The ARC3 is divided into four parts: Introduction; Defining the risk framework; Urban sectors; and Cross-cutting issues. The topics within these sections relate to assessment of urban vulnerability and key climate hazards, mitigation and adaptation responses in urban sectors, and the roles of land use planning and governance in responding to climate change challenges.
Vulnerability and risk assessment
Estimation of spatially and temporally disaggregated risks is a critical prerequisite for the assessment of effective and efficient adaptation and mitigation climate change strategies and policies in complex urban areas.Risk may be considered as the intersection of three vectors – hazards, vulnerabilities, and adaptive capacity. These vectors consist of a combination of physical science, geographical, and socio-economic elements that can be used by municipal governments to create and carry out climate change action plans. Some of these elements include climate indicators, global climate change scenarios, downscaled regional scenarios, changes anticipated in extreme events (including qualitative assessment of high-impact, low-probability phenomena), qualitative assessment of high-impact and low-probability events, associated vulnerabilities, and the ability and willingness to respond. The focus is on articulating differential impacts on poor and non-poor urban residents as well as sectorally disaggregating implications for infrastructure and social well-being, including health.
Urban climate hazards
Cities already experience special climate conditions in regard to the urban heat island and poor air quality.In addition to these, climate change is projected to bring more frequent, intense, and longer heat waves in cities, and most cities are expected to experience an increase in the percentage of their precipitation in the form of intense rainfall events. In many cities, droughts are expected to become more frequent, severe, and of longer duration. Additionally, rising sea levels are extremely likely in coastal cities, and are projected to lead to more frequent and damaging flooding related to coastal storm events in the future.
In regard to critical urban infrastructure, degradation of building and infrastructure materials is projected to occur, especially affecting the energy and transportation sectors. The gap between water supply and demand will likely increase as drought-affected areas expand, particularly for cities located in the lower latitudes, and as floods intensify. While precipitation is expected to increase in some areas, particularly in the mid and high latitudes, water availability is projected to eventually decrease in many regions, including cities whose water is supplied primarily by meltwater from mountain snow and glaciers.
Overall, climate change and increased climate variability will alter the environmental baselines of urban locales, shifting temperature regimes and precipitation patterns.Changes in mean climate conditions and frequency of extreme events will have direct impacts on water availability, flooding and drought periodicity, and water demand. These dynamic changes will affect system processes within multiple sectors in cities interactively, increasing the uncertainty under which urban managers and decision-makers operate.
Energy and buildings
Climate change will affect urban energy through the complex regulatory, technical, resource, market, and policy factors that influence the design and operation of local energy systems. A key attribute for effective climate change response is the ease with which changes can be made to address climate change mitigation and adaptation.The International Energy Agency (IEA) estimates that 67 percent of global primary energy demand – or 7,903 Mtoe* – is associated with urban areas (IEA, 2008). While literature on the impacts of climate change on this sector is still limited, urban energy systems can be dramatically affected by climate change at all parts of the process including supply, demand, operations, and assets (Figure 1.2).
In developed countries, climate change concerns are leading cities to explore ways to reduce greenhouse gas emissions associated with fossil fuel combustion and to increase the resiliency of urban energy systems. In developing countries, cities often lack access to adequate, reliable energy services, a significant issue. In these cities, scaling up access to modern energy services to reduce poverty, promote economic development, and improve social institutions often takes precedence over climate-related concerns.
However, if adoption of mitigation measures brings greater reliance on renewable sources of energy (including biomass-based cooking and heating fuels), some cities may become even more vulnerable to climate change, since production of biomass-based fuel is itself subject to changing climate regimes.
Urban water supply and wastewater treatment
Long-term planning for the impacts of climate change on the formal and informal water supply and wastewater treatment sectors in cities is required, with plans monitored, reassessed, and revised every 5-10 years as climate science progresses and data improve.
Figure 1.2: Impacts of climate change on urban energy systems.
What is needed as well is the development of a new culture of water value, use, and consumption, based on balanced perspectives of its economic, physical, ecological, social, political, and technical dimensions.
Supply, demand, and quality in informal water supply systems in poor cities need to be better understood, with the purpose of improving these systems and their resilience in the context of climate change. More information on comparative performance among cities, as well as on city and regional hydrologic budgets, is required to guide efficient resource allocation and climate change responses in the urban water sector. Integrated water management includes supply, quality, and wastewater treatment both in cities and in their surrounding regions, and effective planning links beyond the water sector to other sectors, such as energy and disaster risk reduction.
The roles of institutions managing formal and informal water resources in urban areas should be analyzed and reassessed, to ensure that institutions are appropriate to changing challenges, including climate change impacts. This may include collaboration between informal and formal sectors where possible. Urban governance issues regarding water supply and demand in both the formal and informal sectors are likely to become increasingly important and contested, and may require changes in water law and management practices.
In regard to immediate adaptation strategies, programs for effective leak detection and repair and the implementation of stronger water conservation/demand management actions -beginning with low-flow toilets, shower heads, and other fixtures – should be undertaken in formal and, to the extent relevant, informal water supply systems. As higher temperatures bring higher evaporative demand, water reuse also can play a key role in enhancing water-use efficiency, especially for landscape irrigation in urban open spaces. Urban-scale water marketing through the informal private sector can be a mechanism by which to increase efficiency, improve system robustness, and facilitate integration of multisector use in some urban circumstances. Water banking (in which water in wet years is saved in, for example, aquifers for use in dry years) by urban water system managers is a way of hedging against uncertainties and improving system robustness. Rainwater capture can also be undertaken as a conservation adaptation to reduce pumped groundwater and related energy use.
City transportation systems
Urban transportation comprises the facilities and services to move people and materials throughout the city and its surrounding region. Cities encompass many modes of transport, including personal vehicles traveling on surface roads and public transport via bus, rail, and airplanes. Rail transit systems are often critically important in urban areas, with very large extents and high rates of passenger service. For instance, the Metro-North railway in New York City serves 1.5 billion passengers annually. In coastal cities, rail transit systems contain many points of climate change vulnerability to enhanced flooding from sea level rise, such as public entrances and exits, ventilation facilities, and manholes. These facilities are vulnerable to inland flooding as well. Most importantly, large portions of transit networks are of a hub-and-spoke design and converge on single points giving relatively little flexibility if any one area is disabled during extreme events, which are projected to increase in the future.
Surface transportation refers to both road-based transit (e.g., buses) and vehicular travel, much of which has high-volume traffic and key infrastructure located near coasts and rivers in many cities and thus vulnerable to sea level rise and inland flooding. Tunnels, vent shafts, and ramps are clearly at risk. Flooding necessitates the use of large and numerous pumps throughout these systems, as well as removal of debris and the repair or replacement of key infrastructure, such as motors, relays, resistors, and transformers.
Besides sea level rise and storm surge vulnerability, steel rail and overhead electrical wire associated with transportation systems are particularly vulnerable to excessive heat. Overheating can deform transit equipment, for example, causing steel rail lines to buckle, throwing them out of alignment, which potentially can cause train derailments. Heat can also reduce the expected life of train wheels and automobile tires. Roadways made of concrete can buckle, and roads of asphalt can melt. This is especially dangerous under congested conditions where heavy vehicles sit on hot surfaces for long periods of time, adding to the stress on materials.
Urban transportation adaptation strategies can focus effectively on both usage and technology. Usage strategies involve the ability to provide alternative means of transport during the periods in which acute climate impacts occur. These include being able to substitute roadways and rail lines for similar facilities in other areas, if possible. Examples of types of adaptation strategies for specific impacts are: changing to heat-resistant materials; sheltering critical equipment from extreme rainfall and wind; raising rail and road lines; increasing the deployment and use of pumps; installing drainage systems to convey water from facilities rapidly; and installing barriers such as seawalls at vulnerable locations.
Urban transportation systems also play an important role in mitigation of greenhouse gas emissions. Such mitigation actions can be implemented via transport and land use policies; transport demand management; and supply of energy-efficient transport infrastructure and services.
Climate change and human health in cities
Climate change can best be conceptualized as an amplifier of existing human health problems, attenuating or aggravating multiple stresses and, in some cases, potentially pushing a highly stressed human health system across a threshold of sustain-ability (Figure 1.3).Protection of the health of the world’s urban populations requires the involvement of all groups (government, business, academia, and communities), levels of government (international, national, regional, and local) and diverse disciplines (health, planning, engineering, meteorology, ecology, etc.).
Figure 1.3: Climate change determinants and urban modifying factors on health outcomes in cities.
Since the infrastructure for health protection is already swamped in many developing country cities, climate change adaptation strategies should focus on the most vulnerable urban residents. Such strategies need to promote "co-benefits" such that they ameliorate the existing and usually unequally-distributed urban health hazards, as well as helping to reduce vulnerability to climate change impacts. This involves health programs developed in partnership with public and private organizations and agencies to guide investments and technology choices that benefit the current health of urban residents at the same time as preparing for and responding to climate change.
Urban land use and climate change
Urban land factors that affect climate change risk include the natural features of a city’s geography, e.g., coasts and flood plains; its urban form, e.g., is the city compact or characterized by "sprawl"; and the nature of the built environment, e.g., what is the extent of impervious surfaces that can exacerbate runoff.
Table 1.1: Urban planning strategies for climate change hazards.
Major urban planning strategies
Temperature, heat waves
Change building codes to withstand greater energy loads.
Precipitation, floods, and droughts
Restrict development in areas prone to floods, landslides, fires; change building codes to encompass more drainage.
Sea level rise and storm surge
Restrict development in coastal areas; change building codes to reduce impact, e.g., elevate buildings, build protective works.
For existing urban areas subject to coastal areas where protection efforts not feasible, develop plans for retreat and new settlements.
A city’s urban planning and management structure also affects its ability to respond to climate change, since planning and management agencies and organizations can contribute greatly to the development of efficient and effective processes for both mitigation and adaptation (Table 1.1). Through urban planning and management, cities determine their land use, neighborhood densities, character of the built environment, parks and open spaces, as well as public infrastructure and facilities. Planning and management departments administer public services and regulate and provide incentives for private infrastructure providers and land markets. A key climate change response mechanism relates to property rights and land tenure. For example, how property rights and land tenure are structured in a city will play a key role in responding to the threat of sea level rise in regard to its coastal development.
Climate change presents city governments with several challenges, including the need for political and fiscal empowerment at the local level to deal with local impacts and specific mitigation measures; the presence of multiple jurisdictions among cities, metropolitan regions, states, and nations; and often weak planning and management structures.These challenges highlight the need for science and evidence-based policy formulation in regard to climate change. However, data and measurement capability are often lacking, especially in cities in developing countries. Beyond the need to develop specific near-term adaptation and mitigation measures, city governments face the challenge of addressing deeper and enduring risks and long-term vulnerabilities. Since city administrations tend to be rather short-lived, long-term risks are often ignored. A final governance challenge involves the need to be inclusive of all communities in an urban area, especially since vulnerability to climate change varies widely among socio-economic groups. To answer these climate change challenges successfully, city governments need to enhance the potential for science-based policymaking, effective leadership, efficient financing, jurisdictional coordination, land use planning, and citizen participation.
The form and function of the ARC3 is designed to be multidimensional. While the core of the effort reflects a cutting-edge climate change assessment focused on cities, it has emerged out of a process that explicitly aims to link on-the-ground scientific expertise in the service of the needs and requirements of local city decision-makers. The presentation and organization of the assessment are designed to bridge the science-policy divide in a range of urban contexts. Key dimensions are the development of risk assessment and management frameworks that take urban climate hazards, sensitivity, adaptive capacity, and agency into account, interactive consideration of mitigation and adaptation in critical urban sectors – energy, water, transportation, and human health – and the inclusion of overarching integrating mechanisms of urban land use and governance. Throughout, the goal is to contribute to effective, ongoing, and beneficial processes in the diverse cities of the world to respond to the risks of current climate extremes and future climate changes. These responses include effective planning to safeguard all urban inhabitants from climate risks equitably, while mitigating greenhouse gas emissions and thus contributing to reduction of the magnitude and impact of future changes.