03.11.2017 change 06.11.2017

Expert: climate change affects cities

Global warming Global warming

Cities are affected by global climate change, but at the same time they also modify the local climate, among other things, through the so-called urban heat islands; they also affect precipitation and humidity - told PAP Prof. Krzysztof Fortuniak from the University of Lodz.

The main factor that determines the climate in a given city are global and regional factors. But cities themselves also affect the local climate - local temperature, precipitation, humidity.

Climatologist Prof. Krzysztof Fortuniak from the Department of Meteorology and Climatology of the University of Lodz reminded that one of the most well known manifestations of the effects of the city on the climate is the so-called urban heat island, the relative temperature increase in the city in relation to the temperature in the surrounding areas. This is a dynamic phenomenon that occurs mostly at night and disappears during the day.

"This phenomenon is observed mainly on clear, cloudless nights when the exchange of energy between the earth and the atmosphere, urban development, heat accumulation in the city and several other factors cause the temperature to remain significantly higher" - noted the climatologist. In extreme cases, these temperature differences can reach a dozen degrees, but usually it is 1-2 degrees C.

In the light of climate change, this phenomenon has its positive and negative sides. The advantage is lower energy consumption for heating apartments in winter. The negative aspects of the urban heat island are felt mainly during the heat waves that increasingly occur due to climate change.

"Because the temperature at night in the city is higher than it is outside, the human body may not have the proper conditions to have some rest from the heat" - noted the expert.

According to the climatologist, cities also influence the modification of the field of wind and precipitation. However, in his view, the precipitation situation is more complex than the temperature, because various physical processes may differently affect the size and intensity of precipitation.

"Here the influence of the city is a little less established; because of the fact that we have air pollution and more condensation cores, smaller water droplets may form and precipitation may not be so strong. But on the other hand, because of elevated temperatures, we have increased convection, or increased upwards airflow, and therefore we would expect an intensification of storms and heavy rains" - he pointed out.

On average, however, higher precipitation in urban areas is observed. It is less clear how cities affect extreme rainfall, although some studies indicate a storms and precipitation increase mainly in leeward districts.

According to Prof. Fortuniak, in terms of adaptation of cities to climate change, simple methods should be used. The possible increase in heavy rains suggests that the urban sewer systems should be modernized so that more water can be discharged. Another way is to create bigger green areas so that the water has a place to sink, and not just flow down the streets.

The issue of adaptation of cities to the urban heat island is, in his view, more complex. "If, in the case of a heat wave, we for example install air conditioning in buildings, then the heat must be transported from the inside of buildings to the outside. And this causes a further temperature increase outside" - assessed the climatologist.

There are various innovative ideas for urban adaptation to climate change. There are, for example, proposals for bright-colour roofs of buildings that reduce the amount of solar energy that the city absorbs. Historic districts in southern Europe are characterized by buildings that, through an appropriate ratio of street width to the height of buildings, make these areas more shaded.

"Materials are introduced that have greater thermal inertia and prevent houses from heating up and cooling down so quickly" - noted the climatologist.

In recent years, Łódź scientists have been studying the surface exchange of greenhouse gases between the Earth\'s surface and the atmosphere and conducting such unique measurements also in cities.

"We measure streams of water vapour, carbon dioxide and methane, the main greenhouse gases. It turns out that cities are very strong, point sources of carbon dioxide and methane emissions to the atmosphere, with very large surface emissions that we are able to measure" - explained Prof. Fortuniak. Measurements of surface methane streams in cities, apart from Łódź, are conducted only in London.

"It turns out that methane emissions from cities are comparable to emissions from marshes of the same size, because we conduct similar surveys in the Biebrza marshes. Marshes are the main land source of methane emissions into the atmosphere, which means that cities are also a very powerful source of that greenhouse gas emissions to the atmosphere, and of course of carbon dioxide" - stressed the expert.

In his opinion, cities influence even the global climate. "Locally, this effect is not very visible, but cities are a source of greenhouse gases, hot spots on the map of the world, and play a big role in the exchange of these greenhouse gases between the surface and the atmosphere" - concluded Prof. Krzysztof Fortuniak. (PAP)

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