Antarctic and arctic meltwater is "bad" because it's dark, but why is transparent liquid on white stuff so dark? The Washington Post's Antarctic heat wave melted 20 percent of an island’s snow cover in days, caused melt ponds to proliferate includes the figure below of meltwater ponds on top of snow/ice.

The article is a good read, but my question is one of optics, namely what makes a pond of meltwater (essentially pure water) on top of something so white become so dark? 

It increases sunlight absorption and heating, which in this case is "bad" but what's the physics or phenomena behind this that makes these so dark in the first place?

If I understand correctly, even shallow ponds or puddles of water can significantly reduce albedo, increase absorption and induce further melting, sometimes in a "runaway" manner. So I don't think it's bulk absorption by the water or contaminants, I think there's something more interesting going on.






  Melt ponds on the Antarctic Peninsula’s George VI Ice Shelf on Jan. 19, 2020. (NASA)
Water has lowest EM absorption in the blue part of light spectrum and increases rapidly  towards both UV and red parts of spectrum. 



As a result in visible light water is blue. Same goes for the ice as it has very similar absorption spectrum. 

While there is a lot of white in the picture, all of it is a thin snow cover on top of blue ice. Once the snow melts you get a blue water on top of thick layer of transparent blue ice that combined absorb most of the light making ponds looks exceptionally dark. Water would be so much lighter in colour if, for example, instead of transparent ice, you'd have sand on the bottom.