> I am surprised no one has mentioned carbon dioxide as a potential problem for hives in some northern locations.


As argued in this study, it may be that increased carbon dioxide is the desired state in a winter cluster to induce hypoxia, which helps conserve winter stores. I get the sense that the basic thermodynamics going on in a winter cluster are only a part of the puzzle and that a measure of bee regulated metabolic control may play an equally important role in successful wintering. If bees are trying to regulate both heat and a balance between atmospheric gases in and around the cluster, it's worth considering what beekeeper provided ventilation may be doing to the latter.  As a beekeeper in a cold climate, I keep thinking about the conditions, physical and otherwise, that a natural bee-selected cavity provides for wintering. As such, I insulate heavily but don't provide any additional ventilation except for what naturally occurs at the lower front entrance.  


>Experiments during three winters have revealed a metabolism controlling function of bee-induced hypoxia in the winter cluster. Permanent low oxygen levels around 15% were found in its core. This hypoxia was actively controlled, probably via indirect mechanisms. Varying ambient oxygen levels demonstrated a causal relationship between lowered oxygen and reduced metabolic rate (MR). Under deeper ambient hypoxia the bees switched to ultra low MR (ULMR), optional-occasional at 15% oxygen, obligatory at 7.5% oxygen. This dormancy status resembled deep diapause in insects. It stayed reversible after at least several days, and was terminated under normal oxygen at 15°C. Reduced MR via core-hypoxia is essential in water conserving thermoregulation of the wintering cluster. It allows bees to reconcile warm wintering in alert state—for defence of stores—with energy saving and longevity. 

>Hypoxia-Controlled Winter Metabolism in Honeybees (Apis mellifera)

Karel Van Neruma and Herman Buelensa

Bill Hesbach
Northeast USA

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