The specter of population control is resurgent because of worldwide concerns about global climate change. Not too long ago the British Medical Journal urged Britons that "the biggest contribution U.K. couples can make to combating climate change would be to have only two children or at least one less than they first intended." (E! Science News) It would be literally suicidal for the Brits to take this advice. To understand why, we need to talk a little about total fertility rates. It may sound boring, but stick with me. The implications may be devastating.

Total fertility rate is the number of births per woman for a given country. For a country to be able to replace its population it needs a TFR of at least 2.1. In the U.K., for example, the TFR is currently 1.84. This means that if nothing changes, such as a sudden and sustained increase in fertility or immigration, the British population will eventually be unable to replace itself.

According to the United Nations, there are 73 countries currently below replacement fertility rates which account for nearly half of the world's population. By 2050 that group of countries will have essentially the same population as today. After 2050, there will begin a precipitous decline in actual populations of those mostly developed countries.

That population decline will occur in developed countries has been understood for many years. American Enterprise Institute scholar Ben Wattenberg wrote "The Birth Dearth" in the mid-1980s describing this phenomenon. More stunning, however, are U.N. numbers that indicate that these same trends are present in less developed countries.

In its "World Population Prospects, the 2006 Revision," the U.N. lays out three scenarios — low, medium and high — to describe fertility rates between now and 2050. Under the "high" forecast, world TFR reaches 2.51, or above replacement rate. Under the "low" and "medium" scenarios, however, world TFR declines to 1.54 and 2.02 respectively. This would mean an absolute decline in world population, with the deepest declines in the more developed regions.

Though the U.N. report does not guess which of the three scenarios is the most plausible, it is difficult to see how the lower TFR estimates are not the most likely. Since the mid-1960s, according to U.N. data, the TFR for less-developed countries has dropped in an unbroken straight line from over six births per woman to less than 2.75 in 2006. This is already at or near the "high" projection for the 2050 time frame and rapidly approaching below replacement rate.

As Wattenberg observes, "never have birth and fertility rates fallen so far, so fast, so low, for so long, in so many places, so surprisingly."