THERE are many places in Illinois where you expect to find a prairie. The roof of City Hall in Chicago is not among them. Yet there it is—20,000 square feet (almost half an acre) of shrubs, vines and small trees, 11 storeys above LaSalle Avenue. Planted in 2000, City Hall's “green roof” reduces the amount of energy needed to cool the building in the summer; captures water during rainstorms, thus reducing the amount of water flowing into Chicago's already overtaxed sewers; and combats the urban “heat island” effect, which makes cities warmer than nearby rural areas. On average, air temperatures above City Hall are 10-15°F degrees lower than those above the adjacent black-tar roof of the Cook County Building; on hot summer days the difference can be as great as 50°F.

Large as it is, City Hall's roof accounts for a small proportion of Chicago's total green-roof space. And those roofs are just one part of Chicago's Climate Action Plan (CCAP), which was launched in September 2008 and was preceded by years of green initiatives during the tenure of Richard Daley, who from 1989 until earlier this year was mayor of Chicago. CCAP aims to reduce Chicago's greenhouse-gas emissions to 75% of their 1990 levels by 2020, and to just 20% of their 1990 levels by 2050. In the two years after CCAP's launch public-transport ridership rose, millions of gallons of water were conserved, hundreds of hybrid buses were added to Chicago's fleet and over 13,000 housing units and nearly 400 commercial buildings were retrofitted for energy efficiency.

These achievements have come not through sweeping social engineering, or by making Chicagoans dine on tofu, sprouts and recycled rainwater while sitting in the dark, but by simple tweaks. City buses inevitably need replacing; so why not replace them with hybrid models that are not only 60% lower in carbon emissions than standard diesel buses, but also 30% more fuel-efficient and will save an estimated $7m a year in fuel and upkeep? Alleys—Chicago has 1,900 miles of them—will inevitably need repaving; why not repave them with permeable, light-coloured surfaces rather than asphalt to reduce water run-off into sewers and reflect rather than retain the sun's light and heat?

New York's ambitious PlaNYC had similar origins. It came about not because New York got green religion, but because the population, unlike that in most of America's other large, old, north-eastern cities, is growing. By 2030 9m people are forecast to live in New York, up from 8m in 2000 and almost 8.2m in 2010. The ways to accommodate such growth in an already dense and developed city inherently tend to be green: improving public transport, redeveloping brownfields, making infrastructure and water supply more reliable and efficient.

Chicago and New York are just two of the ten American cities—the others are Austin, Houston, Los Angeles, New Orleans, Philadelphia, Portland, San Francisco and Seattle—who are members of the Large Cities Climate Leadership Group (mercifully renamed the C40), which now comprises 58 cities around the world. Roughly 297m people, less than 5% of the Earth's total, live in the 40 charter-member C40 cities. But they account for 18% of the world's GDP and 10% of its carbon emissions. In total, cities house more than half the world's population, and account for two-thirds of its energy consumption and over 60% of its greenhouse-gas emissions.