(October 2001) More sizzling summers. Rising sea levels. Increasingly violent storms and floods. These are just a few of the many potential impacts of climate change projected by the Intergovernmental Panel on Climate Change (IPCC), a scientific body created in 1988 by the World Meteorological Association and the United Nations Environment Programme. The IPCC has discovered changes in regional climates, including temperature increases, and expressed “high confidence” that these changes are affecting many different kinds of ecosystems.1 The IPCC has also reported “new and stronger evidence” that most of the warming of the past 50 years is attributable to human activities, particularly emissions of greenhouse gases into the atmosphere.

What does climate change mean for the well-being of human communities around the globe? There are many answers to this broad question — every community is different, with its own vulnerabilities and capacity for adapting to change. Even so, by looking at projected climate changes in combination with expected changes in the world’s human population, it is possible to gain greater insight into this complicated issue.

An Array of Impacts

The IPCC has already described a variety of potential climate change impacts on human communities, without taking into account population trends. Some of the most prominent adverse effects could include

  • A greater risk of flooding for many communities, due to heavier rainfall from more intense storms as well as rising sea levels as polar ice caps melt
  • An increase in deaths due to heat stress
  • A reduction in crop yields in most tropical and sub-tropical regions for most projected increases in temperature
  • Decreased water availability for populations in many water-scarce areas
  • An increase in the number of people exposed to vector-borne (e.g., malaria) and water-borne (e.g., cholera) diseases due to changes in temperature and precipitation patterns
  • Increased energy demand for space cooling.

Notably, not all of the projected impacts are negative. Some of the beneficial effects that have been projected include

  • Increased crop yields in some regions at mid-latitudes for temperature increases of less than a few degrees Celsius
  • Potential increases in timber supply from forests that are managed in ways that take advantage of climate changes
  • Increased water-availability in some water-scarce regions
  • Reduced winter mortality in mid- and high-latitudes
  • Reduced energy demand for space heating due to higher winter temperatures.

These changes, if they come to pass, will not occur in a vacuum. Instead, they will occur in tandem with changes in the size, composition, and distribution of the world’s human population. In many cases, these demographic changes could either exacerbate or alleviate the impact of climate change on human communities. Examining three major population trends — urbanization, aging, and growth in less developed countries — suggests that, on balance, demographic change will make humanity more vulnerable to climate change than it is today.


Each year a higher percentage of the world’s population lives in urban areas. The UN projects that 50 percent of humanity will be living in urban areas for the first time ever by the end of this decade. By 2030, this figure is expected to reach 60 percent. This trend is due both to rural-to-urban migration as well as natural increase (the number of births minus deaths) in urban areas. Urbanization can be seen both in the proliferation of towns and cities, which is the main pattern in China, as well as the expansion of existing urban areas. In 1975, only five cities had more than 10 million inhabitants; by 2015, there will be 24.

Despite a wide range of regional variation, the trend of urbanization is apparent in every major world region (see Figure 1).

 Figure 1
Regional Trends in Urbanization

Note: The definition of “urban” varies by country, so the projections here are suggestive of regional differences in urbanization rates rather than a definitive documentation of them.
Source: UN, World Urbanization Prospects: The 1999 Revision, 2000.

The trend toward urbanization could influence the impact of climate change in several ways. For example, congestion and heat-absorbing asphalt make cities generally hotter than the surrounding countryside. As more people inhabit urban areas, the number of people vulnerable to heat stress is thus likely to rise: a problem that will be compounded by rising temperatures due to climate change. Higher temperatures are particularly difficult for cities that have less experience with intense summer heat. The heat wave that hit Chicago in July 1995, for example, contributed to the deaths of more than 500 people.

Of course, in some cases the flow of people from rural to urban areas may actually increase access to air conditioning, as has happened in cities like Hong Kong and Taipei. Such improvements are critically dependent on increasing affluence and development, however. Sprawling shantytowns, like those that surround Sao Paulo and Jakarta, offer little refuge from rising temperatures.

Since urban areas feature high concentrations of people, they are also more deadly during natural disasters. If climate change leads to more frequent and intense storms and floods, high-density population centers will be among the most exposed. In November 1998, Hurricane Mitch devastated Tegucigalpa, Honduras, and a number of other Central American cities, killing several hundred and possibly thousands of people. One report estimated that the mudslides unleashed by the storm had destroyed more than 48,000 homes in Tegucigalpa, 20 percent of the city’s total. Such urban catastrophes could become more common.

Cities in less developed countries are particularly vulnerable to natural disasters. As noted by Martin Brockerhoff in PRB’s Population Bulletin, “An Urbanizing World,” many cities in the less developed world were established in coastal areas most suitable for trading — Mumbai (Bombay) and Shanghai are prime examples. Since rising sea levels, flooding, and storms disproportionately affect coasts, they represent an even greater threat to the urban residents of these cities. In addition, less developed countries often have less stringent building standards, making their cities particularly dangerous places to be when natural disasters strike. Many of the homes that were destroyed by Hurricane Mitch, for example, were hovels built on the outskirts of Tegucigalpa.


Another key population trend in the 21st century will be aging. Due to the post-World War II baby boom in Europe, the United States, and Japan, as well as falling fertility rates in less developed countries after 1970, many countries will have to contend with populations that contain rising proportions of older citizens. This trend is most pronounced in Japan, but it is also notable in a number of other countries, including both China and India (see Figure 2).

Figure 2

Population Aging Worldwide

Source: UN, World Urbanization Prospects: The 2000 Revision, forthcoming, 2001 (medium scenario).

The implications of population aging for the impact of climate change are not entirely clear, but there are at least two issues that should be considered. First, older people are physically more vulnerable to extreme heat, so population aging could exacerbate increases in heat-stress mortality brought on by climate change. Higher temperatures also exacerbate outdoor air pollution such as ozone, which also disproportionately affects older people. On the other hand, older individuals are also more vulnerable to cold weather, so a warming trend in colder latitudes could decrease winter mortality rates, particularly in countries such as Russia.

A second issue to consider is the pressure posed by an aging population on public finances. Population aging implies a relative decrease in the number of working-age individuals and a relative increase in the number of retirees, a situation that could lead to falling tax revenues and increasing demand for public services. In Japan, the pronounced aging underway has influenced the government’s credit rating as analysts worry about how the country will care for its increasing number of elderly citizens. Even in countries with no social security system, population aging would be likely to erode the government’s financial position as tax revenue declines. The fiscal constraints imposed by aging populations will make it more difficult for governments to finance measures to mitigate the impacts of climate change.

Population Growth in Less Developed Countries

The world’s population currently increases by 83 million people each year, but only 1 million of them live in more developed countries. The population of the less developed world is thus growing much more rapidly than that of more developed regions. Today, citizens of less developed countries outnumber those of more developed countries 4.9 billion to 1.2 billion. In 2050, those figures will be 7.8 billion and 1.2 billion, respectively, according to PRB’s 2001 World Population Data Sheet. Of the nearly 3 billion people the world will add in the next 50 years, South Asia and Africa will each account for roughly 1 billion.

The concentration of population growth in less developed regions means that an increasing number of people live in countries with the least ability to adapt to climate change. The range of measures that could be needed to mitigate the impacts of climate change run the gamut: air conditioner purchases, changes in agricultural practices, construction of houses on stilts, relocation programs, development of storm and flood early warning systems, and replenishment of eroded beaches, to name just a few. Besides lacking the financial resources to undertake many of these measures, many less developed countries also lack the scientific expertise, effective public institutions, infrastructure, and technology needed to make adaptation measures work. Consequently, the brunt of the deleterious impacts of climate change will be borne by countries with an increasing proportion of the world’s population.

Bangladesh provides a dramatic example of this problem. With 134 million people, it is already one of the world’s more populous countries, and its population is projected to surge to 209 million by 2050. The Ganges-Brahmaputra river delta in particular is one of the most densely populated areas in the world. Yet this delta, which occupies much of southern Bangladesh, is also tremendously vulnerable to climate change. The impact of rising sea levels could be disastrous, causing extensive flooding and erosion as well as salinization of agricultural land and freshwater supplies. According to the UN Environment Programme, a 150-centimeter rise in sea levels could displace more than 30 million people. Bangladesh can scarcely afford to tackle these problems on its own: With a gross national income of only $47 billion in 1999, Bangladesh’s economy is the same size as that of Peru, even though it has five times as many people.

To be sure, while the residents of industrialized countries may not be as hard hit as those of less developed countries, climate change would not leave the more developed world unscathed. In the United States, for example, parts of southern Florida and Louisiana would be submerged by a one-meter rise in sea level. And there could be unwelcome indirect effects as well. If less developed countries are disproportionately hurt by climate change, the political backlash against industrialized countries that are leading carbon-emitters (particularly the United States) could be considerable. Less developed countries are already making a variety of demands for compensation, a movement likely to gather strength as climate change continues and its effects are felt more strongly.

On balance, the three population trends examined above — urbanization, aging, and growth in less developed countries — seem likely to increase humanity’s vulnerability to climate change over the next several decades. While certain aspects of climate change could prove beneficial to some communities, the most obvious silver lining — warmer winters at colder latitudes — will not help most people in the less developed world. Given this prospect, the tasks of preventing future climate change and identifying inexpensive means of adapting to it appear all the more pressing.

The Science of Climate Change

Most of the sun’s radiation passes through Earth’s atmosphere to warm the planet. Earth’s surface in turn reflects this energy back toward space in the form of infrared (heat) radiation. Certain gases in the atmosphere, such as carbon dioxide and water vapor, absorb and re-emit this energy, which slows the process of releasing the solar energy back into space. This natural trapping of heat keeps the planet surface warmer than it would otherwise be, making it more hospitable to life.

Over the past two centuries, all kinds of human activities — from powering steam engines or driving a car to air conditioning a house or surfing the World Wide Web — have added to this “greenhouse” effect. The Industrial Revolution, particularly the widespread use of fossil fuels such as oil and coal, has led to the release of enormous amounts of heat-absorbing gases into the atmosphere. The four greenhouse gases whose atmospheric concentrations are most influenced by human activities are

  • Carbon dioxide or CO2 (produced by burning solid waste, wood and wood products, and fossil fuels such as oil, natural gas, and coal)
  • Methane or CH4 (emitted by livestock or by the decomposition of organic wastes in municipal solid waste landfills)
  • Nitrous oxide or N2O (generated by the combustion of fossil fuels and solid waste)
  • Chlorofluorocarbons (also called CFCs, manufactured by industry for use in coolants and insulation).

Global emissions of carbon dioxide from human activities have increased more than 1000-fold, from 660 million to 931 billion metric tons between 1795 and 1995, according to the U.S. Department of Energy’s Carbon Dioxide Information Analysis Center (CDIAC). As a result, atmospheric concentrations of carbon dioxide have risen substantially over the past two centuries.

The implications of rising levels of greenhouse gases in the atmosphere have received a great deal of scientific attention over the past decade. In 1995, the IPCC concluded in its Second Assessment Report that “the balance of evidence suggest[ed] a discernible human influence on the global climate” — a determination that helped lead to the setting of greenhouse gas emissions targets in the Kyoto Protocol.

The IPCC’s Third Assessment Report, completed in October 2000, cited “stronger evidence” than ever before linking human activity to climate change. It also argued that man-made greenhouse gases have probably already “contributed substantially to the observed warming over the last 50 years.” The IPCC also revised its forecast of the amount of global warming that is likely: If greenhouse gas emissions are not curtailed, Earth’s average surface temperatures can be expected to increase by 1.4 to 5.8 degrees Celsius (2.5 to 10.4 degrees Fahrenheit) by 2100, substantially more than the panel estimated in its 1995 report.

In response to a request from the Bush administration, an expert panel at the U.S. National Research Council (NRC) conducted an evaluation of the state of climate change science in early 2001, leading to the publication of Climate Change Science: An Analysis of Some Key Questions. This report agreed with the IPCC’s assertion that greenhouse gases were accumulating in the atmosphere as a result of human activities, leading to rising surface air temperatures and subsurface ocean temperatures. The report also stated that the climate changes observed over the past several decades were “likely mostly due to human activities,” though a “significant part” of these changes might also reflect natural variability. Regarding future climate change, the NRC panel noted that “human-induced warming and associated sea level rises are expected to continue through the 21st century,” although current projections “should be regarded as tentative and subject to future adjustments.”

Bingham Kennedy, Jr. is a former associate editor at the Population Reference Bureau.


  1. Intergovernmental Panel on Climate Change, Climate Change 2001: Impacts, Adaptation, and Vulnerability, a Report of Working Group II of the Intergovernmental Panel on Climate Change (Geneva: IPCC, February 2001), accessed online at www.ipcc.ch, on August 15, 2001.

For More Information

For the Assessment Reports cited in this article, visit the Intergovernmental Panel on Climate Change at www.ipcc.ch

The U.S. Department of Energy’s Carbon Dioxide Information Analysis Center (CDIAC) website can be found at cdiac.esd.ornl.gov