Mexico City, viewed from the air
The urban environment of Mexico City, one of the world's largest urban areas. (Image: UCAR)

Warming global temperatures, sea level rise, and changing availability of water resources are challenges for people who are planning for the future needs of urban settings. Yet it’s not just global changes that affect cities. The cities themselves affect their local climate. Changes in the way land is used, the amount of paved areas and air pollution affect the weather patterns in urban areas, which are growing at a rapid pace.

According to the United Nations, as of October 31, 2011 there were seven billion people living on Earth. The actual population is never known for certain since these numbers rely on national censuses, which are conducted at different times. The United States Census Bureau currently estimates that the world population will increase to more than 9.3 billion people by 2050 and more than 10 billion by the end of this century.

In 1800, there were only about one billion people living on Earth, and only 3% of those people lived in cities. By the end of the 20th century, there were six billion people living on Earth and the percentage in cities had risen to 47%. Today, more than half of the human population lives in urban areas (defined as a population center with more than 50,000 inhabitants).

According to a 2011 United Nations report, the world’s urban population is expected to swell to almost 5 billion by 2030, when three out of five people will live in cities. The world’s urban population is growing at four times the rate of the rural population. By the year 2050, the percentage of urban dwellers worldwide is expected to reach 70 percent.

Although the world’s urban population grew very rapidly (from 220 million to 2.8 billion) during the 20th century, most urban growth today is happening in the developing world.

In the next few decades unprecedented urban growth is expected in developing countries, particularly in Africa and Asia. Overall, in Africa, population is expected to triple by the end of the century. In Asia, population is expected to peak around the middle of the century with over five billion people. By 2030, the towns and cities of the developing world will make up 81 percent of the world’s urban dwellers.

When rural or natural areas are transformed into cities, changes to the landscape can lead to changes in the weather patterns over that landscape. For example, heat from the Sun is absorbed more by asphalt and concrete than by grass and trees. So as a city grows and these materials become more common, more heat is trapped. This is known as the Urban Heat Island effect. Additionally, the heat, the shape of tall buildings, and air pollution can affect weather patterns in and around a city.