The group of technologies widely considered to be “clean energy” include hydropower, geothermal, solar, wind, nuclear, bioenergy (at least in some circumstances), and even some extremely nascent technologies like ocean wave power. These energy sources are “clean” with regard to climate change because—unlike fossil fuels—when they produce energy they do not emit greenhouse gases, the type of pollution that is warming our planet. The most important of these gases is carbon dioxide (CO₂), so “clean” technologies can more precisely be referred to as low-carbon or carbon-free.

Clean energy technologies are in many ways very different from one another, but none directly emit CO₂.¹

“A key word there is directly,” says Jennifer Morris, a principal research scientist at MIT’s Joint Program on the Science and Policy of Global Change and the MIT Energy Initiative. Even if they do not produce emissions during operation, clean energy technologies all have some “embedded emissions,” like those associated with producing their equipment.

“When you start getting into life cycle assessments and you backtrack through all of the steps that it takes to get to the point that you’re producing energy or electricity, then of course there are emissions involved in the different steps along the way,” says Morris. “There’s no such thing as a true, perfectly clean energy source.”