In "What the world needs now, is people — more people" (Opinion Exchange, April 3), economics Prof. Tyler Cowen argues that decreasing fertility rates are a threat to human prosperity. He dismisses the threat from human overpopulation, stating, "If you think the world is overpopulated and has serious environmental problems, you might welcome" decreasing human fertility rates.

The negative effects of human overpopulation are not a matter of what some merely "think," they are a well-documented reality.

In his environmental history, "Something New Under the Sun," John R. McNeill documents, for example, the dramatic shrinking of forests worldwide during the 20th century. Similar phenomena include the collapse of the once-abundant fisheries off the northeast coast of North America due to unsustainable extraction of fish for human consumption, and the massive dead zone in the Gulf of Mexico due to eutrophication, a direct result of agricultural runoff of fertilizer into the Mississippi River.

McNeill's analysis links such relatively recent transformations to greatly increased human population, coupled with increased urbanization. Many such trends, including climatic instability due to carbon dioxide emissions, are starkly obvious when viewed through the broader lens of history.

In recent years, several economists have argued that future population decreases would be economically deleterious. Although Cowen notes this only briefly, others have been more explicit that fewer people, leading to decreased consumption and reduced economic growth, will damage the global economy. This is the case only if humanity stubbornly pursues an 18th-century economic system that understandably did not anticipate the constraints of the 21st-century world.

While capitalism has been a tremendous benefit to the material well-being of many humans, one flaw is in critical need of correction — that the effects of scale are not integrated into the economic model.

As economist Herman E. Daly has articulated clearly and simply, successful economic systems should serve three basic functions: (1) allocation of resources, i.e., determining which goods and services are produced; (2) distribution of those goods and services, i.e., who gets to consume them; and (3) the overall scale of production, i.e., how much is produced and consumed.

Classical free-market capitalism is unparalleled in efficient allocation, only moderately successful in achieving fair distribution and ignores scale by assuming infinite sources of raw materials and infinite sinks to absorb waste.

By ignoring scale, capitalist theory could be predicated upon indefinite economic growth — in other words, unlimited material consumption. Ignoring the effects of scale was permissible when the human population and consumption were small, but that is unequivocally not our situation now.

In recent decades, scientists and engineers at responsible companies in my industry have worked assiduously to reduce the raw materials required to produce a product and the waste generated by that production. Often the improvements have been substantial, but for companies to succeed by our current economic metrics, the overall scale of production must steadily increase nevertheless.

Rather than insisting that humanity must pursue unlimited consumption on a finite planet, we need the world's finest economic minds to determine how capitalism can be modified to function at steady state, while keeping the benefits of free market dynamics. Innovation can, in principle, continue indefinitely; expanding material consumption cannot.

Economic systems are human creations and thus can be changed, while the fundamental physical, chemical and biological characteristics of the Earth cannot.

Cowen laments the potential "failure to take full advantage of the planet's capacity to sustain human life." However, multiple analyses have concluded that the human population has likely exceeded the sustainable carrying capacity of planet Earth already and, without doubt, all of today's world population cannot live as North Americans and Europeans live — there are simply not enough resources.

A large fraction of humanity already struggles to subsist, greatly diminishing the quality of the human experience, and increased population, with the unavoidable parallel increase in natural resource depletion, will only make matters worse. Creating more human lives, without sufficient resources for them, should not be our objective.

Fewer human lives, with each life having adequate resources to allow increased opportunity for meaning and joy, does not paint a bleak future, indeed quite the opposite.

Mark A. Young, of Minneapolis, is a retired chemical engineer.