Browned ice is simply vanishing in situ.
What makes the ice brown? You don't see that on thick ice, so is the ice so thin that the dirt (algae?) on the underside gets exposed? Or are those the remnants from black carbon that fell on the ice?
I remember this being discussed last year, and that algae is indeed a major culprit here. I had kind of forgotten about all that, so thank you.
The following paper may help, and the following quote from it. Sorry, don't have a proper citation
https://arxiv.org/pdf/1702.03325.pdf Light Absorption in Arctic Sea Ice – Chlorophyll and Black Carbon O. Ogunro et al
"In the boreal spring, light absorption by chlorophyll present in certain layers of Arctic sea ice
may thus be significant when compared to the contribution of anthropogenic BC. Some
biological habitats including the bottom and infiltration layers are quite absorptive, approaching
or surpassing the value for black carbon. During the contemporary period, it is suspected that
thinning sea ice will lift radiation restrictions and allow biological activity to increase.
Meanwhile black carbon point sources are being restricted under new environmental regulations
in most developed countries at the Northern Hemisphere [Bond et al., 2010]."
BTW: Anthropogenic carbon has gone down with the global slowdown, but deposited BC from Siberian fires may have gone up.