Beijing has a long history of environmental problems. Between 2000 and 2009 Beijing's urban extent quadrupled, which not only strongly increased the extent of anthropogenic emissions, but also changed the meteorological situation fundamentally, even if emissions of human society are not included. For example, surface albedo, wind speed and humidity near the surface were decreased, whereas ground and near-surface air temperatures, vertical air dilution and ozone levels were increased. Because of the combined factors of urbanization and pollution caused by burning of fossil fuel, Beijing is often affected by serious environmental problems, which lead to health issues of many inhabitants. In 2013 heavy smog struck Beijing and most parts of northern China, impacting a total of 600 million people. After this "pollution shock" air pollution became an important economic and social concern in China. After that the government of Beijing announced measures to reduce air pollution, for example by lowering the share of coal from 24% in 2012 to 10% in 2017, while the national government ordered heavily polluting vehicles to be removed from 2015 to 2017 and increased its efforts to transition the energy system to clean sources.