Studies suggest that much of the Greenland Ice Sheet is accelerating as temperatures increase. Increased temperatures can raise the pressure in the subglacial drainage system as meltwater input increases and the system fails to efficiently evacuate it. Some studies suggest that increased temperatures and meltwater input will slow glaciers down by causing earlier channelization, allowing stable conduit expansion and efficient drainage of input. However, studies also suggest that even channelized systems fail to evacuate meltwater when input is rapid. This study’s modelling of the subglacial drainage system of Petermann Glacier suggests that conduits cannot quickly accommodate rapid changes in meltwater input, such as from lake drainages or warm phases, causing subglacial water pressure to reach ice overburden pressure for as long as 5 days. High ice thickness at Petermann Glacier causes rapid conduit closure, making the sustaining of a sustainable conduit system especially difficult. Along with other research, this study suggests that the subglacial drainage system of Petermann Glacier is key to its dynamics. Lake drainages play a large role in perturbing the system, but their role is limited by the short melt season which means many lakes freeze-over before draining. Warming is expected to cause lake drainage at higher elevations, causing more frequent basal pressure spikes and over a larger area. Petermann Glacier is therefore expected to contribute more to sea-level as temperatures increase.