The world's population exceeded 7 billion in March, 2012 and is expected to surpass 8 billion people by 2025Simply put, the basic cause of global climate change is too many people consuming too much carbon-based energy. The combination of individual impacts times the number of individuals is overwhelming the equilibrium of the Earth's systems.

The graphic below shows the relatively stable human world population through most of our history; and then the beginnings of a steady population increase around 8000 years ago with the advent of the Agricultural Revolution, another rapid increase in the mid-1800's in response to the Industrial Revolution; and, finally the population explosion in the mid-1900's primarily due to lower death rates and increased longevity as the result of the introduction of antibiotics; and the distribution of insecticides, medicines and other health care to the developing nations following the World War II.

Figure 1. World population throughout the history of mankind.

The population dynamics between the developed nations and the developing nations of the world now drastically differ, with a predicted stabilizing and then a slight decline in the developed nations in sharp contrast to continuing rapid growth continuing in the developing nations (Figure 2.) until the second half of the century. At that point the world's population growth will eventually be halted, and reversed, as we will have exceeded the limits of the planet to sustain civilization(2,3,4). There is disagreement among the experts about the population level that will exceed the sustainability limit, but in general the estimates range between 9 and 14 billion. Meanwhile the United Nations estimates(5) that 38.6 million people were living with HIV in 2005, and that 852(6) million people suffer from chronic hunger with over 6 million children a year starving to death.


Figure 2. World population grown in developed and developing nations. From:

The fundamental demographic disparity between the developed and the developing nations presents extremely difficult cultural, technical and moral challenges to any attempt to manage global climate change; a task that will only grow increasingly difficult.  The United Nations recently reported(7, 8) that by the year 2050 the population of the developing nations will increase by 2.5 billion people, while the population of the developed nations remains constant. Astonishingly this increase is equal to the entire world population in 1950. The projected increase is based on the assumption that the birth rate will decline from 2.75 to 2.05. If the birth rate were to remain constant the population increase in the developing nations would be 5.2 billion, more than twice the total world population in 1950. If health care and living conditions improve in the developing nations people will also live longer, increasing population levels and placing additional demands upon the environment.

A rising middle-class in the burgeoning population of the developing nations, with aspirations of a standard of living comparable to that in the developed nations will more than offset the most ambitious greenhouse gas emission control schemes.  The probably of a population of 9 billion people, as predicted by 2040(8) sharing the standard of living currently enjoyed in the developed nations seems wildly impossible unless the consumption of fossil fuel immediately ceases.  The alternative would most likely be greatly increased social strife and and a increasingly uninhabitable planet.


1.) CIA World Factbook (

2). During the 1960's, predictions of the sustainability limit included such limiting factors as global warming due to heat released from human bodies (for example, see: J.H. Fremlin, How Many People Can the World Support? New Scientist, October 29, 1964.) We now know that mankind's release of greenhouse gases is causing global warming at a far lower population level that required by this theory

3.) In 1968 a movement was started by the publication of The Population Bomb by Dr. Paul Ehrlich, and the subsequent organization of ZPG with chapters throughout the United States, and indeed throughout the world, to inform people about the consequences of overpopulation. At the time, the average reproductive rate in the U.S. was approximately 3.4 children per female. While the 1960s and 1970s saw both a cultural and legal shift in reproductive rights it was the technological advance signified by the 1960 FDA approval of the first oral contraceptive that rapidly altered the reproductive landscape. In 1970 the U.S Congress gave married women in all states access to contraception by rewriting the Comstock Law of 1873, which had prohibited the dissemination and mailing of contraceptives under federal anti-obscenity legislation; and in 1973 the U.S. Supreme Court in the Eisenstadt v. Baird decision gave unmarried individuals the right to purchase contraceptives. By 1975 the U.S. reproductive rate had dropped to 1.5 children per female, with most industrial nations showing similar trends (Figure 2).

4.) Population dynamics includes evaluation of both "relative rate" and "absolute numbers", so both rate and starting population number determine the projected population levels. In addition, the time factor of both reproductive cycles and overall life cycle must be included. Thus the impact resulting from the introduction of a new rate altering technology may not be apparent for some years. For example: the reproductive rate in the developed nations had dropped below the replacement rate by the late 1970's . However, it will take almost half a century before the absolute population number stabilizes and then starts to decline. This lag of approximately 3 reproductive cycles is an example of the momentum often built into complex biological systems.

5.) Joint United Nations Programme on HIV/AIDS. (

6.) Food and Agriculture Organization of the United Nations. (

7.) United Nations. Press Release: POP/952. March 13, 2007. New York. NY

8.) Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat, World Population Prospects: The 2012 Revision,  (