| 45031. |
- Sun, Weiyi, et al.
(författare)
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Multi-centennial ENSO-like variability response to solar activity during the holocene
- 2024
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Ingår i: CLIMATE DYNAMICS. - 0930-7575 .- 1432-0894.
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Tidskriftsartikel (refereegranskat)abstract
- Previous reconstructions suggest a possible correlation between solar and El Nino-Southern Oscillation (ENSO) on the centennial-millennial time scales, but the mechanism remains controversial. This study conducts Holocene transient simulations and finds a significant 350-500-year cycle of ENSO-like variability during the mid-late Holocene under solar activity forcing, supported by multiple reconstructions. This multi-centennial ENSO-like variability is caused by the solar-forced low-latitude process, not the high-latitude process or internal variability. When solar radiation increases, the subtropical Asian continent-Indian Ocean thermal contrast is enhanced, increasing precipitation over the India and Bay of Bengal, which generates easterly anomaly over the Indo-Pacific warm pool. Increased solar radiation also enhances the evaporation in the cloud-free regions of western North and South Pacific, and the moisture is transported to the Maritime Continent by easterly. This increases precipitation there and strengthens the easterly anomaly over the equatorial western Pacific, which triggers the La Nina-like state. The leading coupling process for the development of La Nina-like state is zonal advective and upwelling feedback, and poleward heat advection further amplifies the equatorial eastern Pacific cooling. Findings from this study suggests that a centennial-scale El Nino-like condition might occur under solar forcings if the forecast predicts a solar minimum at the end of the 21st century.
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| 45032. |
- Sun, Weiyi, et al.
(författare)
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Northern Hemisphere Land Monsoon Precipitation Increased by the Green Sahara During Middle Holocene
- 2019
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:16, s. 9870-9879
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Tidskriftsartikel (refereegranskat)abstract
- Changes in land cover and dust emission may significantly influence the Northern Hemisphere land monsoon precipitation (NHLMP), but observations are too short to fully evaluate their impacts. The Green Sahara during the mid-Holocene (6,000 years BP) provides an opportunity to unravel these mechanisms. Here we show that during the mid-Holocene, most of the NHLMP changes revealed by proxy data are reproduced by the Earth System model results when the Saharan vegetation cover and dust reduction are taken into consideration. The simulated NHLMP significantly increases by 33.10% under the effect of the Green Sahara. The North African monsoon precipitation increases most significantly. Additionally, the Saharan vegetation (dust reduction under vegetated Sahara) alone remotely intensifies the Asian (North American) monsoon precipitation through large-scale atmospheric circulation changes. These findings imply that future variations in land cover and dust emissions may appreciably influence the NHLMP. Plain Language Summary Northern Hemisphere land monsoon precipitation (NHLMP) provides water resources for about two thirds of the world's population, which is vital for infrastructure planning, disaster mitigation, food security, and economic development. Changes in land cover and dust emissions may significantly influence the NHLMP, but observations are too short to understand the mechanisms. The Sahara Desert was once covered by vegetation and dust emission was substantially reduced during the mid-Holocene (6,000 years BP), which provides an opportunity to test the models' capability and unravel these mechanisms. Here we use an Earth System model and find that when the Saharan vegetation and dust reduction are taken into consideration, the simulated annual mean precipitation over most of the NHLM regions shows a closer agreement with proxy records. The sensitivity experiments show that the North African monsoon precipitation increases most significantly under the regional effects of Green Sahara. The Saharan vegetation (dust reduction under vegetated Sahara) alone also remotely increases the Asian (North American) monsoon precipitation through large-scale atmospheric circulation changes. The knowledge gained from this study is critical for improved understanding of the potential impacts of the land cover and dust changes on the projected future monsoon change.
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| 45033. |
- Sun, Xiaole, et al.
(författare)
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Temporal and spatial variations of rock weathering and CO2 consumption in the Baltic Sea catchment
- 2017
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 466, s. 57-69
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Tidskriftsartikel (refereegranskat)abstract
- This study provides the first estimate of silicate and carbonate weathering rates and derived CO2 consumption rates for the Baltic Sea catchment using river chemistry data of 78 rivers draining into the Baltic Sea. The silicate weathering rates (denoted as total dissolved solids) of individual river basin range from 0.014 Tg/year to 4.55 Tg/year and the carbonate weathering rates range from 0.079 Tg/year to 6.49 Tg/year. The total CO2 consumption across the Baltic catchment is approximately 3.9 Tg C/year and is almost equally shared by silicates and carbonates. Uncertainty associated with the weathering estimate is around 32%, which is mainly caused by incomplete pollution correction for a few major rivers in the south. The calculations for the boreal river basins have higher certainties because of less human impacts. The CO2 consumption rate of individual river basin vary between 0.53 and 5.66 g C/m(2)/year with an average of 2.97 g C/m(2)/year, in which carbonates consume CO2, 1.4 g C/m(2)/year and silicates take 1.5 g C/m(2)/year. This is in the same range as has been reported for the Mackenzie River and Siberian river basins, but at the lower range of tropical rivers, suggesting the Baltic Sea catchment holds solid weathering signals for high-latitude systems, especially in the pristine boreal silicatedominated areas. The amount of CO2 consumed by weathering in the Baltic Sea catchment accounts for approximately 3-30% of the net ecosystem carbon exchange (10-100 g C/m(2)/year), implying that weathering contributes as a significant sink of atmospheric CO2. Although many studies have shown the positive relation between temperature and weathering rates in various river catchments, multiple regression analysis using the 40-year continuous records of river chemistry in the boreal area of the Baltic Sea catchment reveals a strong correlation between weathering flux and precipitation, but no statistically significant correlation between weathering and temperature. This suggests not only that temperature is not necessarily to be primary controlling factor for weathering rates, but also besides precipitation, other factors, like increased soil organic matter contents and water path changes may have high impact on weathering rates. The 40-year data analysis also shows generally increasing weathering fluxes by 10-20% in the pristine boreal area over the past decades. This indicates that increased CO2 consumption by weathering and the resulting elevated dissolved inorganic carbon delivery to the ocean act as a negative feedback for ocean acidification, such as the Arctic Ocean that has become more acidic due to high terrestrial organic carbon delivery together with increased river water input.
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| 45034. |
- Sun, Xiaole, et al.
(författare)
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The Importance of Benthic Nutrient Fluxes in Supporting Primary Production in the Laptev and East Siberian Shelf Seas
- 2021
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Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 35:7
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Tidskriftsartikel (refereegranskat)abstract
- This study presents an assessment of benthic nutrient regeneration and its role for the nutrient budget of the outer Laptev and East Siberian shelf sea. Porewater profiles of the major nutrients dissolved silica (DSi), dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphate (DIP) as well as total dissolved iron (DFe) were evaluated with a one-dimensional reaction transport model to derive net reaction rates and benthic nutrient fluxes from shelf and slope 16 stations. Integrated over the shelf area the benthic fluxes of DSi, DIN, and DIP were found to be 7.1, 1.2, and 0.5 Gmol/year in the Laptev Sea and 29.8, 9.5, and 2.8 Gmol/year in East Siberian Sea, respectively. A comparison of the ratios of the benthic nutrient fluxes with marine and riverine inputs and Arctic plankton stoichiometry indicate substantial benthic nitrogen loss likely due to denitrification relative to DIP and DSi. Our benthic flux estimation is likely a low estimate of benthic nutrient fluxes considering potentially higher regeneration rates of nutrients from more productive, bioturbated near-shore sediments. The estimate emphasizes the role of benthic nutrient fluxes by returning nutrients with a fundamentally different stoichiometry to bottom waters from that of Arctic marine phytoplankton, riverine sources, and open water inflow. With a simple box model, we provide a snapshot of today's nutrient budget in the two seas and estimate that about 10%-20% of nutrients required by primary production are derived from sediments. This proportion is expected to increase for a future warmer Arctic continental shelf in response to increasing primary production.
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| 45035. |
- Sundberg, Iréne
(författare)
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Kiselalger i Gotlands län 2018 : En undersökning av åtta vattendragslokaler
- 2019
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- I Gotlands län undersöktes år 2018 kiselalger på åtta vattendragslokaler. Sta- tusklassningen av provtagningslokalerna gjordes med hjälp av kiselalgsindexet IPS, som visar graden av påverkan av näringsämnen och lättnedbrytbar orga- nisk förorening. Stödparametrarna TDI (mängden näringskrävande arter) och%PT (andelen föroreningstoleranta arter) har beaktats vid bedömningen. IPS-indexet i Ireån (kraftverket) och Gothemån (Åminne) visade klass 2, god status. Båda kan dock sägas ligga i riskzonen för att hamna i klass 3 måttlig status.Själsöån, Snoderån (Borum), Gothemån (Vallstena) och Laxarveån bedömdes tillhöra klass 3, måttlig status (Gothemån efter expertbedömning).I Närkån och Idån var andelen arter som indikerar förekomst av lättnedbrytbar organisk förorening (%PT) mycket stor och IPS-indexet motsvarade klass 4, otillfredsställande status.Surhetsindexet ACID visar vilken pH-regim vattnet tillhör och är framtaget framför allt för att bedöma surheten i vattendrag med pH lägre än 7.Samtliga lokaler i undersökningen bedömdes ha alkaliska förhållanden, vilket tyder på ett årsmedelvärde för pH över 7,3.
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| 45036. |
- Sundqvist, Elin, et al.
(författare)
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Methane exchange in a boreal forest estimated by gradient method
- 2015
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 67
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Tidskriftsartikel (refereegranskat)abstract
- Forests are generally considered to be net sinks of atmospheric methane (CH4) because of oxidation by methanotrophic bacteria in well-aerated forests soils. However, emissions from wet forest soils, and sometimes canopy fluxes, are often neglected when quantifying the CH4 budget of a forest. We used a modified Bowen ratio method and combined eddy covariance and gradient methods to estimate net CH4 exchange at a boreal forest site in central Sweden. Results indicate that the site is a net source of CH4. This is in contrast to soil, branch and leaf chamber measurements of uptake of CH4. Wetter soils within the footprint of the canopy are thought to be responsible for the discrepancy. We found no evidence for canopy emissions per se. However, the diel pattern of the CH4 exchange with minimum emissions at daytime correlated well with gross primary production, which supports an uptake in the canopy. More distant source areas could also contribute to the diel pattern; their contribution might be greater at night during stable boundary layer conditions.
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| 45037. |
- Sundqvist, Hanna S., et al.
(författare)
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Arctic Holocene proxy climate database - new approaches to assessing geochronological accuracy and encoding climate variables
- 2014
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 10:4, s. 1605-1631
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Tidskriftsartikel (refereegranskat)abstract
- We present a systematic compilation of previously published Holocene proxy climate records from the Arctic. We identified 170 sites from north of 58 degrees N latitude where proxy time series extend back at least to 6 cal ka (all ages in this article are in calendar years before present - BP), are resolved at submillennial scale (at least one value every 400 +/- 200 years) and have age models constrained by at least one age every 3000 years. In addition to conventional meta-data for each proxy record (location, proxy type, reference), we include two novel parameters that add functionality to the database. First, climate interpretation is a series of fields that logically describe the specific climate variable(s) represented by the proxy record. It encodes the proxy-climate relation reported by authors of the original studies into a structured format to facilitate comparison with climate model outputs. Second, geochronology accuracy score (chron score) is a numerical rating that reflects the overall accuracy of C-14-based age models from lake and marine sediments. Chron scores were calculated using the original author-reported C-14 ages, which are included in this database. The database contains 320 records (some sites include multiple records) from six regions covering the circumpolar Arctic: Fennoscandia is the most densely sampled region (31% of the records), whereas only five records from the Russian Arctic met the criteria for inclusion. The database contains proxy records from lake sediment (60 %), marine sediment (32 %), glacier ice (5 %), and other sources. Most (61 %) reflect temperature (mainly summer warmth) and are primarily based on pollen, chironomid, or diatom assemblages. Many (15 %) reflect some aspect of hydroclimate as inferred from changes in stable isotopes, pollen and diatom assemblages, humification index in peat, and changes in equilibrium-line altitude of glaciers. This comprehensive database can be used in future studies to investigate the spatio-temporal pattern of Arctic Holocene climate changes and their causes. The Arctic Holocene data set is available from NOAA Paleoclimatology.
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| 45038. |
- Sundqvist, Hanna S., et al.
(författare)
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Evidence of a large cooling between 1690 and 1740 AD in southern Africa
- 2013
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 3
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Tidskriftsartikel (refereegranskat)abstract
- A 350-year-long, well-dated delta O-18 stalagmite record from the summer rainfall region in South Africa is positively correlated with regional air surface temperatures at interannual time scales. The coldest period documented in this record occurred between 1690 and 1740, slightly lagging the Maunder Minimum (1645-1710). A temperature reconstruction, based on the correlation between regional surface temperatures and the stalagmite delta O-18 variations, indicates that parts of this period could have been as much as 1.4 degrees C colder than today. Significant cycles of 22, 11 and 4.8 years demonstrate that the solar magnetic and the El Nino-Southern Oscillation cycle could be important drivers of multidecadal to interannual climate variability in this region. The observation that the most important driver of stalagmite delta O-18 on interannual time scales from this subtropical region is regional surface temperature cautions against deterministic interpretations of delta O-18 variations in low-latitude stalagmites as mainly driven by the amount of precipitation.
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| 45039. |
- Sundsten, Malin, 1974
(författare)
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Hydraulic Conductivity of Saturated Sand-Bentonite mixtures
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The implementation of the EC directive concerning landfill liners and covers sets stricter technical and environmental requirements for landfill barriers. It is therefore essential to identify suitable materials that fulfil these requirements. There is a general ambition to promote a sustainable society, and it would be advantageous, both environmentally and economically, if industrial waste materials could be reused as landfill barriers. The overall objectives of the present study was to facilitate and increase the use of sand-bentonite mixtures, including industrial waste materials and mixtures, in landfill barriers, and to increase the knowledge and understanding of the hydraulic conductivity. For engineering purposes, it would be useful to establish a relationship between the hydraulic conductivity of sand-bentonite mixtures and some more easily determined parameters, as it is complicated and time-consuming to determine the hydraulic conductivity of a landfill barrier in situ. Therefore, a methodology to estimate semi-theoretical relationships between hydraulic conductivity, dry density and bentonite content for sand-bentonite mixtures with different types of bentonite was developed. A number of laboratory and field tests were performed in order to develop and verify the semi-theoretical relationships for sand-bentonite mixtures. The materials tested were bentonite, foundry green sand and quarry sand. In the laboratory, conventional test methods such as flexible wall permeameters (FWP), as well as alternative methods, such as constant rate of strain (CRS) and constant pore pressure gradient (CGT) tests, were investigated, improved and used for determination of hydraulic conductivity. Hydraulic conductivity was also determined in situ by means of sealed single ring infiltrometers (SSRI) and porous probes (BAT permeameters). Generally, there was a good agreement between the laboratory hydraulic conductivity test results and the corresponding semi-theoretical relationship. The field test results were on average somewhat higher, probably due to the macroscopic effects that were present in situ. Based on the semi-theoretical relationships and the test results, design charts were proposed in order to facilitate an environmentally, technically and economically suitable design process for landfill barriers.
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| 45040. |
- Suni, T., et al.
(författare)
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The significance of land-atmosphere interactions in the Earth system-iLEAPS achievements and perspectives
- 2015
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Ingår i: Anthropocene. - : Elsevier BV. - 2213-3054. ; 12, s. 69-84
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Tidskriftsartikel (refereegranskat)abstract
- The integrated land ecosystem-atmosphere processes study (iLEAPS) is an international research project focussing on the fundamental processes that link land-atmosphere exchange, climate, the water cycle, and tropospheric chemistry. The project, iLEAPS, was established 2004 within the International Geosphere-Biosphere Programme (IGBP). During its first decade, iLEAPS has proven to be a vital project, well equipped to build a community to address the challenges involved in understanding the complex Earth system: multidisciplinary, integrative approaches for both observations and modeling. The iLEAPS community has made major advances in process understanding, land-surface modeling, and observation techniques and networks. The modes of iLEAPS operation include elucidating specific iLEAPS scientific questions through networks of process studies, field campaigns, modeling, long-term integrated field studies, international interdisciplinary mega-campaigns, synthesis studies, databases, as well as conferences on specific scientific questions and synthesis meetings. Another essential component of iLEAPS is knowledge transfer and it also encourages community-and policy-related outreach activities associated with the regional integrative projects. As a result of its first decade of work, iLEAPS is now setting the agenda for its next phase (2014-2024) under the new international initiative, future Earth. Human influence has always been an important part of land-atmosphere science but in order to respond to the new challenges of global sustainability, closer ties with social science and economics groups will be necessary to produce realistic estimates of land use and anthropogenic emissions by analysing future population increase, migration patterns, food production allocation, land management practices, energy production, industrial development, and urbanization.
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