Urbanisation's contribution to climate warming in Great Britain

Urbanisation is changing the climate of the world we live in. In Great Britain (GB) 5.8% of the total land area is covered by artificial surfaces, increasing from 4.3% in 1975. Aside from associated loss of farmland, biodiversity and a range of ecosystem services, changing to urban form warms the Earth's surface: the urban heat island (UHI) effect. Standard estimates of temperature changes do not account for urbanisation (i.e. use of rural-only stations or removal of urban bias in observations), meaning that anthropogenic modifications to the land surface may be causing the surface-level atmosphere to warm quicker than those estimates suggest. Using observations from a high-density urban monitoring network, we show that locally this warming (instantaneously) may be over 8 degrees C. Based on the relationships between UHI intensity, urban fraction and wind speed in this network, we create a statistical model and use it to estimate the current daily-mean urban warming across GB to be 0.04 degrees C [0.02 degrees C -0.06 degrees C]. Despite this climate contribution appearing small (94% of GB's land cover for the time-being is still rural), we show that half of GB's population currently live in areas with average daily-mean warming similar to 0.4 degrees C. Under heatwave conditions our high estimates show 40% of GB's population may experience over a 1 degrees C daily-mean UHI. Furthermore, simply due to urbanisation (1975-2014) we estimate GB is warming at a rate equivalent and in addition to 3.4% [1.9%-5.0%] of the observed surface-level warming calculated from background stations. In the fastest urbanising region, South East GB, we find that these warming rates are up to three times faster. The methodology is straightforward and can be readily extended to other countries or updated as future land cover data becomes available.