Heat risks in urban areas

Contact: Keith Oleson

Extreme heat is a leading cause of weather-related human mortality in the United States and in many countries worldwide. Vulnerability to extreme heat is amplified in large cities due to the urban heat island effect and socioeconomic diversity. Several NCAR-led projects have focused on understanding extreme heat, human health, and urban vulnerability in present and future climates.

The SIMMER project, led by NCAR PI Olga Wilhelmi, focused on understanding extreme heat, human health, and urban vulnerability in present and future climates.  The primary goals of the SIMMER were to:

    • Advance the methodology for assessing current and future urban vulnerability from heat waves through the integration of physical and social science models, research results, and NASA data
    • Develop models and tools for building local capacity for heat hazard mitigation and climate change adaptation in the public health sector.

The BRACE project examined differences in impacts between two specific climate futures, those associated with Representative Concentration Pathways (RCPs) 4.5 and 8.5.As part of this project, Keith Oleson (NCAR/CGD) led an assessment of the avoided climate impacts of urban heat/cold waves (Oleson et al. 2015). Heat/cold wave characteristics were derived for U.S. regions from a bias-corrected CESM 30-member ensemble for climate outcomes driven by the RCP8.5 forcing scenario and a 15-member ensemble driven by RCP4.5. This work assessed the avoided climate impacts of urban heat/cold waves in 2061–2080 when following the lower forcing scenario. Urban heat wave days per year increase from six in 1981–2005 to up to 92 (southeast U.S.) in RCP8.5. Following RCP4.5 reduces heat wave days by about 50% (Oleson et al. 2015).

The associated urban heat wave data was also used to develop health-based models and project the future frequency of high-mortality urban heat waves through a collaboration with Brooke Anderson of Colorado State University (Anderson et al. 2016a, b) and used to study future population exposure to heat waves through a collaboration with Bryan Jones of City University of New York (Jones et al. 2016). The health-based models can be used to project future trends in high-mortality heatwaves under different scenarios of a changing future (e.g., climate change, population change). Further, these models are novel in the way they allow exploration of different scenarios of adaptation to heat, as they include as predictive variables heatwave characteristics that are measured relative to a community’s temperature distribution, allowing different adaptation scenarios to be explored by selecting alternative community temperature distributions (Anderson et al. 2016a).

These health-based models were applied to project trends in high-mortality heatwaves, including proportion of all heatwaves expected to be high-mortality, using the definition that a high-mortality heatwave increases mortality risk by ≥20%. We projected these trends in 82 U.S. communities in 2061–2080 under two scenarios of climate change (RCP4.5, RCP8.5), two scenarios of population change (SSP3, SSP5), and three scenarios of community adaptation to heat (none, lagged, on-pace). Projections were most strongly influenced by the adaptation scenario—going from a scenario of on-pace to lagged adaptation or from lagged to no adaptation more than doubled the projected number of and exposure to high-mortality heatwaves. Based on our results, fewer high-mortality heatwaves are expected when following RCP4.5 versus RCP8.5 and under higher levels of adaptation, but high-mortality heatwaves are expected to remain a very small proportion of total heatwave exposure (Anderson et al. 2016b).


Anderson, G.B., K.W. Oleson, B. Jones, and R.D. Peng, 2016a: Classifying heatwaves: developing health-based models to predict high-mortality versus moderate United States heatwaves, Climatic Change, DOI:10.1007/s10584-016-1776-0.

Anderson, G.B., K.W. Oleson, B. Jones, and R.D. Peng, 2016b: Projected trends in high-mortality heatwaves under different scenarios of climate, population, and adaptation in 82 US communities, Climatic Change, DOI:10.1007/s10584-016-1779-x.

Jones, B. et al. Population exposure to heat-related extremes: Demographic change vs climate change. Climatic Change, in preparation.

Oleson, K.W., G.B. Anderson, B. Jones, S.A. McGinnis, and B. Sanderson, 2015: Avoided climate impacts of urban and rural heat and cold waves over the U.S. using large climate model ensembles for RCP8.5 and RCP4.5, Climatic Change, DOI: 10.1007/s10584-015-1504-1.