Rapid ablation zone expansion amplifies north Greenland mass loss.

Since the early 1990s, the Greenland ice sheet (GrIS) has been losing mass at an accelerating rate, primarily due to enhanced meltwater runoff following atmospheric warming. Here, we show that a pronounced latitudinal contrast exists in the GrIS response to recent warming. The ablation area in north Greenland expanded by 46%, almost twice as much as in the south (+25%), significantly increasing the relative contribution of the north to total GrIS mass loss. This latitudinal contrast originates from a different response to the recent change in large-scale Arctic summertime atmospheric circulation, promoting southwesterly advection of warm air toward the GrIS. In the southwest, persistent high atmospheric pressure reduced cloudiness, increasing runoff through enhanced absorption of solar radiation; in contrast, increased early-summer cloudiness in north Greenland enhanced atmospheric warming through decreased longwave heat loss. This triggered a rapid snowline retreat, causing early bare ice exposure, amplifying northern runoff.

Direct measurements of meltwater runoff on the Greenland ice sheet surface.

Meltwater runoff from the Greenland ice sheet surface influences surface mass balance (SMB), ice dynamics, and global sea level rise, but is estimated with climate models and thus difficult to validate. We present a way to measure ice surface runoff directly, from hourly in situ supraglacial river discharge measurements and simultaneous high-resolution satellite/drone remote sensing of upstream fluvial catchment area. A first 72-h trial for a 63.1-km2 moulin-terminating internally drained catchment (IDC) on Greenland's midelevation (1,207-1,381 m above sea level) ablation zone is compared with melt and runoff simulations from HIRHAM5, MAR3.6, RACMO2.3, MERRA-2, and SEB climate/SMB models. Current models cannot reproduce peak discharges or timing of runoff entering moulins but are improved using synthetic unit hydrograph (SUH) theory. Retroactive SUH applications to two older field studies reproduce their findings, signifying that remotely sensed IDC area, shape, and supraglacial river length are useful for predicting delays in peak runoff delivery to moulins. Applying SUH to HIRHAM5, MAR3.6, and RACMO2.3 gridded melt products for 799 surrounding IDCs suggests their terminal moulins receive lower peak discharges, less diurnal variability, and asynchronous runoff timing relative to climate/SMB model output alone. Conversely, large IDCs produce high moulin discharges, even at high elevations where melt rates are low. During this particular field experiment, models overestimated runoff by +21 to +58%, linked to overestimated surface ablation and possible meltwater retention in bare, porous, low-density ice. Direct measurements of ice surface runoff will improve climate/SMB models, and incorporating remotely sensed IDCs will aid coupling of SMB with ice dynamics and subglacial systems.

Distinct patterns of seasonal Greenland glacier velocity.

Predicting Greenland Ice Sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms. We present a 5 year record of seasonal velocity measurements for 55 marine-terminating glaciers distributed around the ice sheet margin, along with ice-front position and runoff data sets for each glacier. Among glaciers with substantial speed variations, we find three distinct seasonal velocity patterns. One pattern indicates relatively high glacier sensitivity to ice-front position. The other two patterns are more prevalent and appear to be meltwater controlled. These patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient, distributed hydrologic networks to efficient, channelized drainage, while others do not. The difference may be determined by meltwater availability, which in some regions may be influenced by perennial firn aquifers. Our results highlight the need to understand subglacial meltwater availability on an ice sheet-wide scale to predict future dynamic changes. KEY POINTS: First multi-region seasonal velocity measurements show regional differencesSeasonal velocity fluctuations on most glaciers appear meltwater controlledSeasonal development of efficient subglacial drainage geographically divided.

Partitioning recent Greenland mass loss.

Mass budget calculations, validated with satellite gravity observations [from the Gravity Recovery and Climate Experiment (GRACE) satellites], enable us to quantify the individual components of recent Greenland mass loss. The total 2000-2008 mass loss of approximately 1500 gigatons, equivalent to 0.46 millimeters per year of global sea level rise, is equally split between surface processes (runoff and precipitation) and ice dynamics. Without the moderating effects of increased snowfall and refreezing, post-1996 Greenland ice sheet mass losses would have been 100% higher. Since 2006, high summer melt rates have increased Greenland ice sheet mass loss to 273 gigatons per year (0.75 millimeters per year of equivalent sea level rise). The seasonal cycle in surface mass balance fully accounts for detrended GRACE mass variations, confirming insignificant subannual variation in ice sheet discharge.

Elevation changes in Antarctica mainly determined by accumulation variability.

Antarctic Ice Sheet elevation changes, which are used to estimate changes in the mass of the interior regions, are caused by variations in the depth of the firn layer. We quantified the effects of temperature and accumulation variability on firn layer thickness by simulating the 1980-2004 Antarctic firn depth variability. For most of Antarctica, the magnitudes of firn depth changes were comparable to those of observed ice sheet elevation changes. The current satellite observational period ( approximately 15 years) is too short to neglect these fluctuations in firn depth when computing recent ice sheet mass changes. The amount of surface lowering in the Amundsen Sea Embayment revealed by satellite radar altimetry (1995-2003) was increased by including firn depth fluctuations, while a large area of the East Antarctic Ice Sheet slowly grew as a result of increased accumulation.