived estimates of isoprene emission over tropical South America and east and south Asia. The model simulates diurnal variability well: correlation coefficients are significant (at the 95% level) for all flux tower sites. The model reproduces day-to-day variability with significant correlations (at the 95% confidence level) at four of the six flux tower sites. At the UMBS site, a complete set of seasonal observations is available for two years (2000 and 2002). The model reproduces the seasonal pattern of emission during 2002, but does less well in the year 2000. The model overestimates observed emissions at all sites, which is partially because it does not include isoprene loss through the canopy. Comparison with the satellite-derived isoprene-emission estimates suggests that the model simulates the main spatial patterns, seasonal and inter-annual variability over tropical regions. The model yields a global annual isoprene emission of 535 +/- 9 TgC yr(-1) during the 1990s, 78% of which from forested areas.
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| 2. |
- Piao, S. L., et al.
(författare)
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The carbon budget of terrestrial ecosystems in East Asia over the last two decades
- 2012
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4189. ; 9:9, s. 3571-3586
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Tidskriftsartikel (refereegranskat)abstract
- This REgional Carbon Cycle Assessment and Processes regional study provides a synthesis of the carbon balance of terrestrial ecosystems in East Asia, a region comprised of China, Japan, North and South Korea, and Mongolia. We estimate the current terrestrial carbon balance of East Asia and its driving mechanisms during 1990-2009 using three different approaches: inventories combined with satellite greenness measurements, terrestrial ecosystem carbon cycle models and atmospheric inversion models. The magnitudes of East Asia's terrestrial carbon sink from these three approaches are comparable: -0.293 +/- 0.033 PgC yr(-1) from inventory-remote sensing model-data fusion approach, -0.413 +/- 0.141 PgC yr(-1)(not considering biofuel emissions) or -0.224 +/- 0.141 PgC yr(-1) (considering biofuel emissions) for carbon cycle models, and -0.270 +/- 0.507 PgC yr(-1) for atmospheric inverse models. Here and in the following, the numbers behind +/- signs are standard deviations. The ensemble of ecosystem modeling based analyses further suggests that at the regional scale, climate change and rising atmospheric CO2 together resulted in a carbon sink of -0.289 +/- 0.135 PgC yr(-1), while land-use change and nitrogen deposition had a contribution of -0.013 +/- 0.029 PgC yr(-1) and -0.107 +/- 0.025 PgC yr(-1), respectively. Although the magnitude of climate change effects on the carbon balance varies among different models, all models agree that in response to climate change alone, southern China experienced an increase in carbon storage from 1990 to 2009, while northern East Asia including Mongolia and north China showed a decrease in carbon storage. Overall, our results suggest that about 13-27% of East Asia's CO2 emissions from fossil fuel burning have been offset by carbon accumulation in its terrestrial territory over the period from 1990 to 2009. The underlying mechanisms of carbon sink over East Asia still remain largely uncertain, given the diversity and intensity of land management processes, and the regional conjunction of many drivers such as nutrient deposition, climate, atmospheric pollution and CO2 changes, which cannot be considered as independent for their effects on carbon storage.
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| 3. |
- Sitch, S., et al.
(författare)
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Recent trends and drivers of regional sources and sinks of carbon dioxide
- 2015
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4189. ; 12:3, s. 653-679
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Tidskriftsartikel (refereegranskat)abstract
- The land and ocean absorb on average just over half of the anthropogenic emissions of carbon dioxide (CO2) every year. These CO2 "sinks" are modulated by climate change and variability. Here we use a suite of nine dynamic global vegetation models (DGVMs) and four ocean biogeochemical general circulation models (OBGCMs) to estimate trends driven by global and regional climate and atmospheric CO2 in land and oceanic CO2 exchanges with the atmosphere over the period 1990-2009, to attribute these trends to underlying processes in the models, and to quantify the uncertainty and level of inter-model agreement. The models were forced with reconstructed climate fields and observed global atmospheric CO2; land use and land cover changes are not included for the DGVMs. Over the period 1990-2009, the DGVMs simulate a mean global land carbon sink of -2.4 +/- 0.7 PgC yr(-1) with a small significant trend of -0.06 +/- 0.03 PgC yr(-2) (increasing sink). Over the more limited period 1990-2004, the ocean models simulate a mean ocean sink of -2.2 +/- 0.2 PgC yr(-1) with a trend in the net C uptake that is indistinguishable from zero (-0.01 +/- 0.02 PgC yr(-2)). The two ocean models that extended the simulations until 2009 suggest a slightly stronger, but still small, trend of 0.02 +/- 0.01 PgC yr(-2). Trends from land and ocean models compare favourably to the land greenness trends from remote sensing, atmospheric inversion results, and the residual land sink required to close the global carbon budget. Trends in the land sink are driven by increasing net primary production (NPP), whose statistically significant trend of 0.22 +/- 0.08 PgC yr(-2) exceeds a significant trend in heterotrophic respiration of 0.16 +/- 0.05 PgC yr(-2) - primarily as a consequence of widespread CO2 fertilisation of plant production. Most of the land-based trend in simulated net carbon uptake originates from natural ecosystems in the tropics (0.04 +/- 0.01 PgC yr(-2)), with almost no trend over the northern land region, where recent warming and reduced rainfall offsets the positive impact of elevated atmospheric CO2 and changes in growing season length on carbon storage. The small uptake trend in the ocean models emerges because climate variability and change, and in particular increasing sea surface temperatures, tend to counteract the trend in ocean uptake driven by the increase in atmospheric CO2. Large uncertainty remains in the magnitude and sign of modelled carbon trends in several regions, as well as regarding the influence of land use and land cover changes on regional trends.
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| 4. |
- Sitch, S., et al.
(författare)
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Trends and drivers of regional sources and sinks of carbon dioxide over the past two decades
- 2013
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Ingår i: Biogeosciences Discussions. - : Copernicus GmbH. - 1810-6277. ; 10, s. 20113-20177
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Abstract. The land and ocean absorb on average over half of the anthropogenic emissions of carbon dioxide (CO2) every year. These CO2 "sinks" are modulated by climate change and variability. Here we use a suite of nine Dynamic Global Vegetation Models (DGVMs) and four Ocean Biogeochemical General Circulation Models (OBGCMs) to quantify the global and regional climate and atmospheric CO2 – driven trends in land and oceanic CO2 exchanges with the atmosphere over the period 1990–2009, attribute these trends to underlying processes, and quantify the uncertainty and level of model agreement. The models were forced with reconstructed climate fields and observed global atmospheric CO2; Land Use and Land Cover Changes are not included for the DGVMs. Over the period 1990–2009, the DGVMs simulate a mean global land carbon sink of −2.4 ± 0.7 Pg C yr−1 with a small significant trend of −0.06 ± 0.03 Pg C yr−2 (increasing sink). Over the more limited period 1990–2004, the ocean models simulate a mean ocean sink of –2.2 ± 0.2 Pg C yr–1 with a trend in the net C uptake that is indistinguishable from zero (−0.01 ± 0.02 Pg C yr−2). The two ocean models that extended the simulations until 2009 suggest a slightly stronger, but still small trend of −0.02 ± 0.01 Pg C yr−2. Trends from land and ocean models compare favourably to the land greenness trends from remote sensing, atmospheric inversion results, and the residual land sink required to close the global carbon budget. Trends in the land sink are driven by increasing net primary production (NPP) whose statistically significant trend of 0.22 ± 0.08 Pg C yr−2 exceeds a significant trend in heterotrophic respiration of 0.16 ± 0.05 Pg C yr−2 – primarily as a consequence of wide-spread CO2 fertilisation of plant production. Most of the land-based trend in simulated net carbon uptake originates from natural ecosystems in the tropics (−0.04 ± 0.01 Pg C yr−2), with almost no trend over the northern land region, where recent warming and reduced rainfall offsets the positive impact of elevated atmospheric CO2 on carbon storage. The small uptake trend in the ocean models emerges because climate variability and change, and in particular increasing sea surface temperatures, tend to counteract the trend in ocean uptake driven by the increase in atmospheric CO2. Large uncertainty remains in the magnitude and sign of modelled carbon trends in several regions, and on the influence of land use and land cover changes on regional trends.
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| 5. |
- Arneth, A., et al.
(författare)
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Historical carbon dioxide emissions caused by land-use changes are possibly larger than assumed
- 2017
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Ingår i: Nature Geoscience. - : Springer Science and Business Media LLC. - 1752-0894 .- 1752-0908. ; 10:2, s. 79-84
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Forskningsöversikt (refereegranskat)abstract
- The terrestrial biosphere absorbs about 20% of fossil-fuel CO 2 emissions. The overall magnitude of this sink is constrained by the difference between emissions, the rate of increase in atmospheric CO 2 concentrations, and the ocean sink. However, the land sink is actually composed of two largely counteracting fluxes that are poorly quantified: fluxes from land-use change and CO 2 uptake by terrestrial ecosystems. Dynamic global vegetation model simulations suggest that CO 2 emissions from land-use change have been substantially underestimated because processes such as tree harvesting and land clearing from shifting cultivation have not been considered. As the overall terrestrial sink is constrained, a larger net flux as a result of land-use change implies that terrestrial uptake of CO 2 is also larger, and that terrestrial ecosystems might have greater potential to sequester carbon in the future. Consequently, reforestation projects and efforts to avoid further deforestation could represent important mitigation pathways, with co-benefits for biodiversity. It is unclear whether a larger land carbon sink can be reconciled with our current understanding of terrestrial carbon cycling. Our possible underestimation of the historical residual terrestrial carbon sink adds further uncertainty to our capacity to predict the future of terrestrial carbon uptake and losses.
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| 6. |
- Ito, A., et al.
(författare)
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Cold-Season Methane Fluxes Simulated by GCP-CH4 Models
- 2023
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 50:14
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Tidskriftsartikel (refereegranskat)abstract
- Cold-season methane (CH4) emissions may be poorly constrained in wetland models. We examined cold-season CH4 emissions simulated by 16 models participating in the Global Carbon Project model intercomparison and analyzed temporal and spatial patterns in simulation results using prescribed inundation data for 2000–2020. Estimated annual CH4 emissions from northern (>60°N) wetlands averaged 10.0 ± 5.5 Tg CH4 yr−1. While summer CH4 emissions were well simulated compared to in-situ flux measurement observations, the models underestimated CH4 during September to May relative to annual total (27 ± 9%, compared to 45% in observations) and substantially in the months with subzero air temperatures (5 ± 5%, compared to 27% in observations). Because of winter warming, nevertheless, the contribution of cold-season emissions was simulated to increase at 0.4 ± 0.8% decade−1. Different parameterizations of processes, for example, freezing–thawing and snow insulation, caused conspicuous variability among models, implying the necessity of model refinement.
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| 7. |
- Cox, A. J. F., et al.
(författare)
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Acclimation of photosynthetic capacity and foliar respiration in Andean tree species to temperature change
- 2023
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Ingår i: New Phytologist. - : Wiley. - 0028-646X .- 1469-8137. ; 238:6, s. 2329-2344
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Tidskriftsartikel (refereegranskat)abstract
- Climate warming is causing compositional changes in Andean tropical montane forests (TMFs). These shifts are hypothesised to result from differential responses to warming of cold- and warm-affiliated species, with the former experiencing mortality and the latter migrating upslope. The thermal acclimation potential of Andean TMFs remains unknown. Along a 2000m Andean altitudinal gradient, we planted individuals of cold- and warm-affiliated species (under common soil and irrigation), exposing them to the hot and cold extremes of their thermal niches, respectively. We measured the response of net photosynthesis (A(net)), photosynthetic capacity and leaf dark respiration (R-dark) to warming/cooling, 5 months after planting. In all species, A(net) and photosynthetic capacity at 25 degrees C were highest when growing at growth temperatures (T-g) closest to their thermal means, declining with warming and cooling in cold-affiliated and warm-affiliated species, respectively. When expressed at T-g, photosynthetic capacity and Rdark remained unchanged in cold-affiliated species, but the latter decreased in warm-affiliated counterparts. R-dark at 25 degrees C increased with temperature in all species, but remained unchanged when expressed at T-g. Both species groups acclimated to temperature, but only warm-affiliated species decreased R-dark to photosynthetic capacity ratio at T-g as temperature increased. This could confer them a competitive advantage under future warming.
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| 8. |
- Elsner, J., et al.
(författare)
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Deep acceptors trapped at threading-edge dislocations in GaN
- 1998
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 58:19, s. 12571-12574
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Tidskriftsartikel (refereegranskat)abstract
- Local-density-functional methods are used to examine the behavior of the oxygen defect, gallium vacancy, and related defect complexes trapped at threading-edge dislocations in GaN. These defects are found to be particularly stable at the core of the dislocation where oxygen sits twofold coordinated in a bridge position. VGa-ON is found to be a deep double acceptor, VGa-(ON)2 is a deep single acceptor, and VGa-(ON)3 at the dislocation core is electrically inactive. We suggest that the first two defects are responsible for a deep acceptor level associated with the midgap yellow luminescence band.
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| 9. |
- Elsner, J., et al.
(författare)
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Structural and electrical properties of threading dislocations in GaN
- 1997
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Ingår i: Defects in semiconductors. - : Trans Tech Publications Inc.. ; , s. 1203-1210
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Konferensbidrag (refereegranskat)abstract
- The atomic structures and electrical properties for threading screw and threading edge dislocations of wurtzite GaN are calculated within the local-density approximation. Both dislocations are electrically inactive with a band gap free from deep levels. These results are understood to arise from relaxed core structures which are similar to (1010) surfaces. The threading screw dislocations are found to be stable with an open-core whereas the threading edge dislocations possess filled cores.
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| 10. |
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