Climate change 2014 - Synthesis Report

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A User’s Guide

Soils act as sources and sinks for greenhouse gases (GHG) such as carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O). Since both storage and emission capacities may be large, precise quantifications are needed to obtain reliable global budgets that are necessary for land-use management (agriculture, forestry), global change and for climate research. This paper discusses exclusively the soil emission-related processes and their influencing parameters. It reviews soil emission studies involving the most important land-cover types and climate zones and introduces important measuring systems for soil emissions. It addresses current shortcomings and the obvious bias towards northern hemispheric data. When using a conservative average of 300 mg CO 2 e m −2  h −1 (based on our literature review), this leads to global annual net soil emissions of ≥350 Pg CO 2 e (CO 2 e = CO 2 equivalents = total effect of all GHG normalized to CO 2 ). This corresponds to roughly 21% of the global soil C and N pools. For comparison, 33.4 Pg CO 2 are being emitted annually by fossil fuel combustion and the cement industry.

Greenhouse gas emissions from soils—A review

Collectively, reservoirs created by dams are thought to be an important source of greenhouse gases (GHGs) to the atmosphere. So far, efforts to quantify, model, and manage these emissions have been limited by data availability and inconsistencies in methodological approach. Here, we synthesize reservoir CH4, CO2, and N2O emission data with three main objectives: (1) to generate a global estimate of GHG emissions from reservoirs, (2) to identify the best predictors of these emissions, and (3) to consider the effect of methodology on emission estimates. We estimate that GHG emissions from reservoir water surfaces account for 0.8 (0.5-1.2) Pg CO2 equivalents per year, with the majority of this forcing due to CH4. We then discuss the potential for several alternative pathways such as dam degassing and downstream emissions to contribute significantly to overall emissions. Although prior studies have linked reservoir GHG emissions to reservoir age and latitude, we find that factors related to reservoir productivity are better predictors of emission.

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Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis

Fish bioenergetics: (Fish and Fisheries 13. Series ed. T.J. Pitcher) Malcolm Jobling Chapman and Hall, London, 1994 ISBN 0-412-58090-X, 24.99 Hard cover, acid-free paper, pp. xiv + 309, 31 tables, 61 figures Author, species and subject indexes

Water-level fluctuations (WLF) of lakes have temporal scales ranging from seconds to hundreds of years. Fluctuations in the lake level generated by an unbalanced water budget resulting from meteorological and hydrological processes, such as precipitation, evaporation and inflow and outflow conditions usually have long temporal scales (days to years) and are here classified as long-term WLF. In contrast, WLF generated by hydrodynamic processes, e.g. basin-scale oscillations and travelling surface waves, have periods in the order of seconds to hours and are classified as short-term WLF. The impact of WLF on abiotic and biotic conditions depends on the temporal scale under consideration and is exemplified using data from Lake Issyk-Kul, the Caspian Sea and Lake Constance. Long-term WLF induce a slow shore line displacement of metres to kilometres, but immediate physical stress due to currents associated with long-term WLF is negligible. Large-scale shore line displacements change the habitat availability for organisms adapted to terrestrial and aquatic conditions over long time scales. Short-term WLF, in contrast, do not significantly displace the boundary between the aquatic and the terrestrial habitat, but impose short-term physical stress on organisms living in the littoral zone and on organic and inorganic particles deposited in the top sediment layers. The interaction of WLF acting on different time scales amplifies their overall impact on the ecosystem, because long-term WLF change the habitat exposed to the physical stress resulting from short-term WLF. Specifically, shore morphology and sediment grain size distribution are the result of a continuous interplay between short- and long-term WLF, the former providing the energy for erosion the latter determining the section of the shore exposed to the erosive power.

/pdf/temporal-scales-of-water-level-fluctuations-in-lakes-and-4f1plsdcx3.pdf

Temporal scales of water-level fluctuations in lakes and their ecological implications

This study investigates the role of surface waves and the associated disturbance of littoral sediments for the release and later distribution of dissolved methane in lakes. Surface wave field, wave-induced currents, acoustic backscatter strength, and the concentration and distribution of dissolved methane were measured simultaneously in Lake Constance, Germany. The data indicate that surface waves enhance the release of dissolved methane in the shallow littoral zone via burst-like releases of methane during the passage of wave groups. The amount of released methane depends on the surface wave field and the water temperature that controls the methane production in the sediments. The dissolved methane concentrations in the shallow littoral zone were always higher than concentrations in the deep water and open water, while methane concentrations in the epilimnion were typically higher than methane concentrations in the metalimnion and upper hypolimnion. The relatively high epilimnetic methane concentrations in the pelagial can be explained by lateral transport of methane from the littoral zone to the pelagic zone. Littoral zones can thus be an important source of methane in lakes. Methane released by surface waves from littoral sediments may cause elevated near-surface methane concentrations in large areas that enhance the flux of methane at the air-water interface and, thus, the overall methane emissions from lakes to the atmosphere.

https://aslopubs.onlinelibrary.wiley.com/doi/epdf/10.4319/lo.2010.55.5.1990

Wave‐induced release of methane: Littoral zones as source of methane in lakes

Freshwater systems contribute significantly to the global atmospheric methane budget. A large fraction of the methane emitted from freshwaters is transported via ebullition. However, due to its strong variability in space and time, accurate measurements of ebullition rates are difficult; hence, the uncertainty regarding its contribution to global budgets is large. Here, we analyze measurements made by continuously recording automated bubble traps in an impounded river in central Europe and investigate the mechanisms affecting the temporal dynamics of bubble release from cohesive sediments. Our results show that the main triggers of bubble release were pressure changes, originating from the passage of ship lock-induced surges and ship passages. The response to physical forcing was also affected by previous outgassing. Ebullition rates varied strongly over all relevant timescales from minutes to days; therefore, representative ebullition estimates could only be inferred with continuous sampling over long periods. Since ebullition was found to be episodic, short-term measurement periods of a few hours or days will likely underestimate ebullition rates. Our results thus indicate that flux estimates could be grossly underestimated (by up to ~50%) if the correct temporal resolution is not used during data collection.

/pdf/pumping-methane-out-of-aquatic-sediments-ebullition-forcing-3rfcfe2pgx.pdf

Pumping methane out of aquatic sediments - ebullition forcing mechanisms in an impounded river

Surface waves and their interactions with sediments and benthic organisms are the main hydrodynamic process affecting littoral ecosystems. Here, we present a long-term data set on surface-wave parameters, which was obtained from the analysis of measurements with a pressure sensor. The data set covers a time period of a year and allows for resolving waves with heights down to less than a centimeter and frequencies up to 0.8 Hz. Wind waves and three different types of ship waves were distinguished by their spectral properties. In Lake Constance, ship-generated waves are as important as wind-generated waves and contribute about 41% of the annual mean wave energy flux to shore. In summer, during the most productive time period, ship waves dominate the wave field in terms of the energy flux to shore and also in their frequency of occurrence. Ship waves cause a diurnal and a seasonal pattern in the frequency of occurrence and in the heights of surface waves, whereas in the case of wind waves these parameters do not vary significantly with season or between nighttime and daytime. In contrast to wind waves that occur only sporadically, ship waves propagate into the littoral zone very frequently at regular time intervals. The different pattern of occurrence of ship and wind waves results in a different pattern of disturbance in the littoral ecosystem.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2008.53.1.0368

The relative importance of wind and ship waves in the littoral zone of a large lake

Changes in the budget of dissolved methane measured in a small temperate lake over 1 year indicate that anoxic conditions in the hypolimnion and the autumn overturn period represent key factors for the overall annual methane emissions from lakes. During periods of stable stratification, large amounts of methane accumulate in anoxic deep waters. Approximately 46% of the stored methane was emitted during the autumn overturn, contributing ∼80% of the annual diffusive methane emissions to the atmosphere. After the overturn period, the entire water column was oxic, and only 1% of the original quantity of methane remained in the water column. Current estimates of global methane emissions assume that all of the stored methane is released, whereas several studies of individual lakes have suggested that a major fraction of the stored methane is oxidized during overturns. Our results provide evidence that not all of the stored methane is released to the atmosphere during the overturn period. However, the fraction o...

https://kops.uni-konstanz.de/bitstream/handle/123456789/29002/Encinas_290023.pdf;sequence=2

Hilmar Hofmann

Papers

Temporal scales of water-level fluctuations in lakes and their ecological implications

Wave‐induced release of methane: Littoral zones as source of methane in lakes

Pumping methane out of aquatic sediments - ebullition forcing mechanisms in an impounded river

The relative importance of wind and ship waves in the littoral zone of a large lake

Importance of the autumn overturn and anoxic conditions in the hypolimnion for the annual methane emissions from a temperate lake.