For the second time in five years, scientists are warning about declining human sperm counts. (I wrote about this issue in "Declining sperm counts: Nature's answer to overpopulation?" early last year.)
Besides confirming the results of an important 2017 study, the authors now note an acceleration in the decline of sperm counts. In other words, whatever is causing that decline is getting worse. The rate of decline has doubled since 2000.
It's important to remember that when the fertility rate declines below replacement—currently 2.1 births per woman in so-called developed countries—populations shrink. This may not be a bad thing at first since overpopulation and overconsumption are huge barriers to building sustainable societies. But there comes a point when if fertility rates don't level off and then rise to replacement, extinction become a possibility.