The geothermal gradient is the rate at which the earth's temperature increases with depth, indicating outward heat flows from a hot interior. Away from tectonic plate boundaries, it is 25-30°C per km of depth in most of the world. Strictly speaking, geo-thermal necessarily refers to the earth but the concept may be applied to other planets. The Earth's internal heat comes from a combination of residual heat from planetry (about 20%) and heat produced through redioactive decay (80%) The major heat-producing isotopes in the Earth are potassium-40,uranium 238, uranium-235, and thorium 232 At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 gpa Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, which was twice that of present-day at approximately 3 billion years ago, would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.
In 2007, there were 1.8 million alternative fuel vehicles sold in the United States, indicating an increasing popularity of alternative fuels There is growing perceived economic and political need for the development of alternative fuel sources. This is due to general environmental, economic, and geopolitical concerns of sustainability.
The major environmental concern, according to an ipcc report, is that "Most of the observed increase in globally averaged temperatures since the mid-20th century is due to the observed increase in artoprogenic greenhouse gas concentrations". Since burning fossil fuels is known to increase greenhouse gas concentrations in the atmosphere, they are a likely contributor to global warming.
Other concerns which have fueled demand revolve around the concept of peak oil, which predicts rising fuel costs as production rates of petroleum enter a terminal decline. According to the hubbert peak theory, when the production levels peak, demand for oil will exceed supply and without proper mitigation this gap will continue to grow as production drops, which could cause a major engery crisis
Lastly, the majority of the known petroleum reserves are located in the middle east. There is general concern that worldwide fuel shortages could intensify the unrest that exists in the region, leading to further conflict and war. (See future engergy development for a general discussion)
In an attempt to increase demand for alternative fuels in the US, the irs began allowing taxpayers to claim a special tax credit for using alternative fuels, known as the Alternative Fuel Vehicle Refueling Property Credit. The definition used for alternative fuel under this credit is: Any fuel containing at least 85 percent of one or more of ethanol natural gas,compressed natural gas, liquefied natural gas, liquefied petroleum gas or hydrogen; or any mixture which consists of two or more of bio diesel fuel, or kerosene, and at least 20% of which consists of biodiesel.
The production of alternative fuels can have widespread effects. For example, the production of corn-based ethanol has created an increased demand for the feed stock, causing rising prices in almost everything made from corn. However, in a compative free market, an increased supply of ethanol reduces the demand for conventional fuels, and thus lowers fuel prices. The ethanol industry enables agricultural surpluses to be used to mitigate fuel shortages.