Human overpopulation
Population planning

Several scientists (including Paul Ehrlich, Gretchen Daily and Tim Flannery) proposed that humanity should work at stabilizing its absolute numbers, as a starting point towards beginning the process of reducing the total numbers. They suggested several possible approaches, including:

There is good evidence from many parts of the world that when women and couples have the freedom to choose how many children to have, they tend to have smaller families.

Some scientists, such as Corey Bradshaw and Barry Brook, suggest that, given the "inexorable demographic momentum of the global human population," sustainability can be achieved more rapidly with a short term focus on technological and social innovations, along with reducing consumption rates, while treating population planning as a long-term goal.

However, most scientists believe that achieving genuine sustainability is a long-term project, and that addressing population and consumption levels are both essential to achieving it.

In 1992, more than 1700 scientists from around the world signed onto a " World Scientists' Warning to Humanity," including a majority of the living Nobel prize-winners in the sciences. "The earth is finite," they wrote. "Its ability to absorb wastes and destructive effluent is finite. Its ability to provide food and energy is finite. Its ability to provide for growing numbers of people is finite. And we are fast approaching many of the earth's limits." The warning noted:

Pressures resulting from unrestrained population growth put demands on the natural world that can overwhelm any efforts to achieve a sustainable future. If we are to halt the destruction of our environment, we must accept limits to that growth.

Two of the five areas where the signatories requested immediate action were "stabilize population" and "ensure sexual equality, and guarantee women control over their own reproductive decisions."

In a follow-up message 25 years later, William Ripple and colleagues issued the "World Scientists' Warning to Humanity: A Second Notice." This time more than 15,000 scientists from around the world signed on. "We are jeopardizing our future by not reining in our intense but geographically and demographically uneven material consumption and by not perceiving continued rapid population growth as a primary driver behind many ecological and even societal threats," they wrote. "By failing to adequately limit population growth, reassess the role of an economy rooted in growth, reduce greenhouse gases, incentivize renewable energy, protect habitat, restore ecosystems, curb pollution, halt defaunation, and constrain invasive alien species, humanity is not taking the urgent steps needed to safeguard our imperilled biosphere." This second scientists’ warning urged attention to both excessive consumption and continued population growth. Like its predecessor, it did not specify a definite global human carrying capacity. But its call to action included "estimating a scientifically defensible, sustainable human population size for the long term while rallying nations and leaders to support that vital goal."

Subsequent scientists' calls to action have also included calls for population planning. The 2020 "World Scientists' Warning of a Climate Emergency" stated: "Economic and population growth are among the most important drivers of increases in CO2 emissions from fossil fuel combustion." "Therefore," the study noted: "we need bold and drastic transformations regarding economic and population policies." "The world population must be stabilized—and, ideally, gradually reduced," it concluded, implying that humanity is overpopulated given current and expected levels of resource use and waste generation.

A follow-up scientists’ warning on climate change in 2021 reiterated the need to plan and limit human numbers to achieve sustainability, proposing as a goal "stabilizing and gradually reducing the population by providing voluntary family planning and supporting education and rights for all girls and young women, which has been proven to lower fertility rates."
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