In just 38 years, the sperm count of the average Western dude has been cut in half.

That’s a “very troubling” statistic, according to Dr. Hagai Levine of Hadassah-Hebrew University. Levine’s research on sperm counts was published Thursday in the journal Human Reproduction Update.

The meta-analysis suggests that, at least among North American, European, Australian, and New Zealand men, sperm count has declined at an alarming rate, and, unless something changes, it will probably continue to do so. Perhaps most alarmingly, the research reaffirms that scientists really don’t understand the forces behind this problematic plummet.

Sperm counts might seem like one of the more humorous aspects of medicine (because, c’mon, sperm), but the little reproductive cells are actually an important indicator of overall health. Or, as the researchers put it, sperm is a “canary in the coal mine” — an obvious indicator of more subtle problems, like heart health. They are also kind of important for keeping the entire human species plugging along.

For his latest study, Levine and his colleagues took existing data from prior studies on sperm counts and ran them through a statistical analysis. Instead of collecting their own samples, which would be complicated and ultimately limited, they brought together studies on sperm from around the world — 185 in total. Levine then compared the rate of decline in two groups: “Western” and “other.”

What they found surprised them. For starters, there was the jaw-dropping 1.4 percent decline in sperm count every year between 1973 and 2011 for Western men. But the researchers also noted that the decline wasn’t seen among those living in Asia, Africa, or South America. This geographic divide suggests environmental factors could be at play.

“We don’t know for sure why this is happening, but our findings should drive massive scientific effort to identify the causes and modes of prevention,” Levine tells Inverse via email. Past research, though far from the final word on this topic, has yielded some insight. Smoking, obesity, pesticides, and other chemicals appear to hurt sperm. And some researchers are looking at structural issues, too, like the poor design of sperm and its all-important bendiness.

Still, Levine says, more science is needed — and sooner rather than later. Not only has there been a halving of sperm counts in the last four decades, but according to the paper, there is “no evidence of leveling off in recent years.”

While Levine has been looking at issues of sperm and fertility for years, and is focused on the personal problems degraded sperm poses to male health, he’s not immune from the pop culture dimensions his work has taken on in recent months. “I am a big fan of the TV adaption of The Handmaid’s Tale, as well as Margaret Atwood’s books,” he says of the series, set in a post-apocalyptic world where all but a handful of women are infertile. “Personally, I believe these fears are valid.”

“We should focus on the immediate clear and present danger and take care of the issue now by research and actions,” Levine says. But our action — or our inaction — “will determine if and when … declining male fertility will threaten the existence of our species.”