<P> Heat flows constantly from its sources within the Earth to the surface . Total heat loss from the Earth is estimated at 44.2 TW (4.42 × 10 Watts). Mean heat flow is 65 mW / m over continental crust and 101 mW / m over oceanic crust . This is 0.087 watt / square meter on average (0.03 percent of solar power absorbed by the Earth), but is much more concentrated in areas where the lithosphere is thin, such as along mid-ocean ridges (where new oceanic lithosphere is created) and near mantle plumes . The Earth's crust effectively acts as a thick insulating blanket which must be pierced by fluid conduits (of magma, water or other) in order to release the heat underneath . More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges . The final major mode of heat loss is by conduction through the lithosphere, the majority of which occurs in the oceans due to the crust there being much thinner and younger than under the continents . </P> <P> The heat of the Earth is replenished by radioactive decay at a rate of 30 TW . The global geothermal flow rates are more than twice the rate of human energy consumption from all primary sources . </P> <P> Heat from Earth's interior can be used as an energy source, known as geothermal energy . The geothermal gradient has been used for space heating and bathing since ancient Roman times, and more recently for generating electricity . As the human population continues to grow, so does energy use and the correlating environmental impacts that are consistent with global primary sources of energy . This has caused a growing interest in finding sources of energy that are renewable and have reduced greenhouse gas emissions . In areas of high geothermal energy density, current technology allows for the generation of electrical power because of the corresponding high temperatures . Generating electrical power from geothermal resources requires no fuel while providing true baseload energy at a reliability rate that constantly exceeds 90% . In order to extract geothermal energy, it is necessary to efficiently transfer heat from a geothermal reservoir to a power plant, where electrical energy is converted from heat by passing steam through a turbine connected to a generator . On a worldwide scale, the heat stored in Earth's interior provides an energy that is still seen as an exotic source . About 10 GW of geothermal electric capacity is installed around the world as of 2007, generating 0.3% of global electricity demand . An additional 28 GW of direct geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications . Because heat is flowing through every square meter of land, it can be used for a source of energy for heating, air conditioning (HVAC) and ventilating systems using ground source heat pumps . In areas where modest heat flow is present, geothermal energy can be used for industrial applications that presently rely on fossil fuels . </P> <P> The geothermal gradient varies with location and is typically measured by determining the bottom open - hole temperature after borehole drilling . To achieve accuracy the drilling fluid needs time to reach the ambient temperature . This is not always achievable for practical reasons . </P>

Why does temperature increase with depth in the earth