Illuminating the pathway to destruction Arctic lakes are an important source of atmospheric CO ₂ and therefore play a role in climate change. It is thus vital to know how the rapid Arctic warming will affect them. Cory et al. now show that light is the biggest culprit in the breakdown of carbon from thawing permafrost soils (see the Perspective by Tranvik). This carbon then moves out into Arctic lakes and streams. Contrary to previous expectations, these photochemical processes cause much more destruction of the organic molecules in fresh water than bacterial respiration does. Science , this issue p. 925 ; see also p. 870