Non-linear urban overheating increments under climate change: Evidence from UKCP18 nighttime temperatures

Climate change has placed populations under increasing nighttime heat. With adaptation planning often organised around discrete global warming levels, we ask how summer nighttime heat and urban heat islands change across policy-relevant warming windows. We address these questions by using Greater London as a case study and analyse bias-corrected UKCP18 Local projections under RCP8.5 for 2006–2015, 2021–2030 and 2036–2045. These fixed analysis windows are treated as baseline, near-term, and early mid-term warming window, broadly representing +1.0, +1.5, and +2.0∘C warming levels above pre-industrial levels. The urban heat island intensity (UHII) was quantified by comparing a 5 km urban core with a 40–50 km rural ring across the three analysis windows. The analysis delivers three results. First, nighttime minimum temperature shows non-linear stepwise warming, with a larger early-step increase: +0.83°C from the baseline window to the near-term warming window, and +0.54°C from the near-term warming window to the early mid-term warming window. Second, seasonal-mean UHII changes little, decreasing by 0.037°C and 0.048°C across the two adjacent steps. However, urban and rural nighttime minimum temperatures increases across the three analysis windows in every summer month. Third, absolute nighttime heat expanded, high-UHII nights remained centred on the compact urban core, and baseline cool grid boxes no longer persisted in later windows. These results support earlier heat-adaptation assessment within a high-forcing UKCP18 Local framework, and the prioritisation of persistent central warm areas for further evaluation.