J. van Huissteden

Methane emissions from permafrost thaw lakes limited by lake drainage

Thaw-lake expansion is enhanced by climate warming, potentially feeding back to boost warming further. A new landscape-scale modelling study of the life cycle of Siberian thaw lakes indicates that drainage strongly limits lake expansion. This results in methane-emission estimates that are substantially lower than previously suggested.

Longer growing seasons do not increase net carbon uptake in the northeastern Siberian tundra

With global warming, snowmelt is occurring earlier and growing seasons are becoming longer around the Arctic. It has been suggested that this would lead to more uptake of carbon due to a lengthening of the period in which plants photosynthesize. To investigate this suggestion, 8 consecutive years of eddy covariance measurements at a northeastern Siberian graminoid tundra site were investigated for patterns in net ecosystem exchange, gross primary production (GPP) and ecosystem respiration (R-eco). While GPP showed no clear increase with longer growing seasons, it was significantly increased in warmer summers. Due to these warmer temperatures however, the increase in uptake was mostly offset by an increase in R-eco. Therefore, overall variability in net carbon uptake was low, and no relationship with growing season length was found. Furthermore, the highest net uptake of carbon occurred with the shortest and the coldest growing season. Low uptake of carbon mostly occurred with longer or warmer growing seasons. We thus conclude that the net carbon uptake of this ecosystem is more likely to decrease rather than to increase under a warmer climate. These results contradict previous research that has showed more net carbon uptake with longer growing seasons. We hypothesize that this difference is due to site-specific differences, such as climate type and soil, and that changes in the carbon cycle with longer growing seasons will not be uniform around the Arctic. (Less)

The Late Miocene and Pliocene climate in East Asia as recorded by grain size and magnetic susceptibility of the Red Clay deposits (Chinese Loess Plateau)

Abstract Grain-size characteristics of the aeolian Red Clay sediment series in China demonstrate the existence of warm–cool alternations during the late Miocene and Pliocene. From the dissimilarity between the temporal patterns of grain size and magnetic susceptibility it is inferred that the finest grain-size fraction in the Red Clay (clay and very fine silt) is transported by the wind, as is the case of the coarser (silty) component. Relatively cool climatic conditions are indicated by increased loess supply (represented by the silty component) from the NW, caused by a stronger winter monsoon, as in the overlying Quaternary system. It is striking that at the same time the amounts of clay also increased, probably due to transportation by intensified westerly circulation. Relatively warm periods are characterized by opposite grain-size properties. These warm–cool alternations on the Asian continent, dated by palaeomagnetic analyses, correlate well with already established changes in sea level and ice sheet volumes up to ∼6 Ma. In addition, time series analysis of the grain-size signal points to an orbitally forced climatic cyclicity (precession, eccentricity and (weak) obliquity).

Modeling regional to global CH4 emissions of boreal and arctic wetlands

Methane (CH4) emission from boreal, arctic and subarctic wetlands constitutes a potentially positive feedback to global climate warming. Many process-based models have been developed, but high uncertainties remain in estimating the amount of CH4 released from wetlands at the global scale. This study tries to improve estimates of CH4 emissions by up-scaling a wetland CH4 emission model, PEATLAND-VU, to the global scale with a spatial resolution of 0.5 degrees for the period 2001-2006. This up-scaling was based on the global circum-arctic distribution of wetlands with hydrological conditions being specified by a global hydrological model, PCR-GLOBWB. In addition to the daily hydrological output from PCR-GLOBWB, comprising water table depths and snow thickness, the parameterization included air temperature as obtained from the ECMWF Operational Archive. To establish the uncertainty in the representations of the circum-arctic distribution of wetlands on the CH4 emission, several existing products were used to aggregate the emissions. Using the description of potential peatlands from the FAO Digital Soil Map of the World and the representation of floodplains by PCR-GLOBWB, the average annual flux over the period 2001-2006 was estimated to be 78 Tg yr(-1). In comparison, the six-year average CH4 fluxes were 37.7, 89.4, 145.6, and 157.3 Tg yr(-1) for different estimates of wetland extends based on the studies by Matthews and Fung, Prigent et al., Lehner and Doll, and Kaplan, respectively. This study shows the feasibility to estimate interannual variations in CH4 emissions by coupling hydrological and CH4 emission process models. It highlights the importance of an adequate understanding of hydrology in quantifying the total emissions from northern hemispheric wetlands and shows how knowledge of the sub-grid variability in wetland extent helps to prescribe relevant hydrological conditions to the emission model as well as to identify the uncertainty associated with existing wetland distributions. (Less)

Detection of rapid climate change in Last Glacial fluvial successions in the Netherlands

Climate change during the Last Glacial is considered as a major forcing factor of fluvial system changes. A continuous succession of fluvial sediments, reflecting adaptations to climate change from the Weichselian Middle Pleniglacial (oxygen isotope stage 3) onwards, occurs in lowland river basins in the Netherlands. A comparison of the Pleniglacial and Late Glacial fluvial record in the Netherlands shows that climatic oscillations of similar magnitude did not produce changes in the fluvial sedimentary system of equal magnitude. The Late Glacial fluvial system proves to be highly sensitive to climate change. By contrast, many of the rapid climate changes that have occurred during oxygen isotope stage 3, according to the Greenland ice core record, are not detectable in the fluvial sediments. This can be explained by differences in the impact of the climate variations on drainage basin vegetation. During the Late Glacial, the tree line repeatedly shifted through the Netherlands, whereas the area remained within the tundra zone during the Middle Pleniglacial. Precipitation variations and permafrost aggradation and degradation have played a secondary role. The Weichselian fluvial succession in the Netherlands demonstrates that detection of a change in the fluvial sedimentary system and relating this change to climate change is subject to methodological limitations. The climatic significance of changes in the fluvial record should be carefully evaluated, as well as their chronology. The possibility that climate did not influence the fluvial system should always be considered as a null hypothesis in studies on fluvial successions.

Modelling the effect of water-table management on CO2 and CH4 fluxes from peat soils

Drainage of peatlands for agriculture causes an increase of CO 2 flux from peat decomposition, contributing to national CO 2 emission. The reverse process, i.e. for re-creation of wetlands, reduces the CO 2 flux, but increases the CH 4 flux. We developed a process model (PEATLAND) to simulate these fluxes from peat soils subject to different water-table management scenarios. The model combines primary production, aerobic decomposition of soil organic matter (including the soil-parent material, peat), CH 4 formation, oxidation, and transport. Model input requires specification of water table and air temperature data sets, vegetation parameters such as primary production and parameters related to gas transport, and basic soil physical data. Validation using closed flux-chamber measurements of CO 2 and CH 4 from five different sites in the western Netherlands shows that seasonal changes in fluxes of CO 2 and CH 4 are correctly modelled. However, the CO 2 submodel underestimates peat decomposition when peat decomposition rates obtained from laboratory incubation experiments are used as input. Field decomposition rates are considerably higher. This is attributed to enhancement of decomposition by the addition of easily decomposable material from root exudation (’priming effect’). Model experiments indicate that 1) drainage increases the CO 2 production from peat decomposition strongly; 2) restoring a high water table may decrease the total greenhouse gas flux by a small amount although the CH 4 flux increases strongly; 3) a warmer climate may cause higher greenhouse gas fluxes from peat soils resulting in a positive feedback to climate warming, and 4) high vegetation productivity in fen meadows may stimulate peat decomposition by the priming effect.

Periglacial fluvial systems in northwest Europe during marine isotope stages 4 and 3

Comparison of fluvial successions in river valleys dating from marine isotope (MI) Stages 4 and 3 in a west–east transect from Britain to Poland shows the spatial and temporal variation in palaeohydrological characteristics of northwest European river valleys during these stages. MI Stage 4 has been a period of deep fluvial incision. Following this erosion, most valleys were filled during MI Stage 3 with successions of gravelly river deposits (Britain) or sand/silt/peat successions (northwest European lowlands). The sedimentology of the deposits suggests strong discharge variations, caused by a nival discharge regime with pronounced spring snowmelt discharges. The MI Stage 3 climate oscillations generally cannot be traced in the fluvial record. An incision phase of Hengelo interstadial age has been found in the Netherlands, but similar incision phases related to other warming events have not been proved convincingly. The restricted reaction of the river systems is caused by the absence of geomorphologically effective vegetation changes and relatively minor variations in precipitation during MI Stage 3. Over longer time scales, silt and peat beds gradually decrease in the younger (post-Hengelo/Upton Warren) part of MI Stage 3, corresponding with evidence of aeolian activity and permafrost. This is related to the longer duration of the stadials in this part of MI Stage 3.

Spatial and temporal dynamics in eddy covariance observations of methane fluxes at a tundra site in Northeastern Siberia

In the past two decades, the eddy covariance technique has been used for an increasing number of methane flux studies at an ecosystem scale. Previously, most of these studies used a closed path setup with a tunable diode laser spectrometer (TDL). Although this method worked well, the TDL has to be calibrated regularly and cooled with liquid nitrogen or a cryogenic system, which limits its use in remote areas. Recently, a new closed path technique has been introduced that uses off-axis integrated cavity output spectroscopy that does not require regular calibration or liquid nitrogen to operate and can thus be applied in remote areas. In the summer of 2008 and 2009, this eddy covariance technique was used to study methane fluxes from a tundra site in northeastern Siberia. The measured emissions showed to be very dependent on the fetch area, due to a large contrast in dry and wet vegetation in between wind directions. Furthermore, the observed short-and long-term variation of methane fluxes could be readily explained with a nonlinear model that used relationships with atmospheric stability, soil temperature, and water level. This model was subsequently extended to fieldwork periods preceding the eddy covariance setup and applied to evaluate a spatially integrated flux. The model result showed that average fluxes were 56.5, 48.7, and 30.4 nmol CH4 m(-2) s(-1) for the summers of 2007 to 2009. While previous models of the same type were only applicable to daily averages, the method described can be used on a much higher temporal resolution, making it suitable for gap filling. Furthermore, by partitioning the measured fluxes along wind direction, this model can also be used in areas with nonuniform terrain but nonetheless provide spatially integrated fluxes. (Less)

Fluvial and aeolian interaction under permafrost conditions: Weichselian Late Pleniglacial, Twente, eastern Netherlands

Abstract Close interaction between fluvial and aeolian deposition led to the formation of a distinct ‘fluvio-aeolian’ depositional environment in river valleys in the Netherlands during the last glacial maximum. We describe three sections illustrating this sedimentary environment. The primary sedimentary structures and periglacial phenomena document the dynamic interaction between fluvial, aeolian and periglacial processes. The rivers were ephemeral in nature, with alternating fluvial and aeolian deposition in overbank environments. Within the sections, successive levels of frost cracks, ice-wedge casts, soil wedges and cryoturbations are found. The sections demonstrate that not all these levels of periglacial structures indicate climatic oscillations, but are caused by lateral migration of fluvial activity. However, one large cryoturbation level truncated by a gravel lag (the Beuningen gravel bed) is evidence of widespread permafrost degradation induced by climatic warming between ca. 16 and 22 ka. This climatic warming is followed by a significant increase of aeolian activity.

Sensitivity analysis of a wetland methane emission model based on temperate and arctic wetland sites

Modelling of wetland CH4 fluxes using wetland soil emission models is used to determine the size of this natural source of CH4 emission on local to global scale. Most process models of CH 4 formation and soil-atmosphere CH 4 transport processes operate on a plot scale. For large scale emission modelling (regional to global scale) upscaling of this type of model requires thorough analysis of the sensitivity of these models to parameter uncertainty. We applied the GLUE (Generalized Likelihood Uncertainty Analysis) methodology to a well-known CH 4 emission model, the Walter-Heimann model, as implemented in the PEATLANDVU model. The model is tested using data from two temperate wetland sites and one arctic site. The tests include experiments with different objective functions, which quantify the fit of the model results to the data. The results indicate that the model 1) in most cases is capable of estimating CH4 fluxes better than an estimate based on the data avarage, but does not clearly outcompete a regression model based on local data; 2) is capable of reproducing larger scale (seasonal) temporal variability in the data, but not the small-scale (daily) temporal variability; 3) is not strongly sensitive to soil parameters, 4) is sensitive to parameters determining CH 4 transport and oxidation in vegetation, and the temperature sensitivity of the microbial population. The GLUE method also allowed testing of several smaller modifications of the original model. We conclude that upscaling of this plot-based wetland CH4 emission model is feasible, but considerable improveCorrespondence to: J. van Huissteden (ko.van.huissteden@geo.falw.vu.nl) ments of wetland CH4 modelling will result from improvement of wetland vegetation data.