The world's population could swell to 10.9 billion by the end of the century, a new United Nations analysis found, raising concerns that adding more than 3 billion people to the planet could further deplete natural resources and accelerate global warming.
The increase, up from the current count of 7.7 billion people, is expected despite a continued decline in the global fertility rate, which has fallen from 3.2 births per woman in 1990 to 2.5 births per woman this year. Experts say the global fertility rate will continue to decline, but the world's overall population will still rise, hitting 9.7 billion by 2050.
The new report predicts slower population growth than the UN's last assessment, released in 2017. That estimate projected that the world population would reach a staggering 11.2 billion by the end of the century. The revised figures reflect the downward trend in the global fertility rate, which means the populations of more countries are shrinking.
Scientists are also concerned about the effect of population growth on climate change. As the global population increases, so will humanity's footprint on the planet.
"Our impact on the climate is tied up with population in lots of different ways — what resources people are using, how much industrial production is going on, how much energy is needed for heating, cooling and transportation," Amy Snover, director of the University of Washington's Climate Impacts Group, said. "All of these things affect greenhouse gas emissions, so the more people we have and the more resources we use, the harder it will be to cope with the risks and impacts of climate change."
In some regions, the increasing population means more people will be vulnerable to rising seas, extreme weather and the spread of infectious diseases, which can all be intensified by climate change.
"People in the least-developed countries already tend to be at higher risk of climate change, so depending on how population growth unfolds across the globe, there's going to be more people in hazardous areas," Snover said.
She added that one of her biggest concerns is whether the planet will have enough resources to meet people's basic needs. Population growth, combined with climate change, could strain agriculture and access to clean drinking water even more.
Scientists say it's difficult to know whether there's a limit to the number of people the planet can sustain — or when we could be approaching it. Still, according to Bradshaw, the 7.7 billion people who inhabit Earth now may already be pushing the planet to a breaking point.
"Even if we maintained the current status quo and not a single other carbon molecule was released because of human activity," he said, "we'd still see the effects of climate change for at least 300 years from just the emissions that are already in the system."
2100年全球人口将达到109亿
联合国最新分析发现，到本世纪末，世界人口可能会增至109亿。人们担心，地球上增加30多亿人口可能会进一步耗尽自然资源，加速全球变暖。
尽管全球生育率持续下降，但预计届时这一数字仍将远超目前的77亿人口。全球生育率从1990年的3.2下降到今年的2.5。专家表示，全球生育率将继续下降，但世界总人口仍将增长，到2050年达到97亿。
这份新报告预测的全球人口增长速度低于联合国2017年发布的上一次评估。此前的评估预计，到本世纪末世界人口将达到惊人的112亿。修正后的数据反映出全球生育率下降的趋势，这意味着更多国家的人口正在减少。
科学家们还担心人口增长对气候变化的影响。随着全球人口的增长，人类将影响地球上更广泛的区域。
华盛顿大学气候影响小组主任艾米·斯诺弗说：“我们对气候的影响在很多方面都与人口息息相关。人们使用了什么资源，在进行多少工业生产，需要多少能源用于取暖、制冷和运输。所有这些都影响到温室气体的排放，所以人口越多，使用的资源越多，就越难以应对气候变化的风险和影响。”
在一些地区，人口增长意味着更多的人将容易受到海平面上升、极端天气和传染病传播的影响，而这些都可能因气候变化而加剧。
斯诺弗说：“最不发达国家的人民已经面临着更高的气候变化风险，因此，将有越来越多的人生活在危险地区，这取决于全球人口增长的趋势。”
她补充说，一个最大的担忧是地球上是否有足够的资源来满足人们的基本需求。人口增长加之气候变化可能会给农业生产和获取清洁饮用水带来更多困难。
科学家表示，很难知道地球能承受的人口上限，或者何时会接近上限。不过，据布拉德肖说，现在居住在地球上的77亿人可能已经把地球推向了崩溃的边缘。
他说：“即使我们维持目前的现状，不再因为人类活动而有更多的碳排放，但由于生态系统中已经存在的碳排放，在未来300年内，我们仍然可以看到气候变化的影响。”