The Green Dimension of a Compact City: Temperature Changes in the Urban Area of Banja Luka

Abstract

Responsible and rational urban planning is reflected in an integral and multidimensional approach to city development. Contemporary theories of sustainable and resilient urban planning support a clear vision and strategy for city development, emphasizing its identity, specifics, and values. Through its vision and action plans, the city of Banja Luka protects its default identity as a green city while recognizing the current development trend toward a compact city. The paper deals with two theories: green and compact cities, both motivated by ecological and energy planning. The paper aims to show the importance of the green dimension of a compact city and the adoption of the principles of compactness in the green city in order to examine its contribution through synergistic action. Through the comparative analysis of the green structure change toward a compact urban area and temperature rise in the last fifty years and on different scales, it is possible to question the set green goals and effects of environmental urban (non) planning.

1. Introduction

In recent decades, we have witnessed increasing consumption of energy and the process of switching to electric systems, as well as increasingly frequent advocacy of savings and the struggle to reduce harmful effects on the environment. The political events of 2022 further intensified the energy crisis present since the 1970s and forced humanity to think even more deeply about the way of life and daily habits that would lead to reduced energy consumption. From the perspective of urban planning and the architectural profession, the question arises of finding more optimal solutions and designs for the new challenges that the entire planet is facing.

An efficient urban form as a permanent social good that shapes the conditions and way of life of its inhabitants is the priority in planning and new city development strategies. Many urban policies dealing with the issue of city form are evolving to promote sustainability, quality of life, and green areas. In this regard, compactly planned urban forms can be an effective air quality improvement tool that reduces traffic congestion (directly affecting fewer accidents, less energy use, and less environmental pollution) while also increasing social interaction and physical activity [1,2]. According to theoretical considerations, the global problem of the environment lies in the development of underdeveloped countries and the increase in the number of urban populations, i.e., the intensive development of urban areas. Apart from the economic and social dimensions of the city, the assumptions that make the city more livable in an ecological sense are also important in modern planning [3].

Approximately 50 years have passed since introducing the term “compact” city with clearly defined parameters and organization, which even from today’s point of view can be called a utopian idea [2]. Such a city as a strictly defined whole has not yet appeared, but we can recognize its principles in cities: urban form with clear boundaries and dense concentration of population less dependent on automobile transport; spatial diversity with mixed functions; social equality and self-sustainability [1,4]. Such a concept of planning and designing in urban planning is based on an efficient system of public transport and the promotion of walking and cycling in order to reduce energy consumption and pollution. Urban forms should be coherent with the economy, while a vital society that is inclusive, just, and healthy should have a high quality of life and environment. Urban form, as the physical form and pattern of land use of cities and settlements, has various attributes [2]. At the level of urban planning, the question of energy needs to be answered precisely by choosing an appropriate spatial model of the city, which does not only include the expansion of urban spaces to new areas but also the reconstruction of existing abandoned industrial zones, barracks, brownfield sites, etc. [5,6].

Although the compact city is conceptually positioned within the debate on sustainable urban form, there is a need to integrate climate change discourses in light of the important contribution that building adaptation can make [7]. This is a broader contextual issue that should affect all urban development and residents around the world but is unfortunately subject to a political context that may or may not mention (or acknowledge) climate change. This is a debate that needs to be engaged now not only because of the increasing scale of climate change effects experienced by the world, mainly developing countries, but also because there is a need to understand ways in which potential conceptual conflicts can be resolved. In order to achieve this, a new policy focus is needed at multiple levels (regional, city, and local) where it is necessary to respond to the practical challenges of climate change mitigation and adaptation.

In the last twenty years, spatial and urban planning as a tool for mitigating and adapting to climate change has become increasingly important. Experts assume that rising global temperatures and sea levels will increase the risks associated with climate change, such as floods, droughts, landslides, and extreme weather events [8,9]. To prepare for these disasters in the future, interdisciplinary and precautionary land management measures are needed. Planning, concerning the development of urban settlements, plays an important role in disaster risk management. The consequences of climate change in the form of longer, hotter, and more frequent temperature waves (which adversely affect the health of the population and cause a shortage of drinking water) require special adaptation measures in the construction sector and planning solutions [9], reducing the impact of heat islands that are formed in cities. Green infrastructure measures (green roofs, green walls, transport corridors) affect not only the reduction of energy consumption for heating and cooling [10], but also water retention, air purification, and increased biodiversity [11]; the preservation and improvement of areas of flood plains; protective forests in mountainous areas; and wet habitats, which, in combination with other infrastructure, can be effective in reducing the possible consequences of floods, landslides, storms, and forest fires [12,13].

Key ideas of environmental protection and energy responsibility relevant to sustainable cities include a multi-scale approach with recognizable patterns, process relationships, and increased physical and functional connectivity at appropriate scales: metropolitan area or city, district or neighborhood, and individual areas or spaces. It is a wrong opinion or insufficient information about the fact that the control of energy use and the greatest savings can be achieved primarily during the design of individual buildings, using appropriate energy-efficient building materials and technological solutions. Dominantly, research in the field of energy efficiency takes place in this area, which is adequately followed by legal regulations. Building energy consumption carbon emission is highly correlated with microclimate, and green structure has long been recognized as the natural way to improve the microclimate. Some results clearly showed that building energy consumption carbon emission was highly correlated with green structure compactness [14]. On the other hand, ecologically effective spaces introduce an ecological index for evaluating the quantity and quality of urban greening, and the green area in direct contact with the ground on the plot directly planted in the natural substrate has the most favorable ecological effects [15]. The climate of the city depends on the city’s geographical location, size and population, urban structure, land use distribution, type of urban activities [12,16], and the relative presence or absence of trees and vegetated green areas within a largely impermeable landscape [14,15,17].

This paper examines the relationship between the development of a compact city and its green dimension and how such development, by changing the green structure, actually changes the city’s climate. In the example of Banja Luka in site-specific conditions, in parallel, green structure changes and temperature changes for the entire city in the last fifty years are monitored, while detail analysis is on three scales: the wider urban area, the narrower urban area, and the largest park in the city center. The focus is on large urban green structures—parks with an area equal to or larger than 1 ha, as a green structure, have an influence on the city’s climate; the temperature values were measured during one year within four of these green urban parks. By analyzing the temperature values inside city parks at different scales of the city, both now and through the previous period, it is possible to recognize the green dimension of Banja Luka as a compact city. The goal is to verify the green dimensions of a compact city by measuring the temperature and how the climate of the city changes from the dimension of green infrastructure—a unique system of interconnected parks—to the dimension of green structure—parks as isolated green islands (Figure 1).

2. The Green City of Banjaluka—Half a Century of Planning and Development towards a Compact City

The compact city, as a concept of urban planning and urban design within which a relatively high density of residential buildings mixed with other urban functions is preferred, is the principle by which Banja Luka develops in its post-socialist and transitional history, from the 1990s to today. The city of Banja Luka does not have a valid General Urban Plan, as the first and only one adopted back in 1975 (UP 1975) is outdated and invalid [18]. The proposals from 2008 (UPp 2008) [19] and 2020 (UPp 2020) [20] were rejected, so the city has been developing from the previously adopted green concept without a general plan towards a compact one for the last fifty years. This trend is readable through other valid documents such as the City Spatial Plan (SP) [21], Regulatory Plans (RP) [22,23], as well as the Action Plan for the Green City of Banja Luka (AP) [24].

In compact cities, walking, cycling, and mass use of public transport are promoted with the aim of reducing air pollution and making more efficient use of energy, requiring a high-quality development plan. As another benefit of a compact city, the greater possibility of social interaction is often highlighted, given the concentration of a large number of residents in a relatively small area. Numerous studies on the effectiveness of this type of urbanization have been conducted on examples of cities around the world and have produced very different results. Increasing the density of the existing built structures contributes to a reduction in energy consumed in traffic, which is necessary for the functioning of new parts of the city. In this sense, well-organized public transport and alternative modes of transport are also important. By increasing the built-up area, the total need for energy in the city is also reduced through the choice of a building type because the energy consumption of a residential unit in a multi-apartment building is significantly lower than that of residential units in free-standing family houses. In the planning process, the elements of locating settlements and buildings (terrain configuration, terrain slope, available infrastructure, etc.) as well as microclimate (wind speed and direction, sunlight, air pollution) and green structures are considered because they are directly or indirectly related to energy consumption. Some of the goals set in the urban planning of Banjaluka are somewhere in between these compact city principles and recognized values and natural potentials for the preservation of a green city: reconstruction and densification of the center of the narrow urban area; arrangement of the banks of the Vrbas and Vrbanja rivers [22] into attractive green and recreational zones [25]; utilization of existing unused buildings and areas as renovated areas for attractive buildings and contents [19,20,21]; and densification of residential construction in the city center by transforming blocks of family housing into mixed blocks [20]. More research is needed on how dense urban neighborhoods incorporate green open areas. These spaces may not always be publicly available, visible, private, or public. Shared green spaces are increasingly prevalent in high-density residential areas due to the large proportion of apartments with limited opportunities to use green space within the block. Where there are opportunities for the formation of green structures, e.g., in the form of green roofs and balconies, there is a need for empirical evidence to examine the actual impact they have not only on cooling during heat waves but also on residents, property values, and biodiversity habitats. There are a growing number of research papers and urban policies on the concept of “green infrastructure” [3]. The development of green infrastructure and its research and development projects vary widely from country to country [12]. In a time of big global issues and a multitude of universal solutions, it is of great importance to understand the local specificities of spatial planning, recognize and prove favorable solutions, and protect and apply systems that are resistant to climate change.

2.1. A Preserved Green Ring—A Green Structure in a Wider Urban Area

The city aims to be an example of innovative, smart, and sustainable utility and traffic infrastructure, supported by an efficient land use system and increased resistance to climate change and other natural disasters. With a healthy and dynamic built environment, a network of green and blue infrastructure will protect and improve water resources, soil quality, and biodiversity, and maintain Banja Luka’s reputation as a “green city” [24].

The city of Banjaluka has excellent bioecological and spatial conditions for the formation of quality green matrices [26]. The current picture of green areas is represented by forest complexes on the east, south, and southeast sides, which are connected to the green areas of the narrower urban area of the city through the vegetation of the rivers. This spatial arrangement of greenery and dominance of the forest landscape, visible in Figure 1 and Figure 2, also create favorable microclimatic conditions for the quality of life of citizens [27]. Sustainable development is of primary importance because it is a prerequisite for quality of life in the city. It implies the identification, rehabilitation, and preservation of natural resources and the creation of conditions for the formation of a quality environment. Effective management and optimal use of benefits and potential is one of the most important goals of all urban plan proposals in principle. The main objectives of the last proposal in the domain of natural conditions are:

• Adaptation to climate change and the development of databases of spatial data and information on local climate changes, including information on climatic extremes and disasters and the vulnerability of certain areas;
• Checking the degree of risk when planning content that may pose a risk for changing the climatic characteristics of the area [20];
• The concept of forest management and development in the area of the Urban Plan is reflected, first of all, in the protection of forest complexes from a permanent change of purpose as well as the prevention of forest degradation, which ensures the functions that these forests perform. The plan confirms the current status of the special purpose forests in the City of Banjaluka, which represent the “lungs of the city” and the “green ring” [20,21] in the context of their use for the needs of active and passive recreation, tourism, scientific and research work, etc. Protection against deforestation and change of use especially applies to privately owned forests, which are more endangered in this matter. Through appropriate measures of land policy and financial incentives in the broadest sense [21,27], the situation in this segment needs to be improved, and certain areas (plots) should be purchased and turned into public green spaces or recreational areas [18,19,20]. Special emphasis is given to encouraging the afforestation of private plots—agricultural land of worse credit rating categories that, due to their natural characteristics, are not promising for further agricultural production or construction and for which it is necessary to provide appropriate financial machinery and advisory support [20,21].

Figure 2. Urban plan for Banja Luka from 1975, green areas and recreation [18].

Figure 2. Urban plan for Banja Luka from 1975, green areas and recreation [18].

Forests and forest lands represent an important natural resource of the city, whose positive influence, first of all, is reflected through the provision of favorable microclimatic conditions for the quality of life of citizens as well as the facilitation of sports—recreational, scientific—research, and tourist functions. The general goals, coming from the need to preserve the forest fund and environmental protection, are protection and improvement of the state of the entire forest fund, regardless of ownership, and preservation of the health status of forest ecosystems through constant monitoring and timely reactions, as well as converting parts of the forest areas in the western and northern parts of the area into public parks and raising awareness of the importance of forest ecosystems [20]. In the process of defining the needs and future activities in the field of further development of the city, planning goals for the development of the entire system of green structures are defined. Special-purpose forests (park forests) within their former and existing boundaries are presented in

Table 1. Special-purpose forests (park forests) in a wider urban area in 1975 (UP1975), 2008 (UPp2008), and 2020 (UPp2020) [28].

Forest in a Wider Urban Area	UP 1975	UPp 2008	UPp 2020
Trapisti	400 ha	344.35 ha	344 ha
Starčevica	1250 ha	1125.62 ha	1108 ha
Šibovi	100 ha	227.77 ha	243 ha
Total	1.750	1.697.74	1.695

, with changes in total area from 1975 to 2020.

Forests and forest parks are goods of common interest, enjoy special state protection, and are used under the conditions and in the manner prescribed by the Law on Forests. According to the data of the forest cadastre as well as the digitization of orthophotos, the total area of forests and forest land in the area covered by the urban plan of the city of Banja Luka occupies an area of 6295 ha, which is 35.2% of the area and which can be considered as having a high forest cover if taken into account that it is a densely populated area. The degree of forest cover obtained (38.23%) is much higher than the same in the Urban Plan from 1975 (20%), which is the result of the expansion of the boundaries of the current area, including larger forest complexes. The green ring of urban forests, which actually constitutes the wider urban zone of the city, is shown in Figure 2 and Figure 3.

2.2. Towards a Compact City—A Green Structure in a Narrow Urban Area

The boundaries and the scope of the wider, that is, the narrower, urban area did not change significantly, but in the direction of the development of a compact city, one of the main goals is to define realistic spatial possibilities for the growth and development of the city within the existing boundaries of the urban area. At the same time, this implies the establishment of a harmonious relationship between the development of the city and the natural and created environment, i.e., the preservation and improvement of the environment. Some objectives of the Urban Plan proposal for 2020 are:

• Optimal use of urban area space, harmonious socio-economic development and spatial distribution of physical structures, rational use of built capacities, and maximum protection of natural and cultural values of the environment as conditions for the natural reproduction of men and society;
• Environmental protection for the purpose of creating optimal conditions for a healthy and pleasant life. It represents the establishment of a creative harmony of natural and created values, nurturing and raising greenery, protecting watercourses as a general social good, and achieving population densities that enable healthy and humane city environments where social contacts are easily established.

The basic measure given in city plans that should be implemented in order to improve the level of urbanity and which refers to green structures is that in urban areas, an appropriate standard of green areas must be provided of 12–15 green area sq. m per capita and the presence of various greenery (parks, squares, rows of trees, forests, forest parks) [19,20,21]. Although the greenery cadastre has been set up, data on the fulfillment of these conditions is unavailable, and individual measures in various documents are difficult to compare. In the last fifty years, since the adoption of the first and only official Urban Plan of the City of Banja Luka (UP 1975), the total area of landscaped green areas has been increasing, but as the number of inhabitants rises, the degree of contentment [m²/in] is actually decreasing in the following Urban Plan Proposal from 2008 (UPp 2008) and Urban Plan Proposal from 2020 (UPp2020) presented in

Table 2. Change in the existing condition of green areas and the degree of contentment according to the plan from 1975 and the proposals of the plans from 2008 and 2020 [18,19,20,28].

	UP 1975	UPp 2008	UPp 2020
Green areas in parks, squares, and other public areas * [sq. m]	196.298	233.979	241.781
Degree of contentment [sq. m per capita]	2	1.39	1.53

* green areas along roads and within residential blocks excepted.

.

There has been a significant increase in the city’s green area throughout the observed period, yet some parks do not exist today.

Table 3. Area [ha] of the largest parks in the city of Banja Luka that existed in the period from 1975 to 2020 [28].

Parks in a Narrow Urban Area	UP 1975	UPp 2008	UPp 2020
Park dr Mladen Stojanović	9.69	9.69	9.69
Poljokanov park	2.12	2.12	-
Picin park (S. Markovića)	0.63	0.63	-
Kupusište	0.94	0.94	0.94
Mičurinov park	2.25	-	-
Park Petar Kočić	0.95	0.95	0.95
Total	16.85	14.6	11.85

shows how, over the past fifty years, certain parks have been lost in total or decreased significantly due to the gradual conversion of their parts into construction land. However, its remains are actually visible today in the form of squares and other public green areas, as shown in the following

Table 4. The overall green structure of narrow urban areas in 2020 [28].

	[m2]	%	[m2/inh]
Parks	97,022	3.38	0.6
Squares	49,463	1.72	0.3
Other public areas	31,021	1.08	0.2
Total	177.486	6.18	1.1
Park—(cultural landscape Kastel)	64,275	2.24	0.4
Green areas along traffic roads	230,000	8.01	1.5
Greenery within urban blocks	516,185	17.98	3.2
Green spaces and playgrounds—educational institutions	512,163	17.84	3.2
Sports and recreation	264,982	9.23	1.7
Coastal vegetation	1,105,903	38.52	7
The overall green spaces	2,871,014	100.00	16.6

.

Data on the areas for the overall green areas of narrow urban areas in 2020 shown in Table 4 and which refer to the same greenery as Table 2 do not match either in the total area, which is larger in Table 4 nor in the corresponding green area sq. m per capita, which is larger in Table 2. Although it is a presentation of the existing situation, precise data cannot actually be obtained from various documents, and plan proposals for unreliable areas and index values are cited.

In a more detailed analysis of each category, one space stands out. In the category “Green Structure and Playgrounds—Educational Institutions” in the higher education section, an area of over 20 acres stands out, larger than the total area of parks in the urban area of the city. The green area of the university campus is 87.90% of the subcategory of educational institutions, that is, 37.85% of green spaces and playgrounds—educational institutions, or 7% of the overall system of green spaces. This area is invisible in the UP 1975 due to its previous military status. Since its inception, due to its military purpose, the complex has been hidden from the public, so only after it came into civilian hands did it open up with all of its natural and created potential.

2.3. Former Brownfield—Park Architecture Monument “University City”

The “University City” Complex of the University of Banja Luka was created as an Austro-Hungarian military facility during the occupation and annexation, and from then until the end of the twentieth century, it had a military purpose on a total area of 28 hectares. In May 2012, the Minister of Spatial Planning, Construction, and Ecology of the Republic of Srpska, in the matter of declaring a protected area, at the request of the University of Banja Luka and according to the previously obtained opinion of the Ministry of Agriculture, Forestry, and Water Management and the Republic Administration for Geodetic and Property Legal Affairs, issued a decision placing the “University City” Complex in Banja Luka under protection as a protected area for resource management [24]. By the decision of the Banja Luka City Assembly in 2016, a protected area with sustainable use of natural resources was declared a Park Architecture Monument, “University City” [23,29]. Due to the abundance of dendriform that was planted in different periods and the fact that the Vrbas River with its natural vegetation and bird fauna is the eastern border of the complex, it is certain that it is an extremely valuable ecological and landscape area in the center of Banja Luka.

According to the classification in the green system of each city, including Banja Luka, tree groves and parks belong to objects of landscape architecture for public use. However, the specificity of the “University City” facility is that it is functionally rounded as a park in a university center, with the botanical garden covering an area of 5 hectares [29], as shown in Figure 4. In addition to its aesthetic, recreational, health, and general ennobling functions, it also has educational, scientific, and cultural-historical significance. Over the last 20 years, university rankings, which allow one to classify and compare the energy and environmental performance of university buildings and campuses, have become more and more widespread, and their diffusion is still increasing by means of established models [30]. Green Metric was set up with the intention of making a contribution to awareness and action on sustainability and exploiting the potential of the university as a site where these things have a chance of realization. Future challenges to civilization include population pressure, climate change, energy security, environmental degradation, water and food security, and sustainable development [31]. Despite many scientific studies and public discussions, governments around the world have yet to commit to a sustainable agenda. The goal of creating a world university sustainability ranking was done with the understanding that the diversity of types of universities, their missions, and their contexts would pose problems for the methodology. Year after year, the number of criteria grows, new possible measurable indices are explored, and the green dimension remains first on the list and is present in several categories.

On the other hand, the campus is arranged on compact principles too. The facilities of the complex of the former “Vrbas” barracks were partially reconstructed and adapted to a new, primarily educational purpose, and in this way, a University Center was created, which today houses: the Rectorate and the Administrative Centre of the University; the Institute for Genetic Resources with Botanical Garden; the Academy of Arts; the Faculty of Medicine (Department of Pharmacy and Dentistry); the Faculty of Political Science; the Faculty of Philosophy; the Faculty of Philology; the Faculty of Agriculture; the Faculty of Physical Education and Sports; the Faculty of Architecture, Civil Engineering, and Geodesy; the Computer Centre; the Sports and Athletic Hall; and other supporting contents and auxiliary objects. Housing within the subject scope is represented in the form of student dormitories, with an accommodation capacity of about 780 places. Next to the pavilion of the student dormitory, a theater and a space for gathering students were built. Business within the subject scope includes service activities within the student canteen facility, a temporary facility, as well as the nursery of the Institute for Genetic Resources, where seedlings are produced [23]. Within the site, there are built-up traffic areas, most of which were inherited from the time when the area was a military barracks. These are internal roads allowing controlled access to all existing facilities, and there are a large number of footpaths within the scope, so the largest percentage of movement within the University City is pedestrian.

3. Fifty Years of Temperature Changes in the City of Banja Luka

Intensive urbanization and global warming are impacting the health and well-being of the urban population. With global changes in climate, making cities climate-proof is becoming increasingly critical. Along with thoughtful planning and monitoring of the development of the urban part of the city, it is also necessary to adopt control measures and analyze temperature change cause-and-effect relationships. As previously shown, Banja Luka has a very favorable natural position, and with a clearly defined border between the narrower and wider urban areas, the expansion of the city is controlled to protect the green ring of the forest park. The green dimension of a city was presented on three scales over the last fifty years, while in this chapter the temperature changes in the same period for the same area are given in order to finally establish the relationship between them and question the effectiveness of the climate-resilient urban green system.

The cooling effect of large urban parks has long been of great interest to urban planners [32]. Thanks to their large area and location in the heart of the city, these parks often have a significant impact on the temperature of city spaces. The cooling effect of these parks depends on several factors, including the size and shape of the park, the type and amount of vegetation cover, and the regional climate. Thus, the green islands in the city in the form of large parks of 20 ha have a temperature of 2 °S to 5 °S lower than that measured in the city [33]. The difference in air temperature between the city center and the rural landscape can reach over 10 °C [11].

Urban green structures are considered a suitable way to reduce the impact of urban heat islands and provide comfort to nearby residents. In addition to cooling the real space, urban green structures can also affect the surrounding space, and this phenomenon is called the “cooling effect” by means of urban green islands [34]. The most important issues regarding the cooling effects of urban green spaces are the intensity and density of cooling, which can play a major role for urban planners in dealing with the urban heat island [33,34]. One study used data from micrometeorological measurements performed in different urban spaces (downtown, urban park, riverside) randomly chosen without mutual connection, simple relationships, or distance in Banja Luka, Bosnia and Herzegovina, on hot summer days in June 2021, concluding the importance of small-scale micrometeorological measurements, the outcomes of which can be incorporated into the climate-responsive urban design [35]. Measurements were performed on three consecutive days in the period 9–18 h, and the results showed that the lowest maximum temperature was measured in the park, while the riverside maximum temperature was 1.2 higher, and the highest maximum temperature in the downtown location was 1.3 higher than that measured in the park [35].

There is a need for further research in the area of planning that would be directed towards the rational use of not only space, which is one of the primary goals, but also towards planning that will enable the rational use of energy precisely through the management of space and act accordingly to the principles of sustainable city planning with constant verification of the effectiveness of such a plan. Understanding the green structure cooling phenomenon is of great importance in the discussion of sustainable urban planning, especially with the knowledge of a constant increase in temperature in recent decades and the fact that heat waves are more persistent and frequent. According to the data obtained by the Republic Hydro-Meteorological Institute, the year 2022 was the warmest year, with a mean measured temperature of 13.47 °C. The mean annual temperature of 13 °C was the highest recorded since 1863, while in 2014 the mean annual temperature did not reach the same value [36].

Table 5. Temperature changes in Banja Luka in the period 1971–2020 by decades [36].

	Mean Annual Temperature	The Lowest Mean Annual	The Highest Mean Annual
1971–1980	10.57	9.88	11.22
1981–1990	10.86	9.71	11.66
1991–2000	11.36	10.28	12.82
2001–2010	11.95	10.71	12.80
2011–2020	12.70	12.16	13.41

presents the data on the mean, lowest, and highest annual temperatures for the period from 1971 to 2020 in five decades, all given in order to understand the temperature behavior in the city of Banja Luka and the change of the city’s climate in the last fifty years.

The temperature changes presented here by decades showed the Tl rising the fastest, compared to the Tm and Th. In fact, all three measurements have a trend of accelerated growth over the decades. Yet, a spike was recorded in the last decade, the highest for the Tl of +1.45 °C. The Tm spike has been +0.75 °C and +0.61 °C for Th in the last decade.

Compact city strategies must be designed to generate a significant synergistic effect with state-of-the-art urban development strategies, not as a primary or alternative model. It is necessary to plan well-coordinated urban functions in a sustainable urban form to prevent the potential negative impacts of compact city policies, including traffic congestion, heat islands, and urban landscape degradation [1,4]. The University Campus, as an example of compact city planning with sustainability principles [37], either in the scale of the buildings inside the campus [38] or its brownfield conversion and park structure [39], represents the largest organized green area for the city itself, and its position in the narrower urban area plays a significant role in cooling.

In 2020, the city of Banja Luka adopted the Action Plan for a Green City, with the goal of improving the ecological situation in Banja Luka in an efficient and financially sustainable manner with maximum benefits for the economy and society. The vision of the city sets a broad course for the green transformation of Banja Luka. The strategic goals that relate directly to green areas in this document are in the same direction: (1) reduce the uncontrolled expansion of the city; and (2) increase the volume, quality, and diversity of green areas and other green infrastructure [24]. In the case of uncontrolled expansion of the city, when it comes to Banja Luka, we have the example of a dispersed city as a model of the expansion of urban areas in the most unfavorable way in terms of sustainability. The expansion of the city, which implies low building densities, results in a number of unfavorable impacts on the natural environment. These are primarily high energy consumption and the constant expansion and occupation of new areas (very often agricultural land). The expansion of the city in this case requires greater investments in the accompanying infrastructure—traffic and communal. Large spatial distances arise that must be overcome in order to satisfy basic life needs other than housing (work, trade, education, culture, etc.). We ignore the fact that it is possible to make large energy savings through the very process of spatial and urban planning by harmonizing the interests of different users in space with the aim of sustainable development. In strategically attractive and high-quality areas of the traditional city center and the wider city center (vacant areas, complexes of abandoned factories, barracks, etc.), neoliberal capital sees an opportunity to create a new urban-architectural resource [38]. The University Campus, represents the largest organized green area for the city itself, and its position in the narrower urban area plays a significant role in cooling. In order to answer the question of the role of such a green structure in the city’s urban area, the measured temperature values were compared for the year 2022. In the annual report of the Hydrometeorological Institute of the Republic of Srpska, the year 2022 was listed as the warmest in the period since the measurements were taken, and during the year, 70 days with temperatures above 30 degrees Celsius were recorded [36].

In order to answer the question of the role of such a green structure in the city’s urban area, the measured temperature values were compared for the year 2022. In the annual report of the Hydrometeorological Institute of the Republic of Srpska, the year 2022 was listed as the warmest in the period since the measurements were taken, and during the year, 70 days with temperatures above 30 degrees Celsius were recorded [36]. Measurements were made at four locations in the city, namely in the park forest of the wider urban area (Banj Brdo), in the University Campus park (Campus), at the Faculty of Natural Sciences and Mathematics (FNMS) in the city park Mladen Stojanović, and at the measuring station Banja Luka on the northeastern border of the narrow urban area in the direction of the Trapisti park forest (Banja Luka), presented in Figure 5.

The section follows the position of the measuring stations, so it is placed in the direction southwest—northeast, covering both the wider and narrower parts of the urban area of the city. As presented, the locations of the measurement stations are widely spread in the city and always within a larger green area.

Table 6. The mean monthly air temperature during 2022 was measured at four different measuring stations [29,36].

	Measuring Station
	Banj Brdo	Campus	FNSM	Banja Luka
March	5.9	6.3	6	6
April	10.2	11.2	11.1	11.2
May	17.3	18.9	18.6	19
June	21.8	24.1	23.6	24.2
July	22.6	24.7	24.2	24.6
August	21.5	23.2	22.9	23.4
September	16.1	17.1	16.9	17.2
October	14.2	14.3	14.1	14.2
November	8	8.2	7.7	8.2
December	5.6	5.5	5.2	5.7
Vegetation period	14.32	15.35	15.03	15.37

presents the values measured for the average monthly air temperature at these four locations.

To comparatively analyze the values of the average measured temperature for the whole year, the average temperature for the vegetation period at all locations of the measuring stations is also shown, due to no temperature records during January and February for the campus measuring station, where the Institute for Genetic Resources measures temperature during the growing season. The highest mean temperature was on the campus, with +1.03 °C higher than the mean temperature measured in the wider urban area on the Banj Brdo measuring station. On campus, there is a difference of +0.3 °C in regards to the mean temperature measured at the FNSM measuring station in Park Mladen Stojanovic in the narrow urban area. A difference of −0.02 °C compared to the mean temperature measured at the Banja Luka measuring station was unexpected. Seven years ago, when the Regulation Plan for the Campus was adopted in 2016, the mean temperature on the campus was −1.92 °C, lower than the average temperature measured at the Banja Luka measuring station for the same year [23,39].

Table 7. The absolute maximum monthly air temperature during 2022 was measured at four different measuring stations [29,36].

	Measuring Station
	Banj Brdo	Campus	FNSM	Banja Luka
March	22.8	23.5	22.7	24
April	26.2	26	25.2	26.5
May	30	32	32	33
June	35.1	36.8	36.3	37
July	38	40.8	39.6	40.7
August	36	38	37.3	38.5
September	32.5	34	33.5	33.7
October	25	25.3	23.5	24.7
November	29.2	30.6	28.4	29.1
December	19.2	21.3	9.6	20.4
Vegetation period	29.4	30.83	28.81	30.76

shows the absolute maximum temperature values during the year 2022 at the same measuring stations.

The highest absolute maximum temperature was measured on the campus, higher by 1.43 °C than the absolute maximum temperature measured in the wider urban area at the Banj Brdo measuring station. On campus, there is a difference of +2.02 °C in regards to the absolute maximum temperature measured at the FNSM measuring station in Park Mladen Stojanovic in the narrow urban area. The measurement results for the campus measuring station showed an absolute maximum temperature of +0.7 °C in regards to the Banja Luka measuring station.

Such results are unexpected, especially when it comes to the location of the campus, previously presented as an example of good planning practice based on the principles of compactness and brownfield reuse in the largest protected park in the city. Regardless, the campus was the hottest place for absolute maximum temperature measurements and had a sharp increase in the mean annual temperature in the past seven years. To understand these illogicalities, it is necessary to return to a larger scale and additionally analyze the changes in the green structure at the locations where the measurements were made.

4. A Well-Planned Green City Cooling System or the Largest and Hottest Park as an Indicator of the Vulnerability of Such a System

The first and only urban plan of the city of Banja Luka from 1975 very clearly laid the foundations for the development of a green city using natural potentials, strengthening them, and preserving them in the planned system. The wider urban area was not called the “green ring of the city”, but the forest complexes from the south, southwest, and east of the city (today the forest parks of the wider urban area) were introduced into the urban fabric (the narrower urban area) in the form of wide belts according to the basic concept of greening the narrower urban area, corridors of the primary traffic network, and watercourses of the Vrbas and Vrbanja rivers [18]. A system of parks and sports-recreational centers, a network of tree rows, and alleys, as an urban fabric vegetative nervure, continue on such a basic green reinforcement of the narrower urban area in a grid of transverse and longitudinal extension. Some of the parks existed but cannot be seen today on the city map, and even those that have survived have reduced areas or can only be seen in some parts. Therefore, a plan that considers the green structure as a system—the uniformity of use and distribution of green areas, which is a continuous and homogeneous spread of greenery throughout the territory—touches all the contents and functions of the city as a system that characterizes Banja Luka as a green city. Figure 3 shows a map of existing and lost green areas in the period from 1975 to the present, and it becomes clear how the idea of the system was lost and the city got a green ring. Today, we can see the essence of such planning and the idea of a system of urban greenery, which aimed to additionally connect and strengthen the connection between the forest park of the southwest and the northeast valley of the Vrbas River.

The paper presents temperature measurements along the river Vrbas, from the measuring station Banj Brdo in the forest park in the southwest of the city in the wider urban area of the city, through the largest green protected park of the University Campus, to the measuring station on the northeastern side of the city in the direction of the Trapisti Forest Park, illustrated in Figure 5. Figure 6 shows the relation between temperature behavior for each measuring station and the size of the preserved green areas from 1975 to the present in the circumambient for each measuring station.

There was no data for the previous measurement at the FNSM measuring station, yet the presented diagrams of temperature behavior for the other three locations confirm the representative fraction set below to show preserved green surfaces’ size in the circumambient for each measuring station. In fact, the relation diagram validates the connection between the increase in temperature and the reduction of green structure in the circumambient. The temperature differences are minor in the parts of the city where the green structure has been preserved to the greatest extent.

The campus represents a good example of the application of the principles of compactness and development of the city with the protection of green structures, but if we look at it only in this way and isolate it in relation to the environment, it cannot preserve the status of a quality green park during heat waves. In fact, of the four green zones presented in this paper, the campus is the most isolated and the warmest place, despite quality planning within its borders. Contemporary research on city cooling actually looks at green structures as islands that cool the environment to a greater or lesser extent, depending on the size and structure of the park [33], the compactness of greenery, and the radius of effectiveness [14], isolated again and on a small scale only. The connection and networking of the existing green structure in order to act as one system is based on the use, strengthening, and protection of the existing natural potentials. The EU Biodiversity Strategy for 2030 states concrete actions for the promotion of nature-based solutions that should be systematically integrated into urban planning. The European Union defines nature-based solutions as “solutions that are inspired and supported by nature, which are cost-effective, simultaneously provide environmental, social, and economic benefits, and help build resilience” [40]. In an urban context, this specifically refers to different typologies of green structures that use nature’s own local resources—vegetation, water, and soil—in a smart and efficient way to tackle different environmental, social, and climate challenges instead of implementing “conventional” gray infrastructure or technological-based solutions. Regarding the campus as a possible candidate for Greenmetric ranking, protection of existing green structures within the limits and sustainable development of the principles of compactness from the inside are seen, but that is only one scale. Such spaces should also be valorized in terms of their importance, sensitivity, and vulnerability on a larger scale—not just the scale of a narrower area but also the scale of the entire city and within other systems of sustainability and city resilience.

5. Conclusions

The compact city development implies quality of life, where high-quality, long-term planning is crucial for sizing the green infrastructure, which, among other things, plays an important role in changing the city’s climate, i.e., the existing trend of increasing temperature in the city. The city of Banja Luka does not have a single long-term strategic plan in which these issues are adequately addressed. Due to changes in urban greenery based on no single system concept of the compact city green dimension, there is an absence of synergistic effects and good results.

The results show that: (1) For the quality development of the city, it is crucial to have a document that directs and regulates the organization of green areas for achieving better living conditions, i.e., a dynamic long-term planning document that provides a conceptual layout to guide future growth and development for a plainly and precisely defined definition of the green dimension of the city, that is, an urban plan, a green infrastructure master plan, and/or a green zoning plan. (2) Green city areas as a social value are used within plans to promote the overall results and local policies and are often completely ignored after the adoption of the document. The weakest link in the planning procedure is the implementation of its adopted principles. (3) The observed dominant trend in the contemporary context of the city is the construction of buildings in open city areas, parks, free green areas, and unorganized spaces under the pressure of the economic factor. (4) The formal approach to urban planning is inherited from the previous or earlier social, political, and economic context. It was based on hierarchy, a “top-down” approach initiated by the state, and based on expertise. A very similar approach to planning is still applied today in the planning discourse. Planners do not have a mechanism that would enable an easier understanding of the complex relationships and numerous interests of different actors. (5) There is no protection of the green structure as a single system, which is why the green structure and the green area have changed significantly over the last fifty years. There is no longer an even distribution of green areas in the morphological structure of the city. A lack of interconnection between the individual elements of the urban green system makes it almost impossible for them to function as a whole. The trend of disappearing green infrastructure can only be stopped by protecting the remaining greenery in the urban area, because compact greenery in direct contact with the ground is an irreplaceable and most valuable part of the city’s green system. (6) Good examples of quality planning of the green dimension and the development of a compact city on a small scale, such as the Campus, do not give good results in their full potential; in the case of inadequate connection and isolation of such spaces, they turn into green islands with reduced cooling capacity and mitigating effects of climate change.

Analysis of green infrastructure Changes within the city’s urban area are presented at different scales as in valid regulatory documents—the wider urban area, the narrower urban area, and the smaller scale of the largest green area in the narrower urban area, the University Campus. Changes are visible only by overlapping the maps due to the unreliability of the available data, which speaks vaguely about the green dimensioning and development trend. The contribution of the paper is in the methodology too, where on one side the changes in the green (infra)structure and on the other the temperature changes for the same period are monitored, i.e., in the recognized natural green system by measuring the temperature within, the effectiveness and the threat to the uniqueness of the set system are examined. The comparison of the temperature measured within different green areas does not answer the questions about the creation of green islands alone, but with a parallel analysis of the maps and temperature, it can answer the question about the (dis)connection in the system’s cooperation in the cooling of the city. The interdisciplinarity of climate change in urban areas as a topic deals with a broad issue and requires the engagement of not only urban planners and spatial planners but also the knowledge of climatologists, meteorologists, and geographers. Besides, the paper emphasizes the importance of understanding the specificity of the place, the nature of the environment, and its potential in responding to global issues, along with constant checking of development through different disciplines and scales with respect and high-quality addition to the laid foundations. There is a necessity for having a general urban plan as a document that would ensure the quality, effectiveness, and framework for “top-down” action for planning the synergistic effect of the compact city and green city theories. Detailed spatial and temporal climate data from diverse, but also urban, environments should be included in the process of climate change-resilient urban planning.

For future research, the question of the connection between individual parts of the green system and their role, size, and effectiveness in the overall green infrastructure remains. There remains the question of one location in the narrow urban area, the measuring station FNSM, with better results compared to the campus, to examine the position and connection of the Mladen Stojanovic Park in the overall green system of the City of Banja Luka.