Urban heat island vs. global warming

Data from the International Panel on Climate Change (IPCC) shows the relationship between urban heat island warming and warming due to climate change in large cities across the world.

(AP Photo/Ted Shaffrey, File)

Urban centers are warmer than outlying areas and can be referred to as urban heat islands.

The urban heat island is caused by a number of factors. Widespread concrete and asphalt surfaces absorb a significant amount of heat compared to surfaces covered with vegetation. The close proximity of buildings traps more heat. A lack of shade-producing trees and other vegetation in cities as well as human activities also add additional heat to urban areas.

Image courtesy the IPCC.

Urban heat islands also typically have a slightly lower relative humidity compared to outlying areas. Wind speeds are also slightly reduced in urban areas. With less wind, pockets of heat can remain trapped and not mixed out.

The urban heat island has clearly exacerbated the effects of global warming in cities, especially during the nighttime hours, but what percentage of the overall, long-term warming in these large cities is being caused by the urban heat island effect?

According to the IPCC report, in cities like Los Angeles and New York, the urban heat island effect contributed to less than 20 percent of the overall warming between 1950 and 2018. However, these values were not consistent worldwide. For instance, the urban heat island effect in Moscow contributed to about 30 percent of the overall long-term warming trend while in Athens, Greece, it was 60 percent.

Key excerpt from the IPCC....

Future urbanization will amplify the projected air temperature change in cities regardless of the characteristics of the background climate, resulting in a warming signal on minimum (nighttime) temperatures that could be as large as the global warming signal.