Which is the ultimate source of energy on Earth?

Concept

The "ultimate" or primary source of energy is the one from which all other usable energy forms are ultimately derived, rather than one that merely stores or carries energy that originated elsewhere. On Earth, almost every energy form can be traced back through a chain of conversions to a single original supplier: the Sun's radiation.

Application

Sunlight is the original input that drives the major energy pathways on Earth:

• Green plants capture solar energy by photosynthesis and store it as chemical energy in biomass.

• Buried, compressed remains of that ancient plant and organism matter became the fossil fuels (coal, oil, natural gas) over millions of years - so their energy is stored solar energy.

• Uneven solar heating of land, sea and air creates pressure differences that set the atmosphere in motion, producing wind.

• The Sun also powers the water cycle (evaporation and rainfall), which feeds hydro energy.

Because each of these can be traced back to the Sun, the Sun is the ultimate source of energy on Earth.

Contrast

• Coal - a fossil fuel; it only stores solar energy that plants captured long ago, so it is a derived/stored source, not the original one.

• Wind - arises from solar heating of the atmosphere, so it is an effect of the Sun rather than the origin.

• Petroleum - like coal, a fossil fuel formed from once-living matter; it carries previously stored solar energy, not a primary source.