<P> Alternative energy is any energy source that is an alternative to fossil fuel . These alternatives are intended to address concerns about such fossil fuels, such as its high carbon dioxide emissions, an important factor in global warming . Marine energy, hydroelectric, wind, geothermal and solar power are all alternative sources of energy . </P> <P> The nature of what constitutes an alternative energy source has changed considerably over time, as have controversies regarding energy use . Because of the variety of energy choices and differing goals of their advocates, defining some energy types as "alternative" is considered very controversial . </P>

What is the need for exploring alternative sources of energy