A stiff upper layer of ice that formed atop of the Greenland Ice Sheet during the Holocene era may be causing the deceleration of ice flow within, a new study suggests. A better understanding of the inner nature of the Greenland Ice Sheet (GrIS) is critical for estimating its mass loss in the future, and thus sea level rise. While the edges of the Greenland Ice Sheet (GrIS) have been melting at an accelerated rate over recent years, some mystery surrounds the center of this massive body of ice, which has grown thicker. It has previously been suggested that differences in snow accumulation affect the thickness of the ice, which in turn affects the ice flow within. To gain more insights, Joseph MacGregor et al. used radar data at various depths of the ice sheet to calculate its velocity over the last 9,000 years, or the last three quarters of the Holocene epoch. Their analysis suggests that the interior of the GrIS is flowing 95% slower now than it was on average during the Holocene. However, snow accumulation alone does not explain their data, the authors say. Rather, the data suggests that during the last glacial period, softer ice formed due to higher levels of atmospheric dust. Yet during most of the Holocene, lower amounts of dust resulted in stiffer ice. The authors propose that this stiffer ice is putting pressure on the soft ice below, stemming the flow of the GrIS. These findings suggest that the GrIS is not only responding to the modern climate, but is still being affected by long-term changes. A Perspective by Christine Schøtt Hvidberg provides more context.