Much of the world’s air pollution originates in large cities, and their smog can be carried thousands of miles, affecting the health and livelihoods of people who live outside urban areas. But big city mayors around the world are stepping up to develop new strategies to control air pollution.

In 2005 the C40 Cities Climate Leadership Group brought together leaders from major cities around the world to pool ideas and resources for solving environmental problems all cities share. Air pollution tops the list.

The 75-member C40 group leads a Climate Positive Development network that supports projects in 17 cities. Each project aims to reduce greenhouse gas emissions from transportation, energy generation and waste disposal through a variety of high- and low-tech approaches. Solutions include planting more trees and putting gardens on roofs, requiring low-emissions vehicles and fuels, and boosting efficient, sustainable energy production.

This Brooklyn, New York, roof garden improves air quality while providing produce for local restaurants. (© AP Images)

Reducing emissions from the ground up

The goal is ambitious: Creating cities that are net carbon-negative. This means they not only ensure emissions of  greenhouse gases like carbon dioxide are offset by equal amounts captured, or by credits for emissions eliminated elsewhere, but also take steps to reduce carbon dioxide in the atmosphere.

“Our experiences already show that smart, low-carbon solutions cut greenhouse gas emissions in cost-effective ways,” said Tomas Gustafsson, sustainability advisor to the city of Stockholm.

Each project is monitored from beginning to completion in clearly defined phases. Among the six projects that have reached the second phase — with plans, partnerships and milestones in place — is Mahindra World City in Jaipur, India. A partnership between the multinational conglomerate Mahindra Group and the government of the state of Rajasthan, Mahindra World City sustainably integrates residential, commercial and other needs into an environment that will house and employ nearly 300,000 people by 2025. Other projects that have reached the second phase are in Sydney and Melbourne, Australia; Sønderborg, Denmark; London; and Oberlin, Ohio, in the United States.

Alternatively fueled vehicles such as these electric cars are keys to reducing urban greenhouse gas emissions. (© Frederic Legrand/COMEO/

Collaborating to get it right

Every city requires its own tailored solutions, but the collaborative city-to-city network and meticulous evaluations aim to develop strategies that can be replicated in different settings.

Cities such as London, Mexico City or Delhi, India, embody centuries of history, so retrofitting old neighborhoods happens alongside new, sustainable construction. Besides facilitating partnerships for creating zero-emissions cities, the C40 group maintains databases of case studies and an interactive open-data portal.

A partnership initiative with the Climate & Clean Air Coalition announced in July will support efforts to clear the air in 50 C40 cities, including Addis Ababa, Ethiopia; Dar es Salaam, Tanzania; Dhaka, Bangladesh; Jakarta, Indonesia; Lagos, Nigeria; Rio de Janeiro and Sao Paulo; and Stockholm.