Scientists were alarmed to learn that for the first time in history, heavy rain was falling on the top of the ice sheet on the Danish island of Greenland. NASA also analyzed the trigger.
Quoted from the official NASA website, the Greenland ice sheet experienced two intense melts in July 2021, and forecasts warn there will be more similar events to follow. The heat of this year's summer triggered another major melt event on August 14-15, 2021. But this time, the melt was exacerbated by rainfall.
Every year from about May to early September, melting occurs in the vast ice sheets that cover Greenland. In addition to contributing directly to sea level rise, the meltwater can flow to the bottom of the ice sheet through fissures and moulins, accelerating the flow of ice into the ocean.
In melting seasons, it is sometimes possible to have more melting events in a short period of time than 'regular' summer days.
The seventh largest melting event on record (by area) occurred on July 28, when the melt covered about 881,000 square kilometers of ice sheet, according to National Snow and Ice Data Center data. The thaw on August 14, which is the peak of an unusual late summer event, was slightly smaller, covering about 872,000 square kilometers.
"The pattern of melting is different for each event. While the late July thaw event was widespread in northern Greenland, the August event was focused on southern Greenland," said glaciologist Lauren Andrews of NASA's Global Modeling and Assimilation Office.
According to Andrews, on August 15, the empty ice area extended further inland. Andrews also noted that the melt extended inland towards the interior of the ice sheet and reached the Summit Station-National Science Foundation research station located near the top of the ice sheet.
"The thaw was triggered by warm air moving over Greenland and producing unusually high temperatures. The map above shows short-term warming in Greenland. It illustrates how much the air temperature on August 14, 2021, was above or below the temperature of the week before (an average of August 5-11 2021)," explained Andrews.
The map above is from the Goddard Earth Observing System (GEOS) model and represents the air temperature at 2 meters above the ground. The modeled data, which uses mathematical equations that represent physical processes, offer a broad and predictable view of a region where ground-based weather stations are rare.
Extensive rainfall in southern Greenland is contributing to the melt. Warm air alone, not rain, caused previous major melt events at Summit Station, including those in 2012 and 2019, according to Christopher Shuman, a glaciologist at the University of Maryland, Baltimore County.
Research has shown that the amount of melting in Greenland caused by rain has increased over the past few years, both in summer and winter.
Major thaw events, including those occurring in the 2021 season, are generally short-lived and contribute relatively small amounts to the total thaw that occurs throughout the season.
However, they can have a lasting effect on the ice sheet. Melting can trigger processes that cause surface ice to darken and modify snow and cypress below, which could exacerbate future melt and runoff, even under normal atmospheric conditions.
"During a thaw event, this process can occur in parts of the ice sheet that don't normally experience melting, making the impact more widespread. Feedback like this is starting to have an impact," he concluded.
