Search for: All records

Abstract Arctic and alpine tundra ecosystems are large reservoirs of organic carbon^1,2. Climate warming may stimulate ecosystem respiration and release carbon into the atmosphere^3,4. The magnitude and persistency of this stimulation and the environmental mechanisms that drive its variation remain uncertain^5–7. This hampers the accuracy of global land carbon–climate feedback projections^7,8. Here we synthesize 136 datasets from 56 open-top chamber in situ warming experiments located at 28 arctic and alpine tundra sites which have been running for less than 1 year up to 25 years. We show that a mean rise of 1.4 °C [confidence interval (CI) 0.9–2.0 °C] in air and 0.4 °C [CI 0.2–0.7 °C] in soil temperature results in an increase in growing season ecosystem respiration by 30% [CI 22–38%] (n = 136). Our findings indicate that the stimulation of ecosystem respiration was due to increases in both plant-related and microbial respiration (n = 9) and continued for at least 25 years (n = 136). The magnitude of the warming effects on respiration was driven by variation in warming-induced changes in local soil conditions, that is, changes in total nitrogen concentration and pH and by context-dependent spatial variation in these conditions, in particular total nitrogen concentration and the carbon:nitrogen ratio. Tundra sites with stronger nitrogen limitations and sites in which warming had stimulated plant and microbial nutrient turnover seemed particularly sensitive in their respiration response to warming. The results highlight the importance of local soil conditions and warming-induced changes therein for future climatic impacts on respiration. 

The densest overflow water from the Nordic Seas passes through the Faroe Bank Channel and contributes to the headwaters to the lower limb of the Atlantic Meridional Overturning Circulation. The upstream pathways of this dense overflow water are not well known. Using data from a high-resolution hydrographic/velocity survey in 2011, as well as long-term moored velocity and shipboard hydrographic measurements north of the Faroe Islands, we present evidence of a current following the continental slope from Iceland toward the Faroe Bank Channel. This narrow current, which we call the Iceland-Faroe Slope Jet (IFSJ), is bottom-intensified and associated with dense water banked up on the slope. North of the Faroe Islands the IFSJ is situated beneath the Faroe Current, and its variability is tightly linked to the flow of Atlantic Water above. The bulk of the IFSJ’s volume transport is confined to a small area in ϴ-S space centered near a potential density anomaly of 28.06 kg m-3. This is slightly denser than the transport mode of the North Icelandic Jet, which follows shallower isobaths along the slope north of Iceland in the opposite direction and feeds the Denmark Strait overflow. However, the similarity of the hydrographic properties suggests that the two currents have a common source. The average transport of water denser than σϴ = 27.8 kg m-3 in the IFSJ is on the order of 1 Sv, which may account for roughly 50% of the overflow through the Faroe Bank Channel.