According to a UN report released yesterday, climate change is already having an effect on human health, agriculture, and water supplies across the globe. Now, a new study released today in PLOS Biology makes the case that even one of the most robust animal populations around — the roe deer of France — are having trouble keeping up with the pace of changing temperatures.

"Increased mortality of newborn fawns."

Using population data spanning the last 30 years, researchers demonstrated that roe deer in the Champagne region of France have not been able to adjust their birthing season to face the earlier onset of spring vegetation (these food sources are now appearing two weeks earlier than they did in 1985). Jean-Michel Gaillard, a French biologist at the University Claude Bernard Lyon 1 and co-author of the study, explained in an email to The Verge that "this increasing mismatch causes increased mortality of newborn fawns," because the deer's food is running out too early. So, populations that once flourished are no longer increasing at the same pace they were just 20 years ago.

The main uncertainty of this study, Gaillard said, is whether genetics plays a role in the time at which a female deer gives birth. But the researchers didn't find statistical evidence of heritability for birth date in roe deer. Moreover, Gaillard said that there is no uncertainty regarding the role of climate change in this population. "We are confident that climate change, via earlier onset of vegetation flush in temperate forests, is the culprit of the increased fawn mortality in recent years."

"The only common factor being climate change."

According to Gaillard, a previous study on the deer population in the Chize forest, in western France, also showed increased fawn mortality. "This occurred despite markedly different environmental conditions between these populations," he said, "the only common factor being climate change."

In the 1980s and 1990s, roe deer was one of the most successful species of vertebrates in Europe. According to Gaillard, roe deer population sizes across most European countries showed at least a fivefold increase during that period. In fact, the researcher said that this boom was so striking that leading specialists wrote a book entitled "The European Roe Deer: The Biology of Success" in 1998. And now, just 16 years later, "this species cannot cope with increasingly earlier onset of spring," Gaillard said. "For ecologists, our results have clearly important consequences," because this shows that "the fate of a given species can change very quickly."

Their numbers continue to increase, albeit at a much slower pace

For now, France's roe deer are still doing well. Their numbers continue to increase, albeit at a much slower pace. But if climate change goes on for several decades, Gaillard said, we will likely see their populations decline.

Gaillard and his team will continue to study these mammals, in the hopes of finding out why the deer have been unable to adapt. He wonders, for instance, if their unusual reproductive cycle — which includes a five-month delayed implantation — might make them more vulnerable to climate change than one would expect. But regardless of the questions surrounding the study's findings, one thing remains crystal clear, Gaillard said. "It seems to me increasingly difficult to be a climate-change skeptic nowadays."