Path-dependent reductions in CO2 emission budgets caused by permafrost carbon release

International audienc

Decadal fingerprints of freshwater discharge around Greenland in a multi-model ensemble

The recent increase in the rate of the Greenland ice sheet melting has raised with urgency the question of the impact of such a melting on the climate. As former model projections, based on a coarse representation of the melting, show very different sensitivity to this melting, it seems necessary to consider a multi-model ensemble to tackle this question. Here we use five coupled climate models and one ocean-only model to evaluate the impact of 0.1 Sv (1 Sv = 106 m3/s) of freshwater equally distributed around the coast of Greenland during the historical era 1965–2004. The ocean-only model helps to discriminate between oceanic and coupled responses. In this idealized framework, we find similar fingerprints in the fourth decade of hosing among the models, with a general weakening of the Atlantic Meridional Overturning Circulation (AMOC). Initially, the additional freshwater spreads along the main currents of the subpolar gyre. Part of the anomaly crosses the Atlantic eastward and enters into the Canary Current constituting a freshwater leakage tapping the subpolar gyre system. As a consequence, we show that the AMOC weakening is smaller if the leakage is larger. We argue that the magnitude of the freshwater leakage is related to the asymmetry between the subpolar-subtropical gyres in the control simulations, which may ultimately be a primary cause for the diversity of AMOC responses to the hosing in the multi-model ensemble. Another important fingerprint concerns a warming in the Nordic Seas in response to the re-emergence of Atlantic subsurface waters capped by the freshwater in the subpolar gyre. This subsurface heat anomaly reaches the Arctic where it emerges and induces a positive upper ocean salinity anomaly by introducing more Atlantic waters. We found similar climatic impacts in all the coupled ocean–atmosphere models with an atmospheric cooling of the North Atlantic except in the region around the Nordic Seas and a slight warming south of the equator in the Atlantic. This meridional gradient of temperature is associated with a southward shift of the tropical rains. The free surface models also show similar sea-level fingerprints notably with a comma-shape of high sea-level rise following the Canary Current

Population evacuation: evaluating spatial distribution of flood shelters and vulnerable residential units in Dhaka with geographic information systems

The objective of this study was to evaluate the spatial distribution of flood shelters in relation to flood hazards in a resource-poor country. Flood hazard estimates were developed from multi-temporal flood-affected frequency and floodwater depth maps. It is intended that the results could support non-structural flood management. In addition, the location of vulnerable housing units was mapped and their accessibility to shelters was computed with the aid of spatial techniques using a geographic information system. A subset of the Dhaka Metropolitan Development Plan zone and Dhaka megacity, covering an area of 878 km2, was used as a case study since this area is likely to experience more frequent and intense flooding in coming years as a result of rapid urbanisation and climatic change. Using three different criteria, the study identified that a total of 5537 buildings, out of 6342 candidate structures, can be used as emergency shelters during floods, and approximately 145,000 dwellings (19.3 % of total residential units) of various types were located in places that are prone to flood. Further, many (3500 of 5537) of the identified shelters were not sufficiently close to vulnerable dwellings to protect approximately 496,000 potential flood victims during an emergency. There were 26.4 % of the total residents living in vulnerable housing units. In addition, 1098 flood shelters were distributed over five catchments in the study area, although in close proximity to vulnerable residents, do not have the capacity to house the number of people who could potentially seek refuge there. This study, the first of its kind in Dhaka, can assist urban planners and emergency managers in developing an effective evacuation plan for an imminent flood disaster as the city currently lacks any disaster management plan