Extreme Summer Heat to Drive Asphalt Rethink

As climate change accelerates, state and local officials will want to account for future heat waves’ impact on asphalt roads

This summer was the hottest summer this century in the US, according to Gro’s US land surface temperature data for all June 21 to September 21 periods since 2000. With traditionally “cooler” states projected to warm at above average rates as climate change accelerates, state and local officials will want to account for future heat waves’ impact on their asphalt roads. 

More frequent and intense heat waves can hasten deterioration and reduce asphalt roads’ service life, particularly in places that experience cold winters. This is because softer asphalts can withstand winter temperatures, but they have lower high temperature ratings. Stiffer asphalts, meanwhile, are better suited to hotter climates due to slower softening during periods of intense heat.  

Nationally, the Gro Climate Indicator Projection (GCIP) of Days Per Year Above 35°C (95°F), weighted to land area over the US, shows a 6.5 day, or 35% increase, in the number of days above 35°C from 2021 to 2050 for SSP2-4.5, the Intergovernmental Panel on Climate Change’s “middle of the road” warming scenario. At the state level, however, a clearer case for reconsidering the composition of asphalt paving emerges. 

North and South Dakota, Iowa, Michigan, Wisconsin, Minnesota and Alaska, which often rank among US’ coldest states, are expected to see mean daily high temperatures rise the most by 2050.  

For example, Minnesota, which routinely sees nighttime temperatures dip below freezing in the winter, by 2050 is projected to see 4.1 additional days above 35°C, compared with its 2021 baseline of 3.3 days. This more than doubling in the number of days above  35°C suggests that the state might need to reevaluate its pavement design recommendations to consider climate change. Currently, the Minnesota Department of Transportation (DOT) recommends PG-58, an asphalt blend that is used in colder climates, for high-volume roadways. By comparison, the Texas DOT recommends PG-70 or -75 for most high-volume roadways. 

Asphalt pavements are given a performance grade (PG) based on the seven-day maximum asphalt pavement temperature, which is often notably higher than the air temperature on a 35°C day. 

By this measure, Minnesota’s performance grade is based on a seven-day maximum temperature band of 58°C (136°F). An asphalt temperature at  58°C is not uncommon on hot summer days when not shaded by trees. For example, in research on heat island mitigation and stormwater management, asphalt pavements temperatures reached 70°C (158°F) in Davis, California over four days in July 2012 when maximum daily air temperatures were between 95-103°F (35-40°C). 

Under SSP2-4.5, weighted by land area, Texas is projected to spend 83 days, or roughly 20 percent of the year, above 35°C by 2050. In addition to trapping heat and exacerbating heat island effects, asphalt emissions from roads and roofs are a climate change accelerator. 

  1. See, e.g., Li et al. (2013).