The world's population is currently growing at a rate of about 79 million people per year. World population surpassed 6.8 billion in early 2009. U.N. projections for the year 2050, depending on various assumptions, range from slightly less than 8 billion to slightly more than 11 billion, with a median of slightly above 9.1 billion (Engelman). Population growth, coupled with consumption, stresses the Earth's natural resources and contributes to environmental degradation, but population cannot be considered in isolation from other demands on resources and the disparities of levels of consumption and resource use between differing parts of the world.
Since having more people means a greater strain on the environment and resources, population is a challenge for sustainability. To be truly sustainable, humankind must live within the limits of the Earth’s resources and use only what the Earth can provide indefinitely. With more and more people needing food, fresh water, wood, fuel, and other resources, sustainability is increasingly difficult to achieve.
Population cannot be considered in isolation from other demands on resources. In general, developed countries use more — sometimes dramatically more — resources than other countries, and per capita growth in these countries affects the environment and global resources much more than growth in less developed countries. For example, per-person CO2 emissions in the U.S. are five times greater than in China and more than 200 times greater than in the world’s poorest countries (Moore). But growing populations in less developed countries can also strain local ecosystems and resources. When local resources are exported through global trade, regions can support fewer people. Additional factors such as climate change, threats of widespread drought, and the diminishment of arable land further reduce the population that any given region can support.
Limiting population remains highly controversial, given global disparities in wealth and exploitation of resources. Some have suggested that poverty, lower life expectancy, and lack of economic security tend to promote population growth. "To bring the human population into balance with economic resources and the environment," Frances Moore Lappé has written, "societies must address the extreme maldistribution of access to resources — land, jobs, food, education, and health care. That is our real challenge" (Lappé).
According to Lester Brown, "Stabilizing population is the key to maintaining political stability and sustaining economic progress. And the keys to stabilizing population are universal elementary-school education, basic health care, access to family planning, and for the poorest of the poor countries, school lunch programs [because they provide an incentive for children to make it to school and because if they are chronically hungry, their attention spans are short]" (Brown).
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