A loss of snow and ice cover are the main reasons for a reduction in the Arctic’s ability to reflect heat, not soot as had been previously thought. The capacity of the Arctic to reflect heat is determined by something known as the albedo effect. This is a measurement of how well a surface, such as snow or ice, bounces sunlight back into space.
Scientists say soot is not the major contributor, as levels have dropped recently, while warming has continued. The Arctic region has warmed significantly since the 1980s, up to three times as much as the average seen elsewhere across the globe. Much of this warming has been attributed to the reduction of the surface albedo effect.
When sunlight hits a white surface such as snow and ice, more of it is reflected back into space without warming its surroundings than when light hits a darker surface. Thus, darker surfaces tend to absorb more heat. As the albedo effect in the Arctic is reduced, there is a positive feedback effect because, as the region warms, more and more ice and snow cover is lost. As a result, more dark areas are left exposed to sunlight.
Source: https://www.bbc.com/news/science-environment-50381328