Scientists define renewable energy as sources that are easily replenished—like the sunlight that will reliably appear tomorrow at dawn (even if temporarily hidden behind clouds) or the wind that will resume blowing soon enough. Or even biomass like wood, which is commonly used to heat stoves in poorer nations. Or geothermal energy, a source of intense heat buried beneath the earth’s surface.

But fossil fuels—like oil, coal and natural gas—come literally from fossils, which may have taken hundreds of millions of years to develop and cannot be replaced anytime soon. Experts in climate change advocate greater use of renewables because their global-warming carbon emissions are dramatically lower than fossil fuels. Nuclear energy has a lower carbon footprint but comes at a high economic cost and considerable dangers to public safety. In this episode of Humankind, we examine the massive shift to greater use of climate-friendly, low-carbon renewable energy, which has gained considerable momentum in recent years. Bill Moomaw, one of the world’s top climate scientists, explains dramatic trends in adoption of renewables; Green Mountain Power CEO Mary Powell tells how an electricity provider in Vermont is trying to facilitate this change; and John Dillon, Vermont Public Radio news director, describes that state’s approach to promoting greater use of renewables, including the limitations of doing so.

I’m optimistic in the sense that the technology is clearly here. It has improved tremendously, in a very short period of time. In fact, it has been adopted on a very large scale… You look at China where they’ve installed a lot of wind [power]; Germany gets 26 percent of its electricity from renewable energy now. And Germany’s a big industrial country. You know, they make Mercedes Benzs, and BMWs, and high-tech MRIs, and all sorts of things.”

—Prof. William Moomaw, Tufts University environmental policy expert and lead author on UN Intergovernmental Panel on Climate Change