Recent studies confirm and quantify higher surface temperatures in urban areas than in surrounding rural areas for a number of reasons, including the concentrated release of heat from buildings, vehicles, and industry. In the United States, this urban heat island effect results in daytime temperatures 0.9°–7.2°F (0.5°–4.0°C) higher and nighttime temperatures 1.8°– 4.5°F (1.0°–2.5°C) higher in urban areas, with larger temperature differences in humid regions (primarily in the eastern United States) and in cities with larger and denser populations. The urban heat island effect will strengthen in the future as the structure, spatial extent, and population density of urban areas change and grow (high confidence).

This finding is from chapter 10 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Key Finding 4 is based on simulated and satellite land surface measurements analyzed by Imhoff et al.c2e4ff74-7a03-40dc-8bbe-e76b51dca729. Bounoua et al.,f7cd3568-59f5-453d-9187-c4a93f393d3b Shepherd,0c6d6e02-87c6-4fd3-a4c4-399958553e81 Seto and Shepherd,f133a8eb-421f-4dda-b415-b3d74fb6745c Grimmond et al.,37e7c46a-a6cd-4a5c-b98d-306af0511114 and Seto et al.3ed366fd-fda3-43e8-9402-2f5e374f966e provide specific references with regard to how building materials and spatio-temporal patterns of urban settlements influence radiative forcing and feedbacks of urban areas to the climate system.f7cd3568-59f5-453d-9187-c4a93f393d3b

Description of evidence base: Urban interactions with the climate system have been investigated for more than 40 years.edc0e699-6b5a-42d4-95a1-c749033c00bf ff31c479-bfb9-4c80-8ea1-9486048e1abb The heat gain due to the storage capacity of urban built structures, reduction in local evapotranspiration, and anthropogenically generated heat alter the spatio-temporal pattern of temperature and leads to the well-known urban heat island (UHI) phenomenon.37e7c46a-a6cd-4a5c-b98d-306af0511114 3ed366fd-fda3-43e8-9402-2f5e374f966e f133a8eb-421f-4dda-b415-b3d74fb6745c The urban heat island (UHI) effect is correlated with the extent of impervious surfaces, which alter albedo or the saturation of radiation.c2e4ff74-7a03-40dc-8bbe-e76b51dca729 The urban-rural difference that defines the UHI is greatest for cities built in temperate forest ecosystems.c2e4ff74-7a03-40dc-8bbe-e76b51dca729 The average temperature increase is 2.9°C, except for urban areas in biomes with arid and semiarid climates.c2e4ff74-7a03-40dc-8bbe-e76b51dca729 f7cd3568-59f5-453d-9187-c4a93f393d3b

New information and remaining uncertainties: The largest uncertainties about urban forcings or feedbacks to the climate system are how urban settlements will evolve and how energy consumption and efficiencies, and their interactions with land cover and water, may change from present times.b05850d3-a9d7-4395-8588-5e27531c160a d6ca431b-0c90-40e6-bb8d-47e5e293198a aa1fec1f-b5c3-48b8-b17e-ca88da35eb4c 3ed366fd-fda3-43e8-9402-2f5e374f966e

Provenance

This finding was derived from figure -.2: Confidence / Likelihood