Hot city. Are we facing unbearable summers?

In its latest report the Intergovernmental Panel on Climate Change (IPCC) has concluded, “It is unequivocal that human influence has warmed the atmosphere, ocean and land.”

234 top climate scientists from all over the world wrote this report and it was open for comments to all scientists. All the 195 Member Governments of the IPCC finally approve the summary of policy makers. This process implies that the IPCC is conservative by design, and despite this, the conclusion above is very clear. All of the scientists and the governments thus agree that humans caused global warming since 1850 and any doubt about the reality of climate change as well as its cause has no valid basis anymore.

The World Weather attribution project has concluded that the Western North American extreme heat was virtually impossible without human-caused climate change. In addition, the IPCC concluded that in Europe the frequency and intensity of hot extremes have increased in recent decades and will continue to increase regardless of the evolution of future human greenhouse gas emissions. Indeed the effect of global reductions of emissions in the next decades will only start to affect temperatures in around 20 years in a discernable way, although other effects, such as reductions in air pollution will happen much faster.

In the country of Luxembourg, we have observed that the annual temperature averages in the period 1991-2020 were 1.6°C higher than in the pre-industrial period (1861-1890), compared to the 1.0°C that have been globally observed over a similar period. This is in line as the land surface is warming faster than the global average. We can expect this warming trend to continue until the 2040s. If at that point in time, global CO2 emissions are falling towards net-zero we could expect a gradual stabilization of temperatures.

Urban centers and cities are warmer[1] than the surrounding rural areas due to what scientists call the urban heat island effect.

This effect results from several factors, including the heat-absorbing properties of many of the building materials used in cities as well as city geometry, which reduces ventilation between buildings or can trap heat. Additionally heat generated directly from human activities, and the limited amount of vegetation contribute to the heat islands.

This implies that cities amplify the increase in heat stress due to global warming. The IPCC assessed for example that the Brussels area has warmed by 0.6°C more from 1950 to 2018 than the surrounding rural areas, but also that cities exacerbate the effect of extreme climate events, such as heatwaves, with more hot days and warm nights adding to heat stress.

[1] source: https://www.ipcc.ch/report/ar6/wg1/downloads/factsheets/IPCC_AR6_WGI_Regional_Fact_Sheet_Europe.pdf

White surface have a higher albedo than darker surfaces, which means that the surface reflects more of the incoming solar radiation and less of it is absorbed and transformed into heat. This means that for example white squares heat up less than darker squares, even though the reflection dazzles persons on the square.

The effect of adding vegetation to a square, in particular trees which give shadow and absorb CO2 from the atmosphere, reduces local temperature much more than the change of color. A combination of both options can help to reduce locally the heat island effect.

These effects have however a very local effect only. Painting roofs or streets in white colors would not be sufficient to counteract the increase in global temperatures due to human-made greenhouse gases.