ALBERT

Description: The high latitudes form an important component of the Earth’s carbon cycle. It is therefore important to capture this in Earth System Models (ESM’s). However, most carbon-cycle development and evaluation in ESM’s focuses on lower latitudes, and therefore there is an urgent need to address Arctic carbon-cycle processes. Here, we run land-surface schemes from ESM’s at the site level at various Arctic sites, performing a detailed evaluation of the carbon dynamics in the models. They are process-based models, and therefore point-scale evaluations contribute directly towards improving the large-scale results. The sites chosen for the simulations are the five principal field sites from the recently-concluded EU project PAGE21. This gives the distinct advantage that detailed data are available. In particular, data on the physical state of the climate and permafrost at these sites, and large datasets of soil carbon stocks and fluxes. The sites cover a range from low Arctic discontinuous permafrost to high Arctic desert, and a range of soil types from thick peat to mineral soils with little organic matter. The models involved are land surface schemes from three European ESM’s: UKESM (UK), IPSL (France) and MPI-ESM (Germany). The models all have improved process representation as part of PAGE21. The simulations are first compared with physical observations from the sites: specifically snow depth, soil temperature, soil moisture and maximum thaw depth. All models capture the physics with a reasonable accuracy, and certainly capture the major differences between sites, with a few exceptions. In particular, we see the importance of simulating the physical properties of the soil organic layer. Comparing simulated soil organic carbon with observations shows the importance of including vertical soil carbon profiles. In one model this is not represented, which results in a failure to capture the differences in soil carbon in different physical conditions. Including cryoturbation mixing is key to simulating the vertical soil organic carbon profile. When vertical mixing is included, the profile of soil organic carbon at mineral soil sites matches very well with observations. However, none of the models are able to simulate the correct profile at sites with organic soils, highlighting the need for further process representation of peat accumulation. Finally, the land-atmosphere carbon fluxes are assessed using different observations, and we discuss the meaning of these measurements in terms of the land surface model output and how they can be most usefully compared. The simulation of carbon fluxes depends on every aspect of the models: The physical state, the soil carbon stocks and most importantly, the vegetation. Large errors result from the models growing the wrong type of vegetation or no vegetation at all, as tundra vegetation types are not represented in two of the three models. Focussed work is required to better represent Arctic vegetation in such models, and our results highlight the next steps to take. The observational datasets are more detailed than those used in past studies and this work will be used both to facilitate and to justify the development of Arctic carbon cycle processes in Earth system models.

Description: Permafrost thermal state and active layer thicknesses have been compared from five different Arctic field sites as part of the EU 7th framework project PAGE21. Data has been compiled for the years 2012- 2014 in the following locations: Samoylov and Kytalyk, both in Siberia, Russia, Zackenberg in northeast Greenland, Adventdalen in Svalbard, and Abisko in northern Sweden. These sites were chosen to exemplify the gradient between sporadic and continuous permafrost. The investigated ground thermal data all come from lowland sediment areas, primarily from ice-wedge polygonal landforms, except in the case of Abisko, where the permafrost only exits in palsas. The two Siberian sites have the coldest permafrost, earliest active layer freeze-back periods, and significant annual thermal variation in the upper permafrost. These observations can be explained by the large annual air temperature amplitude in the region, high ground ice content, and limited snow cover. Permafrost temperatures in northeast Greenland are somewhat warmer than in Siberia, but are cooler than those of Svalbard. Seasonal air temperature variation is smaller in northeast Greenland than northern Russia, and this is reflected in the smaller variation in seasonal permafrost temperatures in the prior location. Svalbard permafrost has similar seasonal variability compared to northeast Greenland, but is warmer overall, showing the effect of mild winters. The warmest permafrost was measured at Abisko, where active layer thawing also occurs the earliest. Active layer thickness (ALT) measurements from the five sites, collected via mechanical probing of CALM sites, reflect site meteorological variability in addition to differences in active layer material. In the past few years, only small scale changes in ALT have been observed (both thinning and thickening). However, examining the entire data series shows a general increase in ALT. The permafrost temperature and ALT data highlight climate as a main driver of regional permafrost conditions, but also demonstrate the influence of site specific factors which must be considered at the landscape scale.

Description: Due to a strong Arctic warming trend, potentially large greenhouse gas emissions from Arctic and sub- Arctic areas are of concern. The Lena River Delta located in north-east Siberia is the largest delta within the Arctic Circle, characterized by wetland ecosystems and wet polygonal tundra environments. These environments are currently thought to be sinks for carbon dioxide and sources of methane. Tower-based eddy covariance is the most widely used direct method for quantifying exchanges of momentum, energy and trace gases between the surface and the atmosphere. However, they cover a relatively small footprint and constitute point measurements relative to the vast extend of tundra ecosystems. To improve spatial coverage and spatial representativeness of these direct flux measurements, airborne eddy covariance flux measurements across large areas are required. We used the helicopter-carried measurement system “Helipod” equipped with a turbulence probe, fast temperature and humidity sensors, and a fast response gas analyzer to measure turbulent fluxes of heat, carbon dioxide, and methane across the Lena River Delta in Russia in 2012 and 2014. The 2014 campaign covered several periods of the season from April to August 2014. Wavelet transforms are used to improve spatial resolution of the flux measurements and footprint analysis is applied to find relations between surface fluxes and biophysically relevant land cover properties. Strong regional differences in trace gas fluxes were detected, indicating a non-uniform distribution of sources especially in wet sedge-, moist grass-, and moss-dominated tundra. In contrast, the sensible heat flux showed less variability across the investigation area. The obtained results are essential in understanding the role of Arctic ecosystems in the greenhouse gas budgets and to evaluate regional scale model simulations.

Description: The following authors were omitted from the original version of this Data Descriptor: Markus Reichstein and Nicolas Vuichard. Both contributed to the code development and N. Vuichard contributed to the processing of the ERA-Interim data downscaling. Furthermore, the contribution of the co-author Frank Tiedemann was re-evaluated relative to the colleague Corinna Rebmann, both working at the same sites, and based on this re-evaluation a substitution in the co-author list is implemented (with Rebmann replacing Tiedemann). Finally, two affiliations were listed incorrectly and are corrected here (entries 190 and 193). The author list and affiliations have been amended to address these omissions in both the HTML and PDF versions. © 2021, This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.

Description: The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.

Description: We investigate permafrost surface features revealed from satellite radar data in the Siberian arctic at the Yamal peninsula. Surface dynamic analysis based on SRTM and TanDEM-X DEMs show up to 2 m net loss of surface relief between 2000 and 2014 indicating a highly dynamic landscape. Surface features for the past 14 years reflect an increase of small stream channels and a number of new lakes that developed, likely caused by permafrost thaw. We used Sentinel-1 SAR imagery to measure permafrost surface changes. Due to limited observation data we analyzed only two years. The InSAR time-series has detected surface displacements in three distinct spatial locations during 2017 and 2018. At these three locations, 60-120 mm/yr rates of seasonal surface permafrost changes are observed. Spatial location of seasonal ground displacements align well with lithology. One of them is located on marine sediments and is linked to anthropogenic impact on permafrost stability. Two other areas are located within alluvial sediments and are at the top of topographic elevated zones. We discuss the influence of the geologic environment and the potential effect of local upwelling of gas. This combined analyses of InSAR time series with analysis of geomorphic features from DEMs present an important tool for continuous process monitoring of surface dynamics as part of a global warming risk assessment.