It takes a lot of energy to nurture a species such as homo sapiens. In the past few centuries this species has emerged as an interconnected global presence in a way that, as far as science knows, has never occurred before on the planet.

The types of energy humans need include electricity to power their homes and industries, biochemical energy to feed their bodies and combustible resources for warmth, transportation and industrial production.

On a broad scale, the earth's ability to provide what humans need depends on five main sources:

• The sun, that giant fusion reactor in the sky, supplies energy in the order of yottawatts (10²⁴ watts) on a 24/7 basis. 
• Water, which is not only essential for life, but which can also be harnessed for energy production.
• Gravity, the mysterious force that creates and destroys stars, is responsible for tides, and it turns water into a source of convertible kinetic energy.
• The earth's movements create daily and seasonal temperature differentials that generate winds and ocean currents which can be converted to electricity. 
• Radioactivity is the natural decay of heavy elements into lighter ones with a resultant release of radiation, The radiation creates heat which can be used to generate electricity.

In addition, an important energy supply for humans is derived from the decomposing bodies of organisms that have flourished and died throughout the eons. Unlike the resources listed above, however, this supply is limited.

Fossil fuels, which include oil, natural gas and coal, are actually another form of solar energy. Eons ago, living organisms converted the sun's light and heat into the carbon-based molecules that formed their bodies. The organisms died, and their bodies sank deep into the ground and to the bottoms of the oceans. Today, the energy locked into those carbon bonds can be released by retrieving what their remains turned into and burning them.

Oil and natural gas comes from microscopic sea plankton that lived millions of years ago. They died and sank to the bottoms of the oceans, where decomposition and other chemical processes turned them into waxy kerogen and tarry bitumen. The ocean beds eventually dried out, and these materials were buried under rock and soil. They have become the raw materials for making, gasoline, diesel fuel, kerosene and a host of other petroleum products.

The traditional way to retrieve crude oil from the ground is by drilling, but hydraulic fracturing, or fracking, has become an often-used modern alternative. In this process, a mixture of sand, water and potentially dangerous chemicals is forced into the ground to displace the petroleum. Fracking is an expensive process, and it has a number of deleterious effects on the bedrock, the water table and the surrounding air.

Coal comes from terrestrial plants that settled into bogs and swamps and turned into peat. The peat solidified as the ground dried out, and it was eventually covered over by rocks other debris. The pressure turned it into the black, rocky substance burned in many industrial plants and power stations. All this began happening some 300 million years ago, when dinosaurs roamed the earth, but contrary to popular myth, coal is not decomposed dinosaurs.

For millennia, humans have been harnessing water power to perform work, and in physics, work is synonymous with energy. Water wheels placed near a stream or waterfall have used the energy generated by moving water to mill grain, irrigate crops, saw wood and do a host of other tasks. With the advent of electricity, water wheels have been turned into power plants.

The water turbine is the heart of a hydroelectric power generating station, and it works because of the phenomenon of electromagnetic induction, discovered by physicist Michael Faraday in 1831. Faraday found that a spinning magnet inside a coil or conducting wire generates an electric current in the coil, and less than 100 years later, the first induction generator came online at Niagara Falls.

Today, hydroelectric plants supply about 6 percent of the electricity consumed worldwide. The burning of fossil fuels to generate steam and spin turbines, on the other hand, generates almost 60 percent of the world's electricity. Most hydroelectric power is generated by dams, not by waterfalls.

A dam, like a stream or waterfall, depends on gravity. The water enters a passage at the top of the dam, flows through a pipe that magnifies its energy and spins a turbine before exiting near the base of the dam. Two of the world's largest hydroelectric dams are the Three Gorges Dam in China, which generates 22.5 gigawatts of energy and the Itaipu Dam on the Brazil/Paraguay border, which generates 14 GW. The largest dam in North America is the Grand Coulee Dam in Washington State, which generates only about 7 megawatts.

The oceans are one of the world's most important energy resources for two reasons. The first is that they have currents, which in conjunction with the winds, form waves. Waves can be turned into electricity. Because they are the result of temperature differentials caused by the heat of the sun, waves and the currents that form them are technically a form of solar energy.

The other energy resource in the oceans is the tides, which are caused by the gravitational influences of the moon and the sun, as well as by the motions of the earth itself. Technologies also exist to convert the energy in the tides into electricity.

Wave generating stations are not yet mainstream, and the prototype, which was deployed off the coast of Scotland, generates only 0.5 MW. Available wave technologies include:

• Floats and buoys, which rise and fall on the waves and generate power with hydraulic devices.

• Oscillating water columns, which allow water to enter a chamber and compress enclosed air, which then spins a turbine.

• Tapered channel systems, which are shore bound. They channel water into elevated reservoirs, and when the water is allowed to fall, it spins a turbine. 

Tidal power stations can use the power of incoming and outgoing tides to directly spin turbines. Water is about 800 times denser than air, so if a turbine is placed on the ocean floor, the tidal movements generate significant power to spin them. Tidal barrage systems are more common, however.

A tidal barrage is a barrier erected across a tidal basin that allows water from the rising tide to enter, then closes and controls the outflow on the ebb tide. The largest such generator is the Sihwa Lake Tidal Power Station in South Korea. It generates about 254 MW.

Two of the best-known ways to generate electricity in a way that doesn't rely on disappearing fossil fuels and doesn't create pollution are to deploy wind turbines or photovoltaic panels. Because the sun is responsible for the temperature differentials that create wind, both are, strictly speaking, forms of solar energy.

Wind generators work just like hydroelectric or wave-powered ones. When the wind blows, it spins a shaft which is connected by gears to a power-generating induction-style turbine. Modern turbines are calibrated to provide AC current at the same frequency as conventional AC power, which makes it available for immediate use. Wind farms throughout the world supply almost 5 percent of the world's electricity.

Solar panels rely on the photovoltaic effect, whereby the sun's radiation creates a voltage in a semi-conducting material. The voltage creates DC current which has to be converted to AC by passing it through an inverter. Solar panels only generate electricity when the sun is out, so they are often used to charge batteries, which store the power for later use.

Solar panels represent perhaps one of the the most accessible methods for generating electricity, but they supply only a small fraction of the world's electricity -- less than 1 percent.

Strictly speaking, the process of nuclear fission is not a naturally occurring phenomenon, but it comes from nature. Nuclear fission was invented soon after scientists were able to understand the atom and the natural phenomenon of radioactivity. Though fission was originally used to make bombs, the first nuclear power plant came online just three years after the first bomb was exploded at the Trinity site in the New Mexico desert.

Controlled fission reactions occur inside all the world's nuclear power stations. It generates heat to boil water, which produces the steam needed to drive electrical turbines. Once a fission reaction starts, it needs little fuel to continue indefinitely.

Almost 20 percent of the world's electrical needs are met by nuclear power generators. Originally considered a cheap source of virtually unlimited power, nuclear fission has serious drawbacks, not the least of which is the possibility of meltdown and the uncontrolled release of harmful radiation. Two well-known accidents, one at Russia's Chernobyl power plant and another at Japan's Fukushima facility, have obviated these dangers and made nuclear power production less attractive than it once was.

Deep inside the earth's crust, pressures and temperatures are so great that they liquify rock into molten lava. This superheated material courses through veins in the crust that occasionally direct it close to the surface. Communities in areas where this occurs can use the heat to generate electricity and to provide warmth for their homes. This is called geothermal energy, and in some cases, it is augmented by radioactive materials in the ground, which also generate heat.

To make use of geothermal energy, developers drill a tunnel into the earth at a suitable site and circulate water through the tunnel. The heated water comes to the surface as steam, where it can be used directly for heating or to spin a turbine. In some cases, the heat is transferred from the water to another substance with a lower boiling point, such as isobutane, and the resulting vapor spins the turbines.

In its simplest form, geothermal energy has provided healing and comfort at natural spas and hot springs for as long as there have been people to frequent them. Japan is one of the most geologically active countries in the world, and it has a large network of natural hot springs and a long history of soaking. Experts estimate that it has enough geothermal resources to meet up to 10 percent of its electricity needs, making its geothermal potential third in the world, behind only the United States and Indonesia.

Some resources are fragile and disappearing, and converting them into usable energy creates pollutants that alter the planetary environment. Other resources depend only on solar and planetary dynamics that promise to remain unchanged for the next few billion years. In the present moment, humanity has an urgent choice to make. Its very survival may depend on its ability to switch its reliance from the former to the latter in a short period of time.