stylistic:tone
Observed surface melting

Currently, the total accumulation of ice on the surface of Greenland ice sheet is larger than either outlet glacier losses individually or surface melting during the summer, and it is the combination of both which causes net annual loss. For instance, the ice sheet's interior thickened by an average of 6 cm (2.4 in) each year between 1994 and 2005, in part due to a phase of ] increasing snowfall. Every summer, a so-called snow line separates the ice sheet's surface into areas above it, where snow continues to accumulate even then, with the areas below the line where summer melting occurs. The exact position of the snow line moves around every summer, and if it moves away from some areas it covered the previous year, then those tend to experience substantially greater melt as their darker ice is exposed. Uncertainty about the snow line is one of the factors making it hard to predict each melting season in advance.

A notable example of ice accumulation rates above the snow line is provided by Glacier Girl, a Lockheed P-38 Lightning fighter plane which had crashed early in World War II and was recovered in 1992, by which point it had been buried under 268 ft (81 + 1 ⁄ 2 m) of ice. Another example occurred in 2017, when an Airbus A380 had to make an emergency landing in Canada after one of its jet engines exploded while it was above Greenland; the engine's massive air intake fan was recovered from the ice sheet two years later, when it was already buried beneath 4 ft (1 m) of ice and snow.

While summer surface melting has been increasing, it is still expected that it will be decades before melting will consistently exceed snow accumulation on its own. It is also hypothesized that the increase in global precipitation associated with the effects of climate change on the water cycle could increase snowfall over Greenland, and thus further delay this transition. This hypothesis was difficult to test in the 2000s due to the poor state of long-term precipitation records over the ice sheet. By 2019, it was found that while there was an increase in snowfall over southwest Greenland, there had been a substantial decrease in precipitation over western Greenland as a whole. Further, more precipitation in the northwest had been falling as rain instead of snow, with a fourfold increase in rain since 1980. Rain is warmer than snow and forms darker and less thermally insulating ice layer once it does freeze on the ice sheet. It is particularly damaging when it falls due to late-summer cyclones, whose increasing occurrence has been overlooked by the earlier models. There has also been an increase in water vapor, which paradoxically increases melting by making it easier for heat to radiate downwards through moist, as opposed to dry, air.

Altogether, the melt zone below the snow line, where summer warmth turns snow and ice into slush and melt ponds, has been expanding at an accelerating rate since the beginning of detailed measurements in 1979. By 2002, its area was found to have increased by 16% since 1979, and the annual melting season broke all previous records. Another record was set in July 2012, when the melt zone extended to 97% of the ice sheet's cover, and the ice sheet lost approximately 0.1% of its total mass (2900 Gt) during that year's melting season, with the net loss (464 Gt) setting another record. It became the first directly observed example of a "massive melting event", when the melting took place across practically the entire ice sheet surface, rather than specific areas. That event led to the counterintuitive discovery that cloud cover, which normally results in cooler temperature due to their albedo, actually interferes with meltwater refreezing in the firn layer at night, which can increase total meltwater runoff by over 30%. Thin, water-rich clouds have the worst impact, and they were the most prominent in July 2012.

Ice cores had shown that the last time a melting event of the same magnitude as in 2012 took place was in 1889, and some glaciologists had expressed hope that 2012 was part of a 150-year cycle. This was disproven in summer 2019, when a combination of high temperatures and unsuitable cloud cover led to an even larger mass melting event, which ultimately covered over 300,000 sq mi (776,996.4 km) at its greatest extent. Predictably, 2019 set a new record of 586 Gt net mass loss. In July 2021, another record mass melting event occurred. At its peak, it covered 340,000 sq mi (880,596.0 km), and led to daily ice losses of 88 Gt across several days. High temperatures continued in August 2021, with the melt extent staying at 337,000 sq mi (872,826.0 km). At that time, rain fell for 13 hours at Greenland's Summit Station, located at 10,551 ft (3,215.9 m) elevation. Researchers had no rain gauges to measure the rainfall, because temperatures at the summit have risen above freezing only three times since 1989 and it had never rained there before.

Due to the enormous thickness of the central Greenland ice sheet, even the most extensive melting event can only affect a small fraction of it before the start of the freezing season, and so they are considered "short-term variability" in the scientific literature. Nevertheless, their existence is important: the fact that the current models underestimate the extent and frequency of such events is considered to be one of the main reasons why the observed ice sheet decline in Greenland and Antarctica tracks the worst-case rather than the moderate scenarios of the IPCC Fifth Assessment Report 's sea-level rise projections. Some of the most recent scientific projections of Greenland melt now include an extreme scenario where a massive melting event occurs every year across the studied period (i.e. every year between now and 2100 or between now and 2300), to illustrate that such a hypothetical future would greatly increase ice loss, but still wouldn't melt the entire ice sheet within the study period.