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Understanding the interactions between urban form, outdoor and indoor spaces, and local climate requIres interdisciplinary interaction

Gerald Mills (University College Dublin) considers the big challenges for cities amid global climate change (GCC) and discusses the need for an inter-disciplinary approach among urban climate sciences to overcome obstacles. A distinction is made between global climate science, which focusses on Earth-scale outcomes, and urban climate science, which refers to processes and impacts at city-scales, including buildings, streets and neighbourhoods.

Cities are an ideal scale at which to address climate change owing to their significant greenhouse gas (GHG) contribution (i.e., their potential to mitigate), their exposure to many of the projected hazards (i.e. their potential to adapt), and  governance structures that can make changes. However, urban climate processes are not currently fully integrated into global climate science, which limits the capacity of cities to make informed climate-sensitive decisions. One reason for this is scale: the relatively small urban-scale processes that drive local responses are not easily included in Earth System Models (ESMs). Moreover, observations of the drivers and impacts of climate change are not routinely made in cities, which are considered too spatially heterogeneous to provide representative information. Consequently, the potential for city-scale management of GCC has been difficult to articulate and evaluate.

This absence needs to be addressed, and the process has begun with the Special Report on Climate Change and Cities that has been outlined by the Intergovernmental Panel on Climate Change (IPCC), which will be compiled over the next year in preparation for the 7^th Climate Change Assessment Report. This short paper outlines recent scientific advances that provide an opportunity for city-scale climate research to be better integrated into GCC science and the challenges that need to be addressed.

Climate mitigation potential from cities

Cities and urban areas are major drivers of global environmental changes and are responsible for over 70% of CO₂ emissions globally. It follows then that achieving zero carbon cities (ZCC) would represent a major step towards meeting the global mitigation goals set by the UN Paris Agreement. Typically, urban GHG emissions are based on inventories of energy use, disaggregated by sector into industrial, transport, commercial and residential sources. These inventories are used to support policies for decarbonisation of public transport systems and building retrofits, for example. Maps of urban GHG emissions (based on production and/or consumption) reveal that sources are spatially heterogenous which can support placed-based policies. However, significant components of the science infrastructure are missing. For example, GHGs are not routinely measured in cities (unlike air pollutants); this is a major gap as we have no means of independently assessing the impact of GHG policies on atmospheric concentrations or demonstrating the value of behaviour changes at urban scales. The evolution of satellite technology that can remotely measure GHG concentrations in atmospheric columns may fill this gap, but these estimates will still need to be validated at the ground. Moreover, while we have specific models to simulate urban building energy use (Reinhart & Davila 2016) or transport demand for example, they are not routinely integrated into an urban system framework. Critically, we need models that can simulate the dynamism of population movement through the urban space (e.g. from home to work), which regulates GHG emissions and their distribution in space and time (Salas et al. 2024), for instance, by using mobile phone data that can map the patterns of population. These models are needed to develop and test climate change management policies.

Climate adaptation potential from cities

Conventional ESMs make climate projections using uniform grid cells that describe the Earth’s surface, that are much larger than cities. As a result, urban climate processes, such as enhanced convection due to the urban heat island (UHI) are not normally included. At best, these projections describe the background climate where the city is located, and these must be adjusted (downscaled) to account for urban effects and impacts. These downscaling methods are based either on statistical relationships that link the background climate to urban variables (such as air temperature) or on urban models that are ‘nested’ within larger scale models; the resulting projections are used to assess future energy cooling and heating energy demand, heat stress and risk management generally. However, at best this approach can provide a general guide to climate hazards that may be used to develop urban adaptation policies.

ESMs are improving rapidly to include more processes at finer resolutions. The next generation of ESMs larger scale climate models are likely to include variable-grid approaches that can resolve urban scale processes directly in the context of global climate change (see The Model for Prediction Across Scales). Reconfiguring ESMs to adopt this approach is a major challenge, given the investment in the development and evaluation of current models and the computational costs required to run simulations. However, new machine-learning techniques may fill this gap in the meantime and provide urban scale simulations based on ESM climate projections (Zhao et al. 2021). These advances will improve the climate projections needed to make decisions at urban scales but will need global information on the character of urbanised landscapes (now and in the future) to make reliable projections. Some of these data are increasingly available from satellite observations (see the Global Human Settlement Layer) but significant gaps remain, especially on the material fabric of cities and on human occupation patterns.

City scale research challenges

Clearly, we are near the point where the knowledge of urban climate sciences can be incorporated into global climate science. This is imperative if we are to make the right decisions at the scale at which we live.  Consider that about one-third of GHG emissions globally are associated with building energy use (Zhong et al. 2021); this means that a significant part of GCC can be attributed to indoor climate management.  Moreover, the dimensions and placement of buildings in cities has a profound effect on the creation of microclimates, which impacts on both the outdoor and indoor environment. The challenges for the disciplines that conduct research at urban scales is to ensure that the science can answer some of the key climate change questions: for example, what is the best form of compact development in particular city? What is the value of vertical greening for indoor and outdoor climates? How do we retrofit neighbourhoods?

Currently, these urban climate sciences and the study of indoor and outdoor spaces are largely conducted independently of each other and are studied by different disciplines which have evolved their own methods (Figure 1). Consequently, what is seen as an optimum solution for one scale or environment may not be at another (e.g. Futcher et al. 2017). To overcome these issues, we need an inter-disciplinary approach that can integrate knowledge and practices at urban scales to support climate-sensitive decisions. As one example, a big challenge is to better link indoor and outdoor climates - the challenge here is to create a framework for studying the urban environment that can deal with these two settings and how they are occupied and managed. This would entail the use of a common set of variables (on the thermal properties of fabric, especially) that could be shared among disciplines and common models that can simulate the energy exchanges within and among indoor and outdoor microclimates.

Conclusions

Global and urban climate sciences must be better integrated if we are to make decisions on cities that have a meaningful effect on climate outcomes. Until very recently, cities (as individual entities) have not been directly included in global climate projections, although their emissions have. The IPCC’s Special Report will provide a sense for the status of our knowledge on cities and climate change. These reports are based on published research; in this case relevant research needs to be published by October 2026 to be included. The report itself will be structured into five chapters:

1.     Cities in the context of climate change
2.     Cities in a changing climate
3.     Actions and solutions to reduce urban risks and emissions
4.     How to facilitate and accelerate change
5.     Solutions by city types and regions.

Clearly, we need inter- (and multi-) disciplinary approaches to address these issues, and we should start to think about the potential of a ‘one-atmosphere’ approach to indoor-outdoor climate management in cities.

References

Futcher, J., Mills, G., Emmanuel, R. & Korolija, I. (2017). Creating sustainable cities one building at a time: Towards an integrated urban design framework. Cities, 66, 63-71.

Reinhart, C.F. &Davila, C.C. (2016). Urban building energy modeling–A review of a nascent field. Building and Environment, 97, 196-202.

Salas, V.R., Etuman, A.E. & Coll, I. (2024). Exploring the linkages between urban form, mobility and emissions with OLYMPUS: A comparative analysis in two French regions. Science of the Total Environment, 919, 170710.

Zhao, L., Oleson, K., Bou-Zeid, E., Krayenhoff, E.S., Bray, A., Zhu, Q., Zheng, Z., Chen, C. & Oppenheimer, M. (2021). Global multi-model projections of local urban climates. Nature Climate Change, 11(2), 152-157.

Zhong, X., Hu, M., Deetman, S., Steubing, B., Lin, H.X., Hernandez, G.A., Harpprecht, C., Zhang, C., Tukker, A. & Behrens, P. (2021). Global greenhouse gas emissions from residential and commercial building materials and mitigation strategies to 2060. Nature Communications, 12(1), 6126.

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