Thinking about sustainable cities is actually a global challenge. In Europe, one of the most urbanized continents, around 75% of the population lives in urban areas, and it is expected that by 2020 the figure will increase to 80%. As a consequence, the demand for land in cities and their surroundings is increasing and, accompanied by this, the consumption of materials and resources and the generation of waste and emissions, as the urban lifestyle is a highly resource-consuming and hardly sustainable. Uncontrolled urban sprawl is reshaping landscapes and affecting the quality of life for people and the environment like never before. If at the beginning of the 20th century only 10% of the world population lived in urban settlements, in 2007 this figure exceeded 50% and it is expected that by 2050 75% of the world population will live in cities. This demographic change also has very different profiles around the globe. The cities with the highest population growth expected in the coming years are located practically outside the developed world: Lagos, Kinshasa, Jakarta, Karachi, Delhi, Dhaka, Nairobi, Manila, Sao Paulo, Guangzhou, Shanghai, Bangalore and a long list of Asian and African cities are already seeing their population grow in a trend that will continue over time. Today there are already more cities with more than one million inhabitants in China (97) or India (40) than in the United States (39), and more in Latin America and the Caribbean (57) or Africa (41) than in Europe ( 40). The 21st century has become an urban era that requires rethinking the way cities will develop in the coming years. When the western world has barely spent a couple of decades trying to incorporate the sustainability discourse and with great difficulty putting it into practice to sustain the model of life that we have known, in different parts of the world it is taking place an urbanization process of unknown scale and speed. The most paradigmatic case is that of China. It is projected that by 2030 more than one billion Chinese will live in cities, which means that a new Beijing will be created every year in the coming years, and most of this urban growth will happen in what are now small cities and not in the large megacities already existing in the country.
The city is, in any case, a fabulous invention, an extraordinary artifact of collective life that makes social progress possible. Cities act as engines of progress promoting innovation and progress in cultural, intellectual, educational and technological issues. They act as economies of agglomeration just as historically they also served as spaces of freedom and protection, offering promises of prosperity and progress to their newly arrived inhabitants and those already installed. However, the costs of this urbanization of the world are evident. This process multiplies on many occasions the conditions of social inequality and creates problems due to the low quality of urban settlements. It is about the great contradiction of life in the city; as the United Nations State of World Population 2007 report stated, “no country in the industrial age has achieved significant economic growth without urbanization. Cities concentrate poverty, but they also represent the main hope to get out of it“.

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Cities are complex coexistence systems and their functioning depends, in metabolic terms, on the input of resources in the form of raw materials (energy, water,…) and materials that are later expelled in the form of waste and emissions. The concentration of activities and people, something so typical of urban life, generates high transport needs for people and goods to move, demands an enormous amount of energy for lighting, heating, air conditioning, refrigeration,… Being very synthetic, this is the general framework for understanding the sustainability problems of cities and from them the consequences in terms of resource depletion are derived -the oil ceiling being the main risk today-, the growing consumption of resources such as water, the demand for greater energy needs, unequal access to resources at a global level and also at a local level, etc.
Sustainability has therefore become a basically urban challenge and the way in which we think about and design the cities of the future -and, let us not forget, especially in emerging countries- will be decisive for to ensure sustainable human development in the future. The main environmental pressures on the environment come from the urban way of life, driven by driving forces (social habits, demography, land use model,…) on which the ability to act is not easy from local governments. The need to rethink how cities work has become even more urgent in order to ensure quality of life globally. Rethinking cities in terms of sustainability is the most established reference model that remains to be fixed and its practical application requires determined and coherent action, and using diversified strategies. New York, Tokyo or other global-cities, which act as nodes of the global economy and have a relatively stable population and a trend of land occupation that tends to deplete it and occupy it by taking suburban forms in the peripheries of urban centers. The same thing happens, in very broad terms, in the cities around us, the large European capitals, which equally participate in the global economy and with dynamics of land occupation, transport and consumption needs of resources and very inertial materials. The challenge of medium-sized cities is different, which are currently struggling to participate on their own scale in the dynamics of economic exchange and the struggle to participate in urban competition at a global level, needing to provide new population growth, new needs for land consumption, etc. And, finally, we have all the “new” cities that arise from the new urban geopolitics, located in politically unstable environments, very unequal on a social level and that have little economic capacity to meet the arrival of new population, which settles in new urban extensions in difficult housing quality conditions.
All these situations, described in a very broad way, are part of the same challenge, which will have a fundamental impact on the quality of life of the inhabitants of the cities and will determine the conditions of survival of the planet. The economic expansion and the expansion of means of transport have made it possible for the morphology and functioning of our cities to change substantially in recent decades, and we have thus moved towards a model of land occupation that has promoted the territorial growth of areas Spanish metropolitan areas, their large cities and also the system of intermediate cities. On all these scales there has been a change with respect to the traditional form of our cities, through the extension of low-density developments, the extension of second homes on the coast, the expulsion of urban centers from functions previously well integrated in the centers and today in the urban peripheries and, finally, the promotion of monofunctional and differentiated developments for economic activities and housing. As we stated at the beginning of the article, it is a phenomenon that is at the heart of the current economic crisis and forces us to reflect on it in terms of a sustainable economic development model.
To be continued with Searching for an urban model for cities in transition
Photo taken from Earth Observatory< /a>