Many nations count on coal, oil and natural gas to supply most of their energy needs, but reliance on fossil fuels presents a big problem. Fossil fuels are a finite resource. Eventually, the world will run out of fossil fuels, or it will become too expensive to retrieve those that remain. Fossil fuels also cause air, water and soil pollution, and produce greenhouse gases that contribute to global warming. Renewable energy resources offer cleaner alternatives to fossil fuels. They are not completely problem-free, but they produce much less pollution and fewer greenhouse gases, and by definition, will not run out. Here are our main sources of renewable energy:

01 of 07 Solar Energy Stocktrek Images/Getty Images The sun is our most powerful source of energy. Sunlight, or solar energy, can be used for heating, lighting and cooling homes and other buildings, generating electricity, water heating, and a variety of industrial processes. The technology used to harvest the sun's energy is constantly evolving, including water-heating rooftop pipes, photo-voltaic cells, and mirror arrays. Rooftop panels are not intrusive, but large arrays on the ground can compete with wildlife habitat.

02 of 07 Wind Energy monbetsu hokkaido/Moment/Getty Images Wind is the movement of air that occurs when warm air rises and cooler air rushes in to replace it. The energy of the wind has been used for centuries to sail ships and drive windmills that grind grain. Today, wind energy is captured by wind turbines and used to generate electricity. Issues periodically arise about where turbines are installed, as they can be problematic for migrating birds and bats.

03 of 07 Hydroelectricity Water flowing downstream is a powerful force. Water is a renewable resource, constantly recharged by the global cycle of evaporation and precipitation. The heat of the sun causes water in lakes and oceans to evaporate and form clouds. The water then falls back to Earth as rain or snow and drains into rivers and streams that flow back to the ocean. Flowing water can be used to power water wheels that drive mechanical processes. And captured by turbines and generators, like those housed at many dams around the world, the energy of flowing water can be used to generate electricity. Tiny turbines can even be used to power single homes. While it is renewable, large-scale hydroelectricity can have a large ecological footprint.

04 of 07 Biomass Energy sA©bastian Rabany/Photononstop/Getty Images Biomass has been an important source of energy ever since people first began burning wood to cook food and warm themselves against the winter chill. Wood is still the most common source of biomass energy, but other sources of biomass energy include food crops, grasses and other plants, agricultural and forestry waste and residue, organic components from municipal and industrial wastes, even methane gas harvested from community landfills. Biomass can be used to produce electricity and as fuel for transportation, or to manufacture products that would otherwise require the use of non-renewable fossil fuels.

05 of 07 Hydrogen Gene Chutka/E+/Getty Images Hydrogen has tremendous potential as a fuel and energy source. Hydrogen is the most common element on Earth—for example, water is two-thirds hydrogen—but in nature, it is always found in combination with other elements. Once separated from other elements, hydrogen can be used to power vehicles, replace natural gas for heating and cooking, and generate electricity. In 2015, the first production passenger car powered by hydrogen became available in Japan and the United States.

06 of 07 Geothermal Energy Jeremy Woodhouse/Blend Images/Getty Images The heat inside the Earth produces steam and hot water that can be used to power generators and produce electricity, or for other applications such as home heating and power generation for industry. Geothermal energy can be drawn from deep underground reservoirs by drilling, or from other geothermal reservoirs closer to the surface. This application is increasingly used to offset heating and cooling costs in residential and commercial buildings.