We are not advocating that glacial geoengineering be attempted any time soon. An ice sheet intervention today would be at the edge of human capabilities. The easiest design that we considered would be comparable to the largest civil engineering projects that humanity has ever attempted, it would be located in a much harsher environment than the ones in which those projects were built, and our results suggest that it would only have a 30 % probability of success. What we are advocating instead is the beginning of an incremental process of design improvement. In Sect. 5, we suggested multiple possible routes forward to improve the design, and there are likely to be many additional possibilities that we have not considered. With decades or perhaps centuries to work on the problem, the scientific community could work towards developing a plan that was both achievable and had a high probability of success.

Most of the research that needs to be done to move this process forward is research that we must do in order to predict future sea level rise anyway: coupled ice–ocean models; field studies of key glaciers; better understanding of basal hydrology, sediment transport, and erosion; oceanographic data from the sub-ice cavity; calving and fracture studies; and more. Glacial geoengineering is a dramatic topic that can capture popular interest (Meyer, 2018), providing a stimulus and popular appetite for more glaciological research. Glacial geoengineering also provides an additional set of questions that can inform the way we think about ice dynamics. How should the citizens of low-lying nations value ocean circulation in the sub-ice cavities of the Amundsen Sea? How much importance should the international community place on the basal water pressure of key outlet glaciers? What exactly is the societal value of changes to the force balance of far away ice shelves? Geoengineering provides a framework for analyzing problems in glaciology that centers and quantifies the relationship between esoteric ice sheet processes and the concrete consequences of those processes for human societies and human lives.

The results that we have presented here are only the first step towards answering those questions. The designs we considered were very simple and our reduced dimensional model may miss important elements of the ice–ocean system. We only fully resolve one dimension in either the ice or the ocean, and we do not include any representation of ocean currents or mixing except in the ice-contact meltwater plume. Our model is the simplest model that can capture the mechanics of MISI; indeed, it is mostly the same as the 1-D model that Schoof (2007) used to define the modern theoretical understanding of MISI. More advanced ice and ocean models are needed to fully explore lateral buttressing and ocean circulation in the sub-ice cavity. The exact values of collapse timing, sea level rise rate, success probability, and “point of no return” (the date at which an intervention would no longer be effective) will change with more advanced models, different forcings, and different intervention designs. The robust conclusions that can be drawn from our results are as follows: (1) regrounding an ice shelf would slow an ongoing collapse, and (2) regrounding is more likely the more warm water is blocked from reaching the ice base. Neither of these two points is controversial (Joughin et al., 2014; Seroussi et al., 2017), but taken together they suggest that consensus ice physics provide an opening for a large-scale civil engineering project to make a meaningful difference in the probability of an ice sheet collapse.

One of the biggest potential failure points that must be addressed in future models is ice shelf disintegration caused by summer surface melt. The intervention we proposed relies on the buttressing force provided by the floating ice shelf in order to work, but surface meltwater damages the structural integrity of ice shelves and can cause them to disintegrate catastrophically, as Larsen B did in 2002 (Scambos et al., 2003). However, some ice shelves are protected by surface rivers that efficiently export meltwater off the shelf (Bell et al., 2017). Future research is required to determine the extent to which surface meltwater reduces the probability of success for glacial geoengineering, to quantify how that probability reduction depends on atmospheric warming and hence on carbon emissions, and to determine whether it would be possible to deliberately modify supraglacial hydrology so as to encourage meltwater export.

Regardless of whether or not the intervention is successful, it is likely to have unintended consequences. One of the advantages of locally targeted geoengineering is that many of those unintended consequences are likely to also be local in nature. In the case of an artificial sill, changes to the local ocean circulation will be extensive by design, and turbidity will be increased during construction. Both of these are likely to have effects on marine biology. Not only must all side effects be addressed in detail before the sill could actually be built, but an additional set of moral and political questions must be addressed as well.

One of those questions is the issue of decision-making. The mass balance of Greenland and Antarctica affects nations around the globe, but no legal mechanism currently exists for deciding how humanity should go about trying to control those ice sheets. Antarctica is governed by the Antarctic Treaty, but the Greenland Ice Sheet is under the sovereign control of a specific nation, with a local population of 58 000 in a semi-autonomous relationship with Denmark (CIA, 2013). We do not know whether authority over geoengineering legally resides with Copenhagen or with Nuuk, but morally we do not believe that geoengineering should proceed in Greenland without the consent of the Greenlandic people.

Another question is moral hazard, the risk that geoengineering may be used as a political argument to justify continued carbon emissions, and that research into it will therefore undermine climate mitigation. We could counter this by pointing out that MISI may have already begun in the Amundsen sector (Favier et al., 2014; Joughin et al., 2014; Rignot et al., 2014). If that is so, then humanity will still have to deal with an ice sheet collapse even if we stopped all emissions tomorrow. However, the point is moot if knowledge of geoengineering does not actually decrease people's support for climate mitigation, and empirical support for the moral hazard hypothesis within the social science literature is mixed (Burns et al., 2016). Properly contextualized discussion of geoengineering can actually increase concern for climate change (Kahan et al., 2015; Merk et al., 2016), consistent with other research demonstrating that positive, practical, or solution-based messaging is more effective at communicating climate science than negative, apocalyptic, or fear-based messaging (Feinberg and Willer, 2011; O'Neill and Nicholson-Cole, 2009).