The mega-city Lagos and three decades of urban heat island growth

Lagos, Nigeria is rapidly urbanising and is one of the fastest growing cities in the world, with a population increasing at almost 500,000 people per year. Yet the impacts on Lagos’s local climate via its urban heat island (UHI) have not been well explored. Considering the tropics already have year-round high temperatures and humidity, small changes are very likely to tip these regions over heat-health thresholds. Using a well-established model, but with an extended investigation of uncertainty, we explore the impact of Lagos’s recent urbanisation on its UHI. Following a multi-physics evaluation, our simulations, against the background of an unusually warm period in February 2016 (where temperatures regularly exceeded 36 oC), show a 0.44 oC ensemble-time-mean increase in night-time UHI intensity between 1984–2016. The true scale of the impact is seen spatially where the area in which ensemble-time-mean UHIs exceeding 1 oC were found to increase steeply from 254 km2 in 1984 to 1572 km2 in 2016. The rate of warming within Lagos will undoubtedly have a high impact due to the size of the population (12+ million) already at risk from excess heat. Significant warming and modifications to atmospheric boundary-layer heights are also found in rural areas downwind, directly caused by the city. However, there is limited long-term climate monitoring in Lagos or many similarly expanding cities, particularly in the tropics. As such, our modelling can only be an indication of this impact of urbanisation, and we highlight the urgent need to deploy instrumentation.