Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory

About 40 percent of Greenland's ice sheet thawed at or near the surface on July 8. Four days later, the melt had dramatically accelerated and an estimated 97 percent of the ice sheet surface had thawed.

Three satellites found that 97 percent of Greenland -- the land mass second only to Antarctica for its volume of ice -- underwent a thaw never before seen in 33 years of satellite tracking, NASA reported Tuesday.

Satellite experts at first didn't trust their readings, especially since they showed an incredible acceleration. Over four days, Greenland's ice sheet -- which covers 683,000 square miles -- went from 40 percent in thaw to nearly entirely in thaw.

"This was so extraordinary that at first I questioned the result: Was this real or was it due to a data error?" Son Nghiem of NASA's Jet Propulsion Lab in Pasadena, Calif., said in NASA's statement about the findings.

Scientists on the ground in Greenland had been reporting an unusually warm summer thaw, including damage at a snow airfield and strong runoff threatening a bridge, Tom Wagner, who manages NASA's ice research programs, told NBC News.

Ice cores from Greenland's highest region do reveal that such island-wide thaws have happened every 150 years or so, at least over the last few thousand years, but the fear now is that it might occur much more frequently due to warming sea and air temperatures.

"We can't lose sight of the fact that Greenland's ice sheet is losing 150 gigatons of ice a year," Wagner said. That translates into raising sea levels by one-one hundredth of an inch. Additionally, the danger of greater warming and greater melt persists. 

"If we continue to observe melting events like this in upcoming years, it will be worrisome," said Lora Koenig, a NASA glaciologist who helped analyze the satellite data.

Monitoring stations on land "showed temperatures above freezing, confirming that the surface was melting for the entire ice sheet," Konrad Steffen, director of the Swiss Federal Institute for Forest, Snow and Landscape Research, told NBC News.

Since then, he added, "temperatures have fallen below freezing for the higher elevations but still are melting below 1500 meters."

The director of the top ice research center in the U.S. said the discovery fits into "the larger picture of a strongly warming Arctic."

"Arctic sea ice extent this summer is so far tracking at very low, near record levels, and the ice cover is unusually diffuse," Mark Serreze of the National Snow and Ice Data Center told NBC News.

On top of that, he said, the seasonal melt that followed the 2012 winter "started unusually early over most of the Arctic Ocean."

The center's latest report, issued Tuesday, noted that" Arctic sea ice continued to track at levels far below average through the middle of July, with open water in the Kara and Barents seas reaching as far north as typically seen during September."

Thomas Mote, a University of Georgia climatologist who looked at the satellite data, said the melt followed an unusual series of warm air ridges over Greenland since late May, with the strongest coinciding with the rapid thaw in mid-July.

Each successive ridge, Mote told NBC News, was "stronger than the previous one" and it looks like the pattern has finally broken down. 

The ridges happened just as a cyclical weather phase known as the North Atlantic Oscillation shifted. "Together, they produced near perfect conditions for this event," Mote added.

Related: Huge Greenland iceberg breaks off glacier

Because they hold so much ice on land, Greenland and Antarctica have the potential to raise sea levels significantly if warming continues or worsens. 

Sea levels have already risen by about 8 inches in the last century, partly due to some ice melt but also thermal expansion caused by warming seas.

The U.N. climate panel estimates sea level could rise between 7 inches and nearly two feet this century -- the latter a scenario that could prove catastrophic for many coastal areas around the globe.

NASA said researchers had not yet determined whether this summer's Greenland thaw would be significant enough to raise sea levels.

Greenland has enough ice to raise sea levels by 23 feet if it all melted off.

A recent study found that it could take a long-term increase in global temperatures of just 2-3 degrees Fahrenheit to completely melt Greenland's ice sheet in 2,000 years. 

More world stories from NBC News:

• Going for gold: British workers cash in on Olympics with strike threats
• Afghan police commander leads defection to Taliban
• Norway to London: One family's Olympic odyssey
• Reports: Workers told to underplay Fukushima radiation
• US F-16 fighter jet crashes off coast of Japan
• Gunman in Afghan police uniform kills 3, wounds several
• Explosion, fire shuts down Turkey-Iraq oil pipeline; PKK blamed
• Assad reportedly directs troops from tribal heartland as rebels flood capital

Follow World News on NBCNews.com on Twitter and Facebook

Kathryn Hansen / NASA

Arctic melt ponds visited during a July 2011 expedition on the U.S. Coast Guard Cutter Healy gave scientists a chance to find "windows from the sky to the ocean" that are perfect for phytoplankton blooms.

Experts were shocked to find a thick, 60-mile-long "phytoplankton megabloom" under Arctic sea ice, announcing in a study Thursday that ice made thinner by warming temperatures has, for now at least, created ideal conditions for the microscopic, single-cell plants to flourish.

More blooms are likely hidden under the ice, making for "ecological shifts" in Arctic waters that favor some species over others since phytoplankton are the base of the marine food chain, Stanford professor and lead researcher Kevin Arrigo told msnbc.com.

Scientists had thought Arctic phytoplankton blooms only happened after sea ice melted in summer, so the discovery is "like finding the Amazon rainforest in the middle of the Mojave Desert," added Paula Bontempi, who manages the ocean biology program at NASA, which funded the research.

"The waters literally looked like pea soup," Arrigo said at a press conference announcing the study in the journal Science. "It was as thick as a 5-year-old child is tall."

The team discovered the bloom in July 2011 in thin sea ice pocketed with ponds of melted ice on the Chukchi Sea off northern Alaska. Arctic sea ice has been shrinking and thinning in summer since 1979, the result of warming temperatures over the region. 

Those melt ponds proved crucial, allowing just enough light to get the growth process started while also protecting the algae from ultraviolet radiation.

"They were the windows from the sky to the ocean," said researcher Don Perovic, an ice scientist with the U.S. Army Corps of Engineers.

"If I were a phytoplankton," Perovic added, "that's where I'd want to live."

Arrigo said in his 25 years of studying phytoplankton blooms he had never seen one this large. Blooms in open water are much smaller, he noted, while very thick ice won't allow any light in to start photosynthesis.

"It's going to be a more productive system," Arrigo said, noting that plankton bottom feeders will benefit as the plankton sinks to the bottom of the Chukchi, much of which is around 160 feet deep.

The researchers didn't expect Arctic sea ice to disappear completely, since winters are still very cold, but they did note some potential downsides.

Some fish species that rely on mid-level nutrients will suffer, Arrigo said, and the bigger issue is that a warming Arctic appears to be triggering phytoplankton blooms earlier.

Species that can't adapt "to be there at the right time of year" will suffer, Arrigo said.

NASA funded the expedition as a way to match the satellite-based data it gathers on the Arctic with data gathered on the ice.

More content from msnbc.com and NBC News:

• Crackdown on painkiller abuse fuels new wave of heroin addiction
• Jerry Sandusky trial: Many jurors have Penn State ties
• Judge grants Zimmerman another bond hearing
• Video: Sex offender running for school board post
• Dog gets starring role in study on what stresses killer whales

Follow US News on msnbc.com on Twitter and Facebook