This year's summer was among the hottest and driest ever recorded in vast parts of Europe, especially in its southern and western regions. In Northern Europe, average summer temperatures were more moderate, yet still warmer than expected — especially in August. In addition, some severe — but luckily not very persistent — heat waves struck the region.

Heat waves are among the deadliest natural catastrophes. Yet, they kill silently — the effects on mortality and morbidity are not immediately seen, unlike with flooding or storms, and they are more complex to detect. Still, the National Institute for Health Development (TAI) named heat waves the leading cause for last year's excess deaths aside from COVID-19.

More frequent and intense heat waves are already being observed in most parts of the world, and are projected to increase in the future. Climate change vastly increases the probability of extreme heat in Europe, as scientifically proven by recent events in 2015, 2017, 2018, 2019, and 2022.

Compared with rural areas, such effects are felt even more strongly in cities. They are characterized by a great extent of (artificial) surface sealing, storing incoming solar radiation much more than natural (green) spaces. Hence, urban spaces are warmer than their surroundings. This effect is called the "urban heat island" effect. Furthermore, urban spaces lead to drier air (lower relative humidity; the "urban dry island" effect) and changes in wind, cloud cover and precipitation patterns.

These effects are felt in Estonian cities as well, but until now, we have not had data to adequately evaluate the severity of the issue.

This May, SEI Tallinn set up a network of 18 weather sensors measuring temperature, relative humidity and precipitation for the B.Green project. Tallinn's weather is usually measured from a station located on the city's outskirts, but we had the opportunity to spread sensors out all over the city — at the seaside, in green spaces as well as in so-called gray spaces, which include buildings and infrastructure where people mostly live and work. These measurements will help better understand how heat waves affect different areas of Tallinn and where green spaces could help create a more livable environment.

Our measurements confirm three dominating factors in Tallinn's city climate: 1. general atmospheric circulation, 2. the impact of the sea, and 3. the impact of green and gray spaces. While we cannot change the first two factors, the share of green spaces is up to us.

Greener spaces cool more overnight

Let's look at a concrete example: an intensive heat wave unfolded from the end of June to early July this year. For seven days, temperatures reached or exceeded 30 degrees Celsius in all or parts of Tallinn. Apartments and offices heated up quickly, affecting concentration at work and sleep quality at night. During the day, the sea breeze cooled down areas near the shore, as water temperatures around Tallinn stayed well below 20 degrees Celsius. Green spaces cooled down considerably more at night than areas with dense infrastructure and buildings.

During the day, temperatures in green and gray spaces are similar, but concrete and asphalt are heated up by incoming solar radiation during the day and release the additional warmth slowly during at night, keeping nighttime temperatures comparably high. When we compare the results of five sensors along the Pollinator Highway — a green corridor located along an old railway — and five sensors in densely built areas, the latter show values of an average of 2 degrees warmer at night than in greener spaces in Tallinn. This effect is most pronounced during clear nights, when differences can reach 5 degrees Celsius and more.

This map shows the average low temperature for seven nights at the end of June and in early July. Temperatures in green spaces along the Pollinator Highway (green line) dropped considerably more than in living quarters, industrial areas, parking lots and the city center — with an average difference of almost 3 degrees Celsius between locations near forests and Tallinn's Old Town. On some nights, those differences exceeded 5 degrees.

While this is not an issue on colder days, we need cooler nights during heat waves to cool down buildings from the inside (by opening windows) as well as outside (by cooling down heated-up walls).

Most challenging are so-called "tropical nights," when nighttime temperatures remain above 20 degrees Celsius all night. Summer 2022 saw 8-11 such nights at our sensor network's "urban gray" locations. Green areas were less affected, especially those with forests nearby. Here, only a few or even no tropical nights were recorded.

Our measurements confirm that trees and green spaces are not only essential for providing shade and mental relief during the day, but also for providing cooler spaces at night, which can help cool down nearby apartments and office spaces.

Live measurements from SEI Tallinn's network are now publicly accessible. Researchers evaluate the data to improve understanding of the interplay of gray and green spaces in urban areas. With data continuously generated every 15 minutes, we start gaining much deeper insight into Tallinn's climate features, can judge the value of green spaces and will be able to give recommendations to ease urban pressure on Tallinn's city climate.

Surface sealing increasing

We can already tell from this summer's measurements that the sensors will give city planners and other urban decision-makers reason to expand green spaces beyond current extensions and add greenery to streets, parking lots and living and working quarters, especially near the city center.

Tallinn is the European Green Capital 2023. While it strives to make green spaces more accessible for its people, surface-sealing by housing and infrastructure development is continuing and even increasing. This sustains and increases the effects that the urban space has on the city climate.

On the other hand, Tallinn is investing in a green band formed by the so-called Pollinator Highway, an old railway track now turned into a green and publicly open space. The guiding principle of the Pollinator Highway is to preserve and increase biodiversity by supporting pollinators feeding from the blossoms around them while offering diverse opportunities for leisure activities and environmentally friendly urban mobility. Moreover, the track supports social inclusion by passing through socioeconomically diverse areas.

The Pollinator Highway is a good example with which to demonstrate the climatic value of green spaces to city officials, planners and the general public in order to increase awareness and impact future city planning.

As the European Green Capital next year, eyes will be on Tallinn either way. This is a good opportunity to make densely built-up areas of the city greener, change the way of thinking behind city planning and give nature a place even in the city center.

Andreas Hoy is a senior expert in climatology and leads the climate, energy and atmosphere program at the Tallinn Center of the Stockholm Environment Institute (SEI Tallinn). This article was originally written in the context of the B.Green project, which supports green infrastructure solutions to help cities reach and upgrade their climate change adaptation and mitigation goals.

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