And early indications point towards lifestyle factors and pesticides possibly being the culprits.
The halving in sperm counts across the western world over the past four decades and fears for the future of the human race has received widespread publicity.
But little attention has been paid to rising cases of testicular cancer has more than doubled in recent decades.
Now Professor Niels Skakkebaek at the University of Copenhagen writing an editorial in the BMJ said we must act now to find out what could be causing such disturbing trends.
He said quality of semen should be interpreted alongside trends in testicular cancers, as reports suggested countries with a high incidence of this cancer have generally lower semen quality and vice versa.
He called for young men to be screened for the cancer to explore why male fertility is declining so rapidly.
And he suggests the decline and rise in cancer cases could all be down to exposure to hormone disrupting chemicals such as pesticides.
The professor of paediatric endocrinology and andrology in the university’s Department of Growth and Reproduction said: “What could be causing such disturbing trends?
“The short answer is that we do not know.
“However, data suggesting that the incidence of testicular cancer has more than doubled in recent decades leaves little doubt that we should look into environmental causes, including lifestyle effects.
“Alterations in our genome cannot explain the observations as changes have occurred over just a couple of generations.
“Environmental exposures can come through food, water, skin, and work and home environments.
“Both wildlife research and experimental studies suggest that modern lifestyles are associated with increased exposure to various endocrine disrupting chemicals such as pesticides that together may be harmful to wildlife and humans even though exposure to individual chemicals is low.
“However, little has been done to explore their potential effects on semen quality and testicular cancer.
“In particular, studies of maternal exposures in pregnancy and the subsequent reproductive function of their sons are needed.
“Should we be worried about our future ability to reproduce ourselves, as some media coverage has claimed?
“This inconvenient question makes sense when we look at what is going on in fertility clinics all over the world - more and more children are now born after in vitro fertilisation, intracytoplasmic sperm injection, and insemination with partner or donor sperm.”
He added we should fear for the future of the humans race as although more and more children are now born after fertility treatments fertility rates in many countries remain well below the replacement rate of an average of 2.1 children per woman.
In many European countries, including Germany, Japan, and Singapore, fertility rates range between 1.0 and 1.5, and fertility has become important in political and economic debates.
He said: “Simple research questions urgently need answers.
“What is the role of exposure to endocrine disrupting chemicals in reproductive trends?
“What is the role of lifestyle factors, including recreational drugs?
“What is the role of dysgenesis of foetal testis caused by maternal exposures?
“Why is the incidence of testicular cancer increasing among young men of reproductive age?
“We have already waited too long. “