Urban development - Megacities

Urban development - Megacities

Some 3.5 billion people reside in urban areas. This number is projected to reach 6.3 billion by 2050, increasing from 50% to more than 70% of the world’s population. Cities are centres of creativity and economic progress but they also face many environmental challenges due mainly to air pollution and weather, climate and water-related hazards.

Since 2007, for the first time in history, the majority of the world's population has been living in urban areas. Megacities, typically defined as cities with a population of over 10 million, cover less than 0.2% of the Earth's land area, however, 1 person in 10 on our planet lives in a megacity. Most urban dwellers live in cities in developing and least-developed countries, often in informal settlements vulnerable to weather, climate and water-related hazards and facing elevated air pollution levels.

In large urban settlements, human activities greatly modify the environment, creating unique meteorological and climatological characteristics. The agglomeration of tall buildings, roadways, green spaces and concrete surfaces produces intricate rain, wind, heat and air-quality patterns. The hard surfaces can shape water flow and aggravate flood risks. The alignment of buildings can create local wind tunnels. Tiny particles emitted by traffic and industry can reduce air quality. The urban heat-island effect can raise temperatures by 5 to 10oC, exacerbating heatwaves.

Populations in urban areas are particularly vulnerable to air pollution, weather extremes, including heatwaves, flooding, droughts and storm surge for those in coastal areas, and climate change impacts. Increasingly dense, complex and interdependent urban systems can leave cities exposed to a domino effect – where a single extreme event leads to general infrastructural breakdown – with lasting consequences. This very interdependence requires an integrated approach to urban weather, environment and climate services aimed at the city's inhabitants and decision-makers: from weather and climate predictions to actions with community involvement and in urban planning.

Integrated weather, climate, hydrology and related environment services for sustainable cities

Integrated weather, climate, hydrology and related environment services for sustainable cities

The accelerating growth of urban populations, especially in developing countries, has become a driving force of human development. Crowded cities are centers of creativity and economic progress but, from polluted air to flooding and other climate impacts, they also face major weather, climate, water and environment-related challenges. Increasingly dense, complex and interdependent urban systems leave cities vulnerable: through a domino effect, a single extreme event can lead to a broad breakdown of a city’s infrastructure. 

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Understanding the Urban Environment

The air quality and weather urban weather patternsTo make climate information urban-relevant necessitates advancements in regional downscaling, seasonal to sub-seasonal predictions and decadal forecasts. Amongst the scientific challenges for climate researchers, cities capture the most interest. The majority of megacities are built on coastlines and are, thus, vulnerable to sea-level rise and storms. All urban areas and their inhabitatnts are vulnerable to the impacts of heavy precipitation, heatwaves or droughts. The WMO Coordinated Regional Climate Downscaling Experiment (CORDEX) aims to bridge the gap between the typically low-resolution global climate models and the high-resolution required for local climate-smart decisions at the urban level.

In order to support the development of urban-scale environmental prediction systems, there is a need for high-resolution models that include city-specific processes, boundary conditions and fluxes, complemented by enhanced observational systems. New delivery platforms must ensure that weather and climate advises and warnings result in appropriate action. Finally, weather and climate service providers need to build and tailor skills and capacities to local urban areas to reflect thier complexity and challenges of their constant evolution and to focus on protecting human health and the environment.

Air Pollution

Live information panel showing air quality, with the SAFAR project supported by GURMEAir pollution leads to 7 million premature deaths every year and has become a silent epidemic in many urban centres, particularly in developing countries and amongst the vulnerable population – the young, the elderly and those with respiratory illnesses. Outdoor air pollution, which causes 3.7 million deaths a year, is interlinked with weather. Rain can wash particles out of the air and heat can exacerbate health problems linked to air quality (from both pollution and pollen). Wind can bring in pollution from remote sources or blow away local air pollutants. The pollution from megacities can be transported thousands of kilometres and contributes to overall hemispheric background pollution.

The WMO Global Atmosphere Watch (GAW) established the Urban Research Meteorology and Environment (GURME) in order to further study the relationship between air quality and weather and other environmental factors. GURME facilitates projects such as System of Air quality and weather Forecasting And Research (SAFAR), which was setup for the 2010 India Commonwealth Games to deliver location-specific air pollution forecast. SAFAR now provides air quality information for up to 3 days ahead for Pune, Delhi and Mumbai.

The WMO and the International Global Atmospheric Chemistry (IGAC) project produced the GAW/IGAC Impacts of Megacties on Air Pollution and Climate (GAW Report No. 205) that focuses on the impact of megacities on air pollution and climate.

Protecting Health

The UN protecting civiliansThe concentration of people in cities, often in informal and unsanitary settlements, creates challenges for health agencies, however, weather and climate information and services can assist them in protecting public health. Knowledge about the intensity and frequency of droughts, heatwaves and floods in coming months or years is critical to ensuring that medical services and supplies are available where and when needed and that they are secure from natural hazards. For example, knowing that there will be lower-than-average rainfall that will increase the risk of nutritional and respiratory problems or above-average rainfall that may lead to outbreaks of vector-borne diseases such as malaria or waterborne diseases such as cholera would give health agencies time to plan for such risks.

In cities, populations are vulnerable to weather, climate and water-related disasters, which can cause deaths, injuries or mental traumas. Floods and storms are the most frequently occurring of these, and the most threatening to cities built on flood-prone plains and/or along coastlines. Extreme temperatures such as cold spells can also affect cities and citizens for weeks. But heatwaves, amplified by the urban heat-islands, are the most menacing to urban dwellers. Sanitary water and appropriate sewage for citizens is also intricately linked to the hydrological cycle and to weather and climate.

Observing and monitoring cities

Setting up urban observation networksBecause human activities and structures can severely compromise measurements, weather and climate observing stations are often located outside of urban environments. However, urban environments require the development of tailored high-density observation networks in order to provide user-relevant information and inform weather and climate predictions.

Satellites can provide such meteorological and environmental observations at high-resolution on both urban surfaces and vertical structures. New and emerging technologies can also be used to harmonize, integrate and densify observations in urban areas. Crowd sourced data collection and networks of mobile sensors embedded in cell phones or cars, for example, can provide low-cost and flexible solutions to complement more traditional sources of observations: 90% of the of the globe’s inhabited areas are covered by mobile phone antennas. To ensure information is efficiently used before, during and after any environmental event requires integrating data storage, management and dissemination with observing networks and information systems.

Natural Hazards and High Impact Weather

Meteorologist at Indonesia Meteorological and Geophysical Agency, BMKGThe number of reported hydro-meteorological disasters has steadily increased over the past few decades. Improvements in weather forecasting and emergency planning have drastically lowered the resulting loss of life, but related economic losses increased to almost US$ 1 trillion over the first decade of the 21st century.

WMO recommends that policy-makers in urban areas consider adopting Multi-Hazard Early Warning Systems (MHEWS) to minimize losses from hazards. This approach allows a single cost-effective system to deliver warnings on a wide range of hazards, including storms, temperature extremes and air pollution. By employing impacts-based warnings, which describe the physical impacts of a hazard, rather than difficult-to-interpret measurements such as “100 km/h wind” or a “3metre sea-surge,” these systems can be used to communicate more clearly to people about what actions they should take in view of impending hazard.

Developing a Multi-Hazard Early Warning System requires analyzing and mapping city-specific information on population patterns, hospitals and other infrastructure, evacuation routes, and other relevant factors. It also requires political commitment, clear authority and roles, and standardized procedures. This inclusive process needs to be applied across all urban weather, environment and climate services from air pollution to sea-level rise.

In early 2016, the WMO also launched HIWeather: a cooperative international research to achieve a dramatic increase in resilience to high impact weather, worldwide, through improving forecasts for timescales of minutes to two weeks and enhancing their communication and utility in social, economic and environmental applications.

Climate Adaptation and Mitigation

Concentrated growth, increased risk. Photo by Brigitte Leoni, UNISDRBecause of climate change, it is no longer possible to rely on past experience when planning for future natural hazards. Sea-level rise and changing patterns for extreme weather are already increasing infrastructure and public health risks. To build truly resilient cities, urban planners must look beyond the next storm and plan for long-term changes in the weather and climate. Fortunately, recent advances in our understanding of urban climate processes, together with higher resolution predictions, allow climate services to provide the information and projections cities need to take long-term decisions for reducing vulnerability and promoting sustainable development.

To support this, the Global Framework for Climate Services (GFCS) provides a worldwide mechanism for coordinated actions to enhance the quality, quantity and application of climate services.

Observing emission sources and air composition can support cities with monitoring and meeting targeted emission reduction strategies: the Integrated Global Greenhouse Gas Information System (IG3IS) aims to combine atmospheric composition and socio-economic activity data in order to quantify the progress of emissions reduction agreements, to reduce uncertainty of emission inventory reports and to inform additional mitigation actions. Emissions can be reduced by moving energy and food production within cities thanks to solar pannels and urban vegetable gardens. In addition, because burning fossil fuels and biomass for power generation, cooking, heating, manufacturing and transport also releases health-damaging aerosols, mitigating greenhouse gases provides the co-benefit of improved air quality.

The WMO helps cities face the future

A meteorological officer from Indonesia's meteorological and geophysical agency, BMKG, analyzing weather prediction information.Urban development is now a cornerstone of the United Nations 2030 Sustainable Development Goals. It has its own sustainable development goal (SDG 11):


SDG 11: Make cities inclusive, safe, resilient and sustainableUN 2030 SDG Goal 11


Amongst other things, this goal aims to significantly reduce the number of people affected and killed by disasters and to substantially decrease the associated economic losses by 2030. It also seeks to reduce the adverse environmental impact of cities, including air quality and waste management by 2030, and to strengthen disaster management and resilience by 2020. This needs to be done in an inclusive manner, protecting the poor and vulnerable, ensuring safe and affordable housing, basic services, and upgrading slums (often most vulnerable to natural hazards and high-impact weather).

But knowledge about weather, climate and water is also key for cities. To adapt and mitigate climate change, for innovation and infrastructure development, to support sustainable energy, to ascertain access to clean water and sanitation and to care for its citizens’ good health. The WMO is now bringing its range of expertise together to focus on urban development and urban dwellers, and to contribute to the development of a new United Nations Urban Agenda during Habitat-III.

Building sustainable and climate resilient cities requires integrated urban weather, climate, hydrology and related environmental services that are based on reliable scientific knowledge. The key is to ensure high-quality, consistent and relevant information for urban communities in the areas of weather, climate, water and all environmental aspects. This includes impact-based forecasts and multi-hazard early-warning systems for cities to brace for extreme weather events, floods, air pollution, heat-and cold waves. An integrated approach also includes city-tailored guidelines and standardizations for an appropriate response to such forecasts and early warning. WMO is dedicated to provide the science-based integrated urban services supporting the safe, healthy and resilient cities of the future.