Population and climate change.

To review, the four broad dimensions of any complex human problem, including climate change, are the human population, economics, culture, and environment. These dimensions interact with one another in all directions and on many time-scales. From 2010 to 2050, the human population is likely to grow bigger, more slowly, older, and more urban. It is projected that by 2050 more than 2.6 billion people (almost 94% of global urban growth) will be added to the urban population in today's developing countries. That works out to 1.26 million additional urban people in today's developing countries every week from 2010 to 2050. Humans alter the climate by emitting greenhouse gases, by altering planetary albedo, and by altering atmospheric components. Between 1900 and 2000, humans' emissions of carbon into the atmosphere increased fifteenfold, while the numbers of people increased less than fourfold. Population growth alone, with constant rates of emissions per person, could not account for the increase in the carbon emissions to the atmosphere. The world economy grew sixteenfold in the twentieth century, accompanied by enormous increases in the burning of gas, oil, and coal. In the last quarter of the twentieth century, population grew much faster in developing countries than in high-income countries, and, compared with population growth, the growth of carbon emissions to the atmosphere was even faster in developing countries than in high-income countries. The ratio of emissions-to-population growth rates was 2.8 in developing countries compared with 1.6 in high-income countries. Emissions of CO2 and other greenhouse gases are influenced by the sizes and density of settlements, the sizes of households, and the ages of householders. Between 2010 and 2050, these demographic factors are anticipated to change substantially. Therefore demography will play a substantial role in the dynamics of climate changes. Climate changes affect many aspects of the living environment, including human settlements, food production, and diseases. These changes will affect poor people more severely than rich, and poor nations more severely than rich. Yet not enough is known to predict quantitatively many details that will matter enormously to future people and other species. Three kinds of responses are related to demographic issues that affect climate changes: universal secondary education, voluntary contraception and maternal health services, and smarter urban design and construction. These responses may prevent, reduce, or ameliorate the impacts of climate changes. They are as relevant to rich countries as to poor, though in ways that are as different as are rich countries and poor. They are desirable in their own right because they improve the lives of the people they affect directly; and they are desirable for their beneficial effects on the larger society and globe. They are effective responses to the twin challenges of reducing poverty and reducing greenhouse gas emissions.