By Sue Grimmond1 and the WMO Secretariat
Over the past few hundred years, people have increasingly clustered in large settlements, to the point where the world’s urban population now exceeds its rural population. These cities of varying sizes are concentrated in 1–3 per cent of the Earth’s land surface. The number of cities with more than 5 million people is growing: from 4 in the 1950s it is expected to reach 59 in 2015. Many of these cities are in developing countries, many have high levels of air pollution. In 2009, 16 per cent of the world’s population was living in cities with more than 5 million inhabitants.2 A large proportion of the movement of population into cities can be attributed to young people, less than 35 in age. Cities present a vibrant backdrop to innovation, cultural interaction and economic progress. They also attract youth due to educational and job opportunities.
Such large entities depend heavily on their underlying infrastructure, including transport systems (road, rail, pedestrian, bicycles, etc.), water and power supply, sanitation and drainage systems, and communication networks. The complexity of this infrastructure, together with its vulnerability, increases in a non-linear way with size. Doubling the size of a city may increase its complexity and, therefore, its vulnerability several times. Large, fast-growing cities are major drivers of global economic growth (80% of future growth)3, but such growth can often be rapid and unbalanced as many new urban populations are often poor.
The dramatic demographic shifts associated with the growth of cities have wide-ranging implications. Few are felt more by residents than the deterioration of air quality. Cities in poor developing countries often do not place restrictions on emissions of the sort that are more common in North America and Europe. For example, London and Los Angeles have implemented policies and strategies to curb air pollution. Until recently, the changes in air quality resulting from increasingly dense urban centres have not been quantified in detail, and their effects on regional climates and global warming are still not systematically documented.
The urban weather and climate footprint
There are two main mechanisms by which cities will further affect local, regional and global climates. Firstly, urban features such as morphology and heat emissions will continue to influence local temperatures, air circulation, precipitation and the frequency and intensity of thunderstorms. Secondly, changing chemical emissions and feedbacks resulting from atmospheric pollutants will alter weather and climate, both locally and further afield.
Many features in cities can influence atmospheric flow, its turbulence regime, and the microclimate. These features can modify the transport, dispersion, and deposition of atmospheric pollutants, both within and downstream of urban areas (one form of which is acid rain). Key examples include:
• The distribution of buildings and other obstacles (or more generally of all roughness elements) affects the turbulence regime, speed and direction of the flow.
• The extensive use of impermeable materials and the frequent reduction in vegetation in urban areas affects the hydro-meteorological regime and pollutant deposition.
• The release of anthropogenic heat by human activities (such as transportation and the heating and cooling of buildings) affects the thermal regime.
• The release of pollutants (including aerosols) affects the transfer of radiation, the formation of clouds, and precipitation.
• Street geometry (‘street canyons’) affects the flow regime and heat exchange between different surfaces (such as roads and walls).
The net result may be strong urban heat islands – areas of warmer temperatures – which can lead to cities with air temperatures several degrees warmer than nearby rural areas. Such temperature differences can disturb regional air circulation. Wind patterns may be disrupted even further because of ever more numerous high-rise buildings. The disturbances can in turn lead to altered levels of precipitation, air pollution and thunderstorm frequencies.
In addition, the contribution of cities to global warming through greenhouse gas (GHG) emissions is substantial, mostly due to plumes of carbon dioxide (CO2) emissions from urban or nearby supporting areas, although on a per capita basis their emissions intensity may be slightly lower than rural areas.
Megacity air quality and climate change
A number of recent international studies have been initiated to explore these issues.6 These studies aim to assess the impacts of megacities and large air-pollution hotspots on local, regional and global air quality; to quantify feedback mechanisms linking megacity air quality, local and regional climates, and global climate change; and to develop improved tools for predicting air pollution levels in megacities.
Mean tropospheric NO2 column density (1015 molec/cm2 ) from measurements of the
SCIAMACHY instrument on board the ESA satellite ENVISAT, for the years 2003-2007.
While important advances have been made, new interdisciplinary research studies are needed to increase our understanding of the interactions between emissions, air quality, and regional and global climates. Studies need to address both basic and applied research and bridge the spatial and temporal scales connecting local emissions, air quality and weather with climate and global atmospheric chemistry. WMO has established the Global Atmosphere Watch (GAW) Urban Research Meteorology and Environment (GURME) project7 to help enhance the capabilities of national meteorological services to handle meteorological and related aspects of urban pollution.
Megacities and other densely populated regions emit significant amounts of pollution into the atmosphere. The local effects are especially evident within the boundaries of well-known polluted megacities, such as Beijing and Delhi. The pollutants are usually derived from urban transport, energy production and other types of industry, and they have effects on the environment that are harmful to health. However, this pollution is not confined within the boundaries of the megacities themselves but can be transported over large distances, so that it contributes to the overall hemispheric background pollution.
The sources and processes leading to high concentrations of the main pollutants, such as ozone, nitrogen dioxide and particulate matter, in complex urban and surrounding areas are not fully understood. This limits our ability to forecast air quality accurately. Three major global emissions inventories, alongside two city-level inventories, were compared in the MEGAPOLI study.8
This showed that the sources and degrees of emissions vary hugely between megacities, in particular, by geographical region. For example, much of the megacity emissions in Europe and the Americas are associated with road use, whereas in Asia and Africa the output largely stems from residential energy.
Predicting how global climate change will impact cities requires studies to understand the large-scale and longterm processes such as ocean temperature and current, changes in land cover and slow-changing atmospheric variables. Ocean and land surface changes can produce climate fluctuations that potentially are predictable at seasonal and inter-annual time scales. To provide targeted climate-prediction products, prediction models for temperature, rainfall and high-impact events such as heat waves and floods need to be developed. To meet the special needs of cities, refined climate change products can be produced through the regional downscaling of integrated climate-chemistry or Earth-system models.
Main linkages between megacities, air quality and climate, with the main feedbacks, ecosystem, health and weather impact pathways,
and mitigation routes.9 The relevant temporal and spatial scales are also included.
Predicting how global climate change will impact cities requires studies to understand the large-scale and longterm processes such as ocean temperature and current, changes in land cover and slow-changing atmospheric variables. Ocean and land surface changes can produce climate fluctuations that potentially are predictable at seasonal and inter-annual time scales.
To provide targeted climate-prediction products, prediction models for temperature, rainfall and high-impact events such as heat waves and floods need to be developed. To meet the special needs of cities, refined climate change products can be produced through the regional downscaling of integrated climate-chemistry or Earthsystem models.
Research needs and a strategy for the future
The needs and requirements of each city should be informed by a holistic identification of impacts and hazards in order to map the city’s specific vulnerabilities and identify the services that would be most beneficial. Coastal cities have different concerns to land-locked cities; similarly, the requirements of an urban area in the ropics are different to those of cities affected by severe winter weather. Data sharing arrangements between city institutions are a fundamental building block for authorities when they identify priority services and also when they design and establish urban observational networks that capture the phenomena of interest at the spatial and temporal resolution required.
City services are heavily reliant on high-resolution, coupled nvironmental-prediction models that include realistic city-specific processes, boundary conditions and fluxes of energy and physical properties. New urbanfocused observational systems are needed to drive these models and provide the high-quality forecasts to be used in these new services. The use of new, targeted and customized means of communicating with users is required to ensure that services, advice and warnings lead to appropriate action and to feedback that improves the services. New skills and capacities will be required to make the best use of new technologies to produce and deliver new services in a challenging and evolving city environment.
Supporting platform for building climate resilient societies
National meteorological services are encouraged to establish sound working relationships with municipal authorities. They should then jointly identify and agree on the priorities for joint services and the resources required for sustained service delivery and improvement. Considering the global importance of urbanization and the growing number of megacities and large urban complexes, WMO Members would do well to include this phenomenon as a high-level priority. They should consider how best to include the unique climate service requirements of the urban environment in the Global Framework for Climate Services (GFCS). WMO Members may also wish to showcase and share their urban experiences and establish best practices for how to serve the urban dweller, who is now rapidly becoming a majority stakeholder in urban weather, climate, water and related environmental services.
Integrated Urban Weather, Environment and Climate Service
A broad set of concepts defines the development of Integrated Urban Weather, Environment and Climate Service. These concepts relate to the conditions faced by urban populations, the impacts of environmental conditions on megacity and urban societies, the need for a legal framework and clearly defined government agency interactions to enable the creation and maintenance of such services, and the scientific and technological advances required to develop and implement them.
The delivery of urban weather and climate information also needs to be considered. For example, youth are keen on using new methods of communication, thus social media will need to play an increasing role in developing and providing weather and related environmental services.
The numerical models most suitable for integrated urban weather, air quality and climate forecasting operational systems are the new generation of limited-area models with coupled dynamic and chemistry modules (so called Integrated Meteorology-Chemistry Models (IMCM). These models have benefited from rapid advances in computing resources plus extensive basic science research.10
Current state-of-the-art IMCMs encompass interactive chemical and physical processes, such as aerosolsclouds- radiation, coupled to a non-hydrostatic and fully compressible dynamic core that includes monotonic transport for scalars, allowing feedbacks between the chemical composition and physical properties of the atmosphere. However, simulations using fine resolutions, large domains and detailed chemistry over long time durations for the aerosol and gas/aqueous phase are still too computationally demanding due to the models’ huge complexity. Therefore, IMCM weather and climate applications must still make compromises between the spatial resolution, domain size, simulation length and degree of complexity for the chemical and aerosol mechanisms.
A typical model run at the weather scale for an urban domain uses a reduced number of chemical species and reactions because of its fine horizontal and vertical resolutions, while climate runs generally use coarse horizontal and vertical resolutions with reasonably detailed chemical mechanisms.11 There are initiatives to expand the related services of large forecast centres. For example the MACC-II – Monitoring Atmospheric Composition and Climate - Interim Implementation – project12 currently serves as the pre-operational atmosphere service on the global and European scale; it could be extended and downscaled to megacities and urban agglomerations.
Representation of the urban land surface and urban sub-layer has undergone extensive development, but no scheme is capable of dealing with all of the surface exchanges.13 To complicate this further, the increasing resolution of models, combined with the large size of urban buildings in many cities, challenges the limits of current understanding. Key questions include: Should
buildings be directly resolved? What can be simplified to make the computations tractable in realistic modelling time? At what scale can the current land surface schemes and model physics be applied?
Other research needs relate to secondary organic aerosols and their interactions with clouds and radiation, data assimilation that includes chemical and aerosol species, dynamic cores with multi-tracer transport efficiency capability, and the general effects of aerosols on the evolution of weather and climate. All of these areas are concerned with an efficient use of models on massively parallel computer systems.
Operational centres that base their products and services on IMCMs need to closely follow the evolution of the research and development of these coupled models, but they also need to interact with these activities. Research on basic physical and chemical processes and the development of numerical models and tools are integral and central components of reliable and accurate forecast products and services. Nevertheless, because operational personnel will not be fully responsible for these research and development activities, strong and long-term partnerships should be established between researchers and internal and external operational groups. These partnerships should promote the development of methods for measuring improvements in forecast skills and benefits.
WMO Secretariat contributors
• Tang Xu, Director, Weather and Disaster Risk Reduction Services Department
• Alexander Baklanov, Atmospheric Research & Environment Branch, Research Department
1 Department of Meteorology, University of Reading
2 UN, 2010: UN’s World Urbanization Prospects: The 2009 Revision. UN, Department of Economic and Social Affairs, Population Division
3 Göbel, B., 2004: Urbanization and Global Environmental Change. International Human Dimensions Programme on Global Environmental Change (IHDP)
4 UN, 2012: UN’s World Urbanization Prospects: The 2011 Revision. UN, Department of Economic and Social Affairs, Population Division. March 2012
5 In this article, megacities have a threshold population of 5 million but a threshold of 10 million is often used.
6 See MILAGRO, MEGAPOLI, CityZen, ClearfLo, WISE (Seoul), and SUIMON (Shanghai). A comprehensive worldwide overview of impacts of megacities on air pollution and climate and corresponding projects is available at WMO/IGAC, 2012
8 Denier van der Gon, et al., 2011: Discrepancies Between Top-Down and Bottom-Up Emission Inventories of Megacities: The Causes and Relevance for Modeling Concentrations and Exposure. In D. G. Steyn& S. T. Castelli (Eds.), Air Pollution Modeling and its Application XXI, NATO Science for Peace and Security Series C: Environmental Security (Vol. 4, pp. 194-204).
9 Baklanov, A., et al., 2010: MEGAPOLI: concept of multi-scale modelling of megacity impact on air quality and climate, Adv. Sci. Res., 4, 115-120doi:10.5194/asr-4-115-2010.