District of Shizuishan, Ningxia) and is said to have been burning since the Qing Dynasty (before 1912). Detection Before attempting to extinguish a near-surface coal-seam fire, its location and underground extent should be determined as precisely as possible. Besides studying the geographic, geologic and infrastructural context, information can be gained from direct measurements. These include: Temperature measurements of the land surface, in fissures and boreholes, for example using pyrometers Gas measurements to characterize the fire ventilation system (amount and velocity) and the gas composition, so that the combustion reactions can be described Geophysical measurements on the ground and from aircraft to establish the extent of conductivity or other underground parameters. For example, conductivity measurements map humidity changes near the fire; measuring magnetism can determine changes in the magnetic characteristics of the adjacent rock caused by heat Remote sensing from aircraft and satellites. High resolution optical mapping, thermal imaging and hyperspectral data play a role. Underground coal fires of several hundred to over a thousand degrees Celsius may raise the surface temperature by only a few degrees. This order of magnitude is similar to the temperature difference between the sunlit and shadowed slopes of a slag heap or sand dune. Infrared detecting equipment is able to track the fire's location as the fire heats the ground on all sides of it. However, remote sensing techniques are unable to distinguish individual fires burning near one another and often lead to undercounting of actual fires. They may also have some difficulties distinguishing coal-seam fires from forest fires. Combining in-situ data with remote sensing data does allow for monitoring of coal fire intensity over longer periods using time-series analyses. Underground coal mines can be equipped with permanently installed sensor systems. These relay pressure, temperature, airflow and gas composition measurements to the safety monitoring personnel, giving them early warning of any problems. Environmental impact Besides destruction of the affected areas, coal fires often emit toxic gases, including carbon monoxide and sulphur dioxide. China's coal fires, which consume an estimated 20 – 200 million tons of coal a year, make up as much as 1 percent of the global carbon dioxide emissions from fossil fuels. One of the most visible changes will be subsidence. Another local environmental effect can include the presence of plants or animals that are aided by the coal fire. The prevalence of non-native plants can depend upon the fire's duration and the