The human race uses a tremendous amount of energy--particularly with the expansion of population and the advance of living standards during the past century. The worldwide human population is estimated to have been about 1.6 billion in 1900; today it is more than four times that at some 6.7 billion. But energy use has increased even faster--much faster--than the general population numbers. In 2005, total worldwide energy consumption was estimated to be 5 x 10^20 joules (equivalent to 5 million Hiroshima-size atomic bombs); this was some 20 million times more than a century earlier. How long can the planet sustain this tremendous energy usage?
The two principle sources of energy on Earth are sunlight that falls on the surface of the planet and radioactive uranium-238 that heats the Earth's innards. In addition there are some other minor sources such as gravity (of the Moon and Sun--and also the gravitational collapse of the Earth itself), the rotational momentum of the Earth, other radioactive decays (such as uranium-235, potassium-40, and thorium-232), and other bombardments from outer space (like cosmic rays and meteors). But these are dwarfed by the big two--sunlight and U-238.
The total amount of energy from sunlight that falls on the Earth each day is truly staggering--some 8,000 times the total present energy used by humans. Most of it falls on the oceans where it warms the water causing evaporation (and therefore rain, and rivers, and hydroelectric power, and all that). Some of it falls on land or sea areas where it happens to hit the chlorophyll of green plants. There it can convert water and carbon dioxide into sugars that feed into food chains providing metabolic energy for all the plants and animals around us. Some of the sunlight that is converted by chlorophyll to usable biological energy winds up sequestered for millions of years in "fossil fuels"--in coal, natural gas, and oil--so even those energy sources originally came from sunlight. (And they are a major source of energy for humans: about 85% of the energy consumption of the human species is currently derived from fossil fuels.) Of course the movements of the winds and the currents of the oceans are also churned up and driven by energy from the Sun, with some contribution from the angular momentum of the rotation of the planet.
Then there is geothermal energy, mainly derived from the radioactive decay of uranium-238. Over the past few decades we have been amazed to discover that vast colonies of exotic plants and animals live in the depths of the oceans around hydrothermal vents from which they get their energy--not from sunlight, but from heat welling up from the inner core and mantle of the Earth and from sulfide compounds this heat releases. Things are very hot down there: temperatures in the Earth's core reach 11,000 degrees Fahrenheit, hotter than the surface of the Sun. And even in the mantle around the core, the temperature averages about 4,000 degrees F. Water around the thermal vents is several hundred degrees F.--much hotter than it could be at surface pressures where water boils to become steam at 212 degrees F.
Perhaps as surprising as the discovery of this exotic biomass--using energy and metabolic transformations entirely different from those we use on the surface--is its extent. After some cursory deep-water surveys, it is believed that there is more living material down there than there is in all the surface creatures around us combined--all the animals (including fish) and plants (including algae--which outweigh the rest of the plant kingdom) and bacteria (which outweigh all other surface life combined, including all plants and animals that live on the surface of the continents and in the Sun-fed, near-surface levels of the oceans). Clearly there is a tremendous biomass preying on the subterranean heat, just as there is pressure of utilization on surface sunlight.
What about the question of running out of these sources of energy? On the one hand, the Sun is expected to radiate livable amounts of energy towards the Earth for several billion years. On the other, uranium-238 has a half-life of 4.47 billion years and even though the renewal of the Earth's central heating is gradually getting behind--it loses more energy outward each year than it generates inside--this is only at the overall rate of about one hundred degrees every billion years.
Granted, we cannot be so profligate in our use of energy as we have come to be in the recent past. Conservation of energy use and improved energy efficiency must be a major part of our future industrial and general economic development. But our basic energy supplies on Earth, both sunlight and U-238, are immense. If we learn to use them wisely, the energy future of the planet should be secure for quite some time to come (far, far longer, in fact, than human beings are likely to be around to worry about it).
Bun Gladieux, president of the Presssure Positive Company, has a blog with an interesting series of topics.
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