Sperm Counts Really are Declining

Major new meta-analysis of sperm counts worldwide find the same thing as earlier studies:

After adjusting for factors like the subject's age and time without ejaculation, they found that the sperm concentration in western men has fallen from an average of 99 million per milliliter in 1973 to 47.1 million per milliliter in 2011—a 52.4 percent drop. Total sperm counts, or the number of sperm in an entire sample, fell by almost 60 percent. Yet similar drops were not found in the non-western samples.

This isn't anything like a crisis yet, since 47 million per milliliter is still quite fertile. But what is causing this? And will the trend continue?