Like clothing fashions and musical styles, our ideas about what the future will bring change over time. In the early days of the 20th Century, the visions of science fiction writers like Jules Verne and H.G. Wells informed our picture of the years to come. Years later, after the Soviet Union launched Sputnik into orbit, the future became an extraordinary place, teeming with silver rockets and robot butlers. Today, as we face the twin dangers of climate change and dwindling natural resources, much talk of the future centers on alternative energy.

But when we speak of alternative energy, what do we mean? Thinking about it, there may seem to be an abundance of viable options when it comes to power generation: Oil, coal, natural gas... the list goes on.

But in a world beset by pollution and international tensions, continuing to rely on a supply of fossil fuels that grows smaller every day is an exercise in futility. Our ability to develop newer and cleaner methods of powering our homes and vehicles is carries with it an implication that those methods be free from the limitations we currently face. The subtext of alternative energy is sustainability, and with it the drive to increase the ubiquity of renewable fuels.

So what does the future hold? Making any predictions at all is risky; nobody wants to be wrong, lest they end up looking as silly as one of the post-Sputnik robot butlers. But based on emerging technologies, we can determine what kinds of alternative energy might become popular in the next few decades.

Solar Power. The most ready and visible source of alternative energy in the solar system, our sun is expected to last roughly another four and a half billion years before it consumes the last of its hydrogen gas. Solar power utilizes photovoltaic cells to convert sunlight into usable energy. As with many alternative energy sources, businesses and utilities in the United States have been slow to embrace solar power, but scattered steps have been taken: A southern California utility recently announced plans to spend nearly $900 billion for a network of 250 megawatts of photovoltaic power generation in the Mojave desert, and home improvement retailer Home Depot has begun offering solar installation service to its customers.

Hydrogen Fuel Cell. For as long as most of us can remember, we’ve been powering most of our vehicles with internally-combusted gasoline. But when Toyota introduced its Prius gas-electric hybrid to the market, it seemed like history went out the window. The growth of hybrids makes business and industry pursue more and better ways to fuel cars than with gasoline. One of the answers is hydrogen gas, which can be drawn from both conventional and alternative energy sources (such as diesel and biodiesel) to power a fuel cell, where it is reacted with oxygen to produce electricity, which powers the car’s motor. The only by-product is water.

Biomass and Biofuels. Every time you build a campfire, you’re using biomass to create energy. In the very low-tech case of a campfire, dead and dried biological material is used to produce heat for warmth and cooking (as well as light). Biomass is useful in processes like pyrolysis, which burns organic products to produce synthetic gas. Biomass can also be converted to liquid fuels that can be burned in an internal combustion engine, or have their hydrogen gas extracted and used via fuel cell. It’s important to note that burning biomass can produce excess carbon, which can contribute to global warming. But unlike fossil fuels, biofuels are sustainable.

Wind Energy. The conversion of wind into a more useful form of energy is one of the oldest forms of alternative energy in the world. Throughout North America, farmers have used windmills to pump water to irrigate their crops since the 1800s. Today, windmills are used as giant, electricity-generating turbines, and “wind farms” dot the American southwest in states like Texas and California.

Wave Power. The world’s first commercial wave farm was established in 2007 off the coast of Portugal. Though still under review, the Aguaçadora Wave Park is only the first in a series of installations across Europe intended to generate wave power. The key is the Pelamis Wave Energy Converter, a device at the forefront of wave energy generation. Still in its relatively early stages, the Pelamis device is designed to survive at sea – which means it only absorbs a small amount of energy from each wave.