Heat pumps – heating and cooling systems that run entirely on electricity – have been getting a lot of attention recently. They’ve been called the “most overlooked climate solution” and “an answer to heat waves.” And the technology is finally experiencing a global boom in popularity. Last year, 117 million units were installed worldwide, up from 90 million in 2010. As temperatures and greenhouse gas emissions rise, heat pumps, which can be easily powered by renewable energy, promise to provide a pathway to carbon-free home heating. Environmental activist Bill McKibben even suggested sending heat pumps to Europe to help wean the continent off Russian natural gas.