It is one of the great dilemmas of climate change: We take such comfort from air conditioning that worldwide energy consumption for that purpose has already tripled since 1990. It is on track to grow even faster through mid-century - and assuming fossil-fuel–fired power plants provide the electricity, that could cause enough carbon dioxide emissions to warm the planet by another deadly half-degree Celsius. A paper published Tuesday in the Nature Communications proposes a partial remedy: Heating, ventilation and air conditioning (or HVAC) systems move a lot of air. They can replace the entire air volume in an office building...