Some of it can come from social engineering instead of HVAC technical breakthroughs -- automated thermostats like the Nest, hand-held body-coolers like http://

Some of it can come from social engineering instead of HVAC technical breakthroughs -- automated thermostats like the Nest, hand-held body-coolers like http://news.mit.edu/2013/madmec-design-competition-1017 http://news.mit.edu/2013/madmec-design-competition-1017 Better summer clothes will help too. I've been pretty impressed by advances in cooling undergarments that are designed to wick away moisture and encourage air flow. Anyway, all of these seemingly unrelated things contribute to improved AC efficiency.