来源：《科学美国人》2023年3月号
In 2022 the world’s population hit eight billion. Population growth has been steady over the past few decades, with billion-person marks coming every dozen years or so. But that pattern is changing. Growth is beginning to slow, and experts predict the world’s population will top out sometime in the 2080s at about 10.4 billion.
2022年，世界人口达到80亿。在过去的几十年中，人口增长一直保持稳定，大约每十几年就增加10亿人口。但这种模式正在改变。增长开始放缓，专家预测，世界人口将在2080年代的某个时候达到104亿左右的峰值。
That slowdown is partly the result of a shift toward fewer offspring—a phenomenon that is happening almost everywhere around the world, though at different rates. High-income nations now have the lowest birth rates, and the lowest-income nations currently have the highest birth rates. “The gap has continued to widen between wealthy nations and poorer ones,” says Jennifer Sciubba, a social scientist at the Wilson Center in Washington, D.C. “But longer term,” she says, “we’re moving toward convergence.” In other words, this disparity among nations’ birth rates isn’t a permanent chasm. It’s a temporary divide that will narrow over the coming decades.
人口增长放缓的部分原因是少子化的趋势——这一现象几乎在世界各地都在发生，尽管速度不同。高收入国家目前的出生率最低，而最收入国家目前的出生率最高。“富裕国家和贫穷国家之间的出生率差距一直在扩大，”华盛顿特区威尔逊中心的社会科学家詹妮弗·休巴说，“但从长远来看，”她说，“我们正在走向趋同。”换句话说，各国出生率的差异并不是一个永久的鸿沟。这是暂时的分歧，在未来几十年将会缩小。
Many factors contribute to the waxing and waning of the world’s population, such as migration, mortality, longevity and other major demographic metrics. Focusing on fertility, however, helps to illuminate why the total number of humans on Earth seems set to fall. Demographers define fertility as the average total number of live births per female individual in a region or country. The U.S.’s present fertility rate, for example, is about 1.7. Demographers consider a fertility rate of 2.1 to be the replacement rate—that is, the required number of offspring, on average, for a population to hold steady. Today birth rates in the wealthiest countries are below the replacement rate. About 50 percent of all nations fall below the replacement rate, and in 2022 the region with the lowest fertility rate (0.8) was Hong Kong. Over the coming decades most of the rest of the world’s countries will likely follow suit.
许多因素导致世界人口的增减，如移民、死亡率、寿命和其他主要人口统计指标。然而，关注生育率有助于解释为什么地球上的人类总数似乎注定会下降。人口学家将生育率定义为一个地区或国家每名女性的平均活产总数。例如，美国目前的生育率约为1.7。人口学家认为2.1的生育率是人口更替率，也就是说，平均而言，人口保持稳定所需的后代数量。如今，最富裕国家的出生率低于人口更替率。全球约有50%的国家出生率低于人口更替率，2022年生育率最低的地区是香港(0.8)。在未来几十年里，世界上大多数其他国家的出生率也可能会低于人口更替率。
But humanity’s future clearly depends on many things besides fertility. For example, people in wealthier nations may produce fewer children, but those offspring tend to consume more resources—so rich countries can still have outsize planetary impacts despite their dwindling populations. Organizations such as the United Nations Department of Economic and Social Affairs—which tracks and predicts human population numbers—are working toward policy-based solutions for how all of us can have healthy, satisfying and sustainable lives on Earth. A clear-eyed understanding of population shifts is critical for reaching that bright future.
但人类的未来显然取决于许多因素，而不仅仅是生育能力。例如，富裕国家的人可能生育更少的孩子，但这些孩子往往会消耗更多的资源——因此，尽管人口不断减少，富裕国家仍然可以对地球产生巨大的影响。联合国经济和社会事务部等跟踪和预测人口数量的组织正在努力寻求基于政策的解决方案，以使我们所有人都能在地球上过上健康、满意和可持续的生活。要实现这一光明的未来，对人口变化的清醒认识至关重要。