There's a Dark Zone on Greenland's ice sheet, and new drone surveys are showing that it's growing ever darker. On top of that, it also seems to be melting faster than it should - and scientists finally have a better idea why.

The Dark Zone is a stripe of fast-melting ice towards the south-west of the ice sheet. It's around 400 kilometres (248 miles) in length and 100 kilometres (62 miles) at its widest point.

It's also clearly visible in satellite imagery.

Apart from being strangely dark, it's also known as the ablation zone, because this area is melting faster than other parts of the ice sheet.

In 2014, field researchers attributed this melting to mineral and microbial debris on the surface of the ice.

Now new drone research conducted by the Norwegian Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) confirms that data - and shows how widespread the problem really is.

"What we show is that the Dark Zone is covered in a finely distributed layer of dust, and black carbon, which provide nutrition for dark coloured algae. These are the main cause of the darkening," explained co-author Alun Hubbard, a glaciologist with CAGE.

Far from being a problem just for Greenland, this speedy melting is actually bad news for the entire planet, especially the areas bound to be affected by a sea level rise.

Right now, global sea levels are creeping up by about 3 millimetres per year, but that figure is not steady - every 12 months it jumps by another 0.08 millimetres, and researchers know that the melting of Greenland is a particularly strong contributor to this awful trend. 

The mechanism behind the Dark Zone's accelerated melting is simple. Because darker hues absorb more radiation and lighter hues reflect more, when the ice is darker, it absorbs more heat from the Sun, and ends up melting faster.

How much light is reflected by any given surface is called its albedo - a high albedo means high reflectivity. The relatively low albedo of the Dark Zone means the ice is melting up to five times faster than pure white snow.

When the ice melts, it also loses reflectiveness, and liquid water fosters the growth of algae. Even a slight increase in temperature can make the ice melt, and the algae bloom. This is common for spring and summer.

"The algae need nutrients and food, essentially dust, organic carbon, and water," Hubbard said.

"In summer, these are plentiful and the algal bloom takes off. Because algae are dark in colour – they reinforce the dark zone. Thereby you get a positive feedback effect where the ice sheet absorbs even more solar radiation producing yet more melt."

Using drones has several advantages over satellite data. You can control a drone on the spot and manoeuvre it to the areas you want it to examine - and they're much closer to the ground. This means that they are able to capture fine details that satellites in orbit are too far away to pick up.

By combining the two technologies, the team was able to get a better picture of the Dark Zone than ever before.

They determined that meltwater sloshing across the surface of the sheet along with cryconite - the darkest type of dust in some pockets - play a relatively small role, even though previously they were thought to be larger factors in lowering the ice sheet's albedo.

Meanwhile dust, black carbon (soot from wildfires and combustion) and algae are responsible for 73 percent of the variability in albedo, they have found.

"We propose that the ongoing emergence and dispersal of distributed impurities, amplified by enhanced ablation and biological activity, will drive future expansion of Greenland's dark zone," the researchers wrote in their paper.

However, they note, further research is required to determine the extent of the effect melt-off and carbon nutrients on algae growth and concentration.

The team's paper has been published in the journal Nature Communications.