No abstract

Global sea level provides an important indicator of the state of the warming climate, but changes in regional sea level are most relevant for coastal communities around the world. With improvements to the sea‐level observing system, the knowledge of regional sea‐level change has advanced dramatically in recent years. Satellite measurements coupled with in situ observations have allowed for comprehensive study and improved understanding of the diverse set of drivers that lead to variations in sea level in space and time. Despite the advances, gaps in the understanding of contemporary sea‐level change remain and inhibit the ability to predict how the relevant processes may lead to future change. These gaps arise in part due to the complexity of the linkages between the drivers of sea‐level change. Here we review the individual processes which lead to sea‐level change and then describe how they combine and vary regionally. The intent of the paper is to provide an overview of the current state of understanding of the processes that cause regional sea‐level change and to identify and discuss limitations and uncertainty in our understanding of these processes. Areas where the lack of understanding or gaps in knowledge inhibit the ability to provide the needed information for comprehensive planning efforts are of particular focus. Finally, a goal of this paper is to highlight the role of the expanded sea‐level observation network—particularly as related to satellite observations—in the improved scientific understanding of the contributors to regional sea‐level change.

No abstract

Methane (CH4) emissions from climate‐sensitive ecosystems within the northern permafrost region represent a potentially large but highly uncertain source, with current estimates spanning a factor of seven (11–75 Tg CH4 yr−1). Accelerating permafrost thaw threatens significant increases in pan‐Arctic CH4 emissions, amplifying the permafrost carbon feedback. We used airborne imaging spectroscopy with meter‐scale spatial resolution and broad coverage to identify a previously undiscovered CH4 emission hotspot adjacent to a thermokarst lake in interior Alaska. Hotspot emissions were confined to <1% of the 10 ha lake study area. Ground‐based chamber measurements confirmed average daily fluxes from the hotspot of 1,170 mg CH4 m−2 d−1, with extreme daily maxima up to 24,200 mg CH4 m−2 d−1. Ground‐based geophysical measurements revealed thawed permafrost directly beneath the CH4 hotspot, extending to a depth of ∼15 m, indicating that the intense CH4 emissions likely originated from recently thawed permafrost. Hotspot emissions accounted for ∼40% of total diffusive CH4 emissions from the lake study site. Combining study site findings with hotspot statistics from our 70,000 km2 airborne survey across Alaska and northwestern Canada, we estimate that pan‐Arctic terrestrial thermokarst hotspots currently emit 1.1 (0.1–5.2) Tg CH4 yr−1, or roughly 4% of the annual pan‐Arctic wetland budget from just 0.01% of the northern permafrost land area. Our results suggest that significant proportions of pan‐Arctic CH4 emissions originate from disproportionately small areas of previously undetermined thermokarst emissions hotspots, and that pan‐Arctic CH4 emissions may increase non‐linearly as thermokarst processes increase under a warming climate.