An estimated 60% of the presently observed sea level rise is attributed to melting glaciers, ice caps and ice sheets. The contribution of Arctic glaciers and especially the Greenland ice sheet (GrIS) has more than doubled over the period 1961-2008, and is likely to increase further when global temperature increases over the coming century. A first order effect of increasing temperatures on the glacier mass budget is an increase in surface melt. However, not all melt contributes to mass loss via runoff, because part of the meltwater refreezes in the cold snow. For the GrIS, an estimated 30-50% of all meltwater refreezes. Infiltration of water and subsequent refreezing is not well included in existing mass balance models, and estimates of refreezing are not well validated due to a lack of observations. Given the importance of refreezing for the mass budget of Arctic glaciers, in this proposal we focus on improving refreezing estimates for the GrIS and therewith other Arctic ice masses, by using a combined observational and modelling approach. Existing satellite and in-situ observations, complemented with detailed snow temperature observations to be carried out in western Greenland (K-transect), will be used to evaluate the mass balance terms and refreezing modelled with the regional climate model RACMO2, which includes a multi-layer snow model. After improvements to the snow model have been implemented and evaluated, RACMO2 will be run for the period 1958-present-2100 in order to study the role of refreezing in the mass balance of the GrIS in a changing climate.

North-Atlantic and Indian Ocean climate variability appears to be tightly related. The observed increase of the amplitude of the North-Atlantic Oscillation (stronger westerlies) over the past decades is directly related to increased atmospheric convection in response to a warming Indian Ocean. Warm Indian Ocean waters flow into the South Atlantic and further northward as part of the global overturning circulation. Paleoceanographic records indicate that this inter-ocean connection around South Africa fluctuated considerably and abruptly in during (inter)glacial change, as southward transport was the only oceanic pathway for the Indian Ocean to dispose of its excess heat. On millennial time-scales that could explain why warming over the North Atlantic coincided with cooling of the Antarctic sector and vice versa. Observations and modeling suggest an important impact of the Indian-Atlantic inter-ocean connection on the strength and stability of the Atlantic overturning circulation, controlled by the (sub)tropical flows feeding it from upstream. These flows converge in the Mozambique Channel and the southern East Madagascar Current (EMC). North Atlantic and Antarctic cold water masses flow in opposite direction in the deep Indian Ocean. To understand their dynamics, an array of moored instruments will measure interannual variability across the Mozambique Channel and remain in operation for several years. A second array will be placed across the EMC. Combined with satellite data and high-resolution ocean-model simulations a complete picture should emerge of the varying flow in the western Indian Ocean presently feeding into the Indian-Atlantic Ocean connection. Further simulations will be executed under both present and glacial conditions, and assessed against new paleorecords of western Indian Ocean climate change over the past 60,000 years from sediment cores. Finally, simulations with the global climate model EC-EARTH will address the processes controlling the global atmospheric and oceanic (tele)connections between the Indian and Atlantic ocean-climate systems, particularly during abrupt North Atlantic climate change.

This projects aims to improve current estimates of mass changes of the Greenland ice sheet by coupling an Earth System model (EC-Earth) in time slice mode to an ice sheet model (ANICE), where the surface mass balance, in particular the ablation is calibrated against results from a regional climate model (RACMO), which has much better skills with respect to ablation calculations than a GCM. Changes in geometry/topography of the ice sheet and their effect on the mass balance are accounted for by a new spatial mass balance gradient approach. The ice sheet model includes the full coupling of the sea level equation enabling relative sea level calculations and the feedback of changing sea level on the dynamics of the ice sheet. Runs are performed from the LGM into the next centuries in order to capture the current and future imbalance, including transient effects, and validate the model with spatial and temporal bedrock uplift and sea level observations. External results from dedicated outlet glacier models are used to quantify the impact of changes in outlet glaciers on the total mass balance of the entire ice sheet. Runs for the next centuries will be based on the CMIP5 results from The EC-Earth model.

Finding the missing methane emissions Recent publications in the media and scientific literature claimed that the officially reported methane emissions only account for three-quarter of the known total methane emissions world-wide, with especially a mismatch in countries with a limited on-ground data collection. In the project CHEF we will develop an inversion algorithm, not only to find the hotspots of emission, but to derive a complete gridded field of methane emissions per region, which allows us to identify and quantify these missing emissions. CHEF will be embedded in the KNMI-Global initiative to support countries of the Global South.

The applying researchers developed a unique series of projections for twenty-first century regional sea level change - a valuable addition to the widely published global mean projections. These patterns display several characteristic common features, like an up to 20-30% larger rise along the North Atlantic coasts and in (sub-)tropical regions, and only 50% of the average rise in the subpolar North Atlantic Ocean and off the western Antarctic coast. However, significant uncertainties in the patterns´ amplitude remain, mainly due to uncertainties in the ice sheet contributions. Here we aim to assess the quality and merits of these projections by a detailed analysis of satellite observations, as that twenty-year record must have captured ~10-30% of the projected twenty-first century rise. Notably, the observed regional trends in sea level do not resemble the projections, as trends induced by global warming are masked by natural climate variability like El Niño-events. We will extract the long-term regional sea level change from the altimetry record by filtering out the dominant modes of natural variability identified from a statistical decomposition of the observations and fifty-year model re-analyses. The resulting filtered signal, which expectedly contains the sought-after forced regional trends, is compared to the sea level projections. Moreover, taking advantage of our detailed knowledge of the sea level patterns associated with the individual contributions (ocean expansion, glacier melt, Greenland and Antarctic ice sheet mass losses, and groundwater extraction) we analyze which scenario is most likely to be ongoing. The outcomes will benefit policy makers involved in coastal management worldwide.