Interest in so-called renewable sources of energy has risen in concert with rising concerns in the U.S. and worldwide about climate change. Ample scientific evidence exists that links the greenhouse gases and other compounds emitted from the combustion of nonrenewables such as fossils fuels (coal, oil and natural gas) with undesirable effects on both the world's climate and human health.

There are five basic types of renewable energy sources. These are biomass, hydropower, geothermal, wind and solar. Renewable resources have the advantage of being self-replenishing: The world will never run out of them. They carry the disadvantage, however, of being "flow-limited," meaning that humans cannot simply ramp up the supply of these fuels in response to growing demand. If a hydroelectric plant is built on a river with a flow that inexorably diminishes over time, there is little to nothing engineers can do to drive more water through the hydro turbines at the plant.

When the U.S. population was far smaller than it is today and energy technology was in its relative infancy, burning wood, although labor-intensive, was sufficient to meet the nation's energy needs. Through the mid-1800s, there were no electrical appliances, and heating and cooking needs were the main drivers of seeking out combustible fuels of any sort. Then the Industrial Revolution and the development of electrical power followed, and for the last 150 years or so, fossil fuels have provided the overwhelming majority of humankind's energy needs both in the U.S. and worldwide.

Renewables have been a major "should" in conversations about energy sources for decades, but only in the 1990s did their use really start to take off in the U.S. As of 2017, 11 percent of all energy and 17 percent of electricity was produced using a renewable resource, and 57 percent of renewable energy was dedicated to generating electrical power.

A list of renewable resources and the amount of energy derived from each can be found on the Energy Information Administration site in the Resources.

Energy from the sun can be collected and converted to heat and electricity in a number of different ways. The obvious pitfall with relying on this type of renewable resource is that the sun is not always visible, and even in the half-day or so that the sun is above the horizon in most places, cloud cover can render the amount of radiant solar energy negligible on some days. Because electricity cannot be stored in large amounts (batteries, while useful, hardly represent a substantial electricity reserve), solar power is not as useful for around-the-clock needs. Still, arrays of photovoltaic (PV) cells in sunny areas can provide enough power for a small community.

Hydro power (or hydropower, as it is sometimes written) is power generated by the kinetic energy of flowing water. Water has mass, often lots of it, and flowing water obviously possesses some measure of velocity; energy is nothing more than the product of mass times the square of velocity multiplied by a constant. Like sunlight, the amount of water flowing into a given area is not entirely predictable, although hydro projects usually fraught with less uncertainty than solar or wind in terms of availability of the resource.

Hydro power was the primary renewable energy resource in the U.S. as of 2018, although its share among renewables is declining as renewables as a commodity become more prevalent overall. The major consideration with this type of power is that it can disrupt ecosystems and wildlife habitats. Since many hydro projects involve dams, the resulting artificial lakes can literally flood creatures out of their homes.

Wind is the movement of air, and this movement is caused by the fact that the Earth's surface varies a great deal from place to place (e.g., water here, a desert there, mountains over there) and these different surfaces absorb and release heat from the sun in different ways. Generally, air over land warms and rises, and cool air from over oceans rushes in to replace it; in the evenings, the wind blows back toward the water. Wind is therefore really a form of solar energy, although the physical rotation of the planet on its axis does contribute to wind currents to some extent.

Wind power is wonderfully inexpensive, but alas, the unpredictability of wind patterns makes it a less than optimal choice for power generation on significant scales.

Also called biomass, biofuels represent a diverse and rapidly expanding form of renewable energy. Various material from living things can be converted to energy, from decaying plant matter (including wood and the waste from wood-processing centers) to garbage to manure and sewage. Biofuels such as ethanol (a biogas) can assume some of the the same roles as traditional gasoline and diesel fuel.

Not only do these fuels reduce the "carbon foorprint" of the municipality or entity using them, they also dispose of waste in an extremely useful way, making for a win-win. Whereas fossil fuels release long-stored carbon dioxide into the atmosphere when they are burned, plants, a major contributor to biofuels, actually take up carbon dioxide that is released when biofuels are burned, making for a more cyclical scheme.

This kind of power is derived from heat energy released from deep within the Earth itself thanks to radioactive decay processes in rocks far beneath the planet's surface. Its high reliability and the fact that it can be generated locally make it an increasingly attractive renewable resource option.

Heat moves from the center of the Earth (the core) upward through the mantle and finally to the 3- to 5-mile-thick crust. People can tap the resulting hot underground springs and use the heat to power a variety of processes. This renewable is, by definition, not going away, but it is perhaps more potent than many people realize: The center of the Earth is, believe it or not, warmer than the surface of the sun!

A strict renewable resources definition would omit nuclear power from consideration, because nuclear power relies on uranium, an element that is not in infinite supply. Instead, nuclear power is grouped with the renewables in the sense of being "clean," or free of waste products that contribute to pollution and global warming.

In this kind of power generation, uranium atoms are split in a process called nuclear fission that releases enormous quantities of energy per unit mass. This energy is used to drive steam turbines. The specter of radioactive fallout reaching the environment as a result of nuclear reactor mishaps has plagued the industry for decades, but that has not stopped its overall progress and development.

So if you're interested in "going green" yourself but have no idea where to begin, how do individuals and businesses go about implementing renewables in their own everyday lives?

One obvious, though not always practical, way is to generate energy from renewables yourself, in the location where it will be used. This could mean installing PV solar cells on the roof of your home or, if you're a developer or administrator, an office or school building. Private geothermal heat pumps and heat and power derived from biomass are other options. You may also be able to buy renewable energy from your electric company if it offers a "green pricing" or "green marketing" option. Coordinating with your municipal government is a great place to start here.