Cities generate enough heat to alter their own climate

Heat produced by human lifestyles could affect urban climates as much as fossil fuel emissions do. Regional climate models of Moscow, published in Izvestiya, Atmospheric and Oceanic Physics, show that thermal air pollution raises both temperatures and wind speeds across the city.

Cities are major sources of greenhouse gases, contributing to climate change on a global scale. But cities can also alter their own local climate by heating up the urban atmosphere. “Mankind pollutes the atmosphere in many ways, from climate-altering greenhouse gases to microscopic particles that are harmful when inhaled,” says Alexander Ginzburg, head of the Laboratory of Mathematical Ecology of the A.M. Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences in Moscow. “But the effects of thermal pollution on urban climate are less well understood.”

Estimating the extent of thermal pollution across a city is difficult, because of the vast number of heat sources, from our bodies, to transport, industry, offices, homes, and less obvious hotspots such as garbage dumps.

To overcome this, Ginzburg and colleague, Sergei Dokukin, used the latest information from the Climate Limited-area Modelling Community (COSMO-CLM) database on energy consumption per person and population densities to calculate thermal emissions across several major cities, including London and New York. The resultant maps suggest Moscow is one of the hottest megacities in the world.

The pair then used the COSMO-CLM European regional climate model to model the effect of thermal air pollution on meteorological conditions, including air temperature, humidity, wind speed, and wind direction across Moscow between December 2015 and January 2018.

The model, which accounts for specific urban effects such as surface permeability and turbulence over buildings, showed that thermal pollution increased average annual wind speeds in Moscow by 3.6 kilometres per hour. “It is well known that thermal pollution can affect wind in cities, but we were amazed that such a simple model could demonstrate the effect so easily,” he adds. The model revealed the biggest effect of thermal pollution in Moscow was on January 12th, 2017, under specific weather conditions, when calculated anthropogenic heat fluxes ‘stopped’ the cold northern atmospheric front due to the warming of the air in Moscow by 7°C, compared to calculations without considering anthropogenic heat fluxes.

Global warming will affect how cities use energy as people try to stay within their thermal comfort zone. But while warmer winters may reduce heating demands, hotter summers will increase the need for air-conditioning and refrigeration, processes that emit enough heat to raise outdoor temperatures further. “We anticipate that thermal pollution will increase in the summer as the extra energy used to keep cool intensifies the urban heat island,” says Ginzburg. Understanding how thermal pollution affects the urban climate, and vice versa, will help city authorities forecast their energy needs throughout the year.

Next, Ginzburg’s team intend to explore the physiological aspect of climate change. “We will begin by investigating how people actually feel during certain weather conditions,” he says. “This would enable us to determine how much energy is needed to provide the right level of thermal comfort in each building.”