Here's the deal, guys: Go out and get a vasectomy today and one group will throw in a T-shirt, adorned with a polar bear carrying a pair of scissors. It's World Vasectomy Day and the Tucson, Arizona-based Center for Biological Diversity wants men to "Get Whacked for Wildlife."

But not everybody is on board with this strategy for cutting back on planetary destruction.

The campaign is purposefully designed to be "so ridiculous that everybody has the chance to step back, laugh, and really talk about" a subject that can be difficult to broach, said Leigh Moyer, the group's specialist on population matters.

"Choosing to have fewer, or no children, is one of the best ways to reduce your carbon footprint, but right now men are often left out of that conversation," Moyer said. "If we want to leave room for wildlife on an already crowded planet, we need to get both genders involved in preventing unplanned pregnancies. The fact is: vasectomies are the most effective form of male birth control, but many people feel awkward talking about it."

With the mood lightened, Moyer said, the conversation can turn to how vasectomies are a relatively simple, quick, and effective procedure. But the broad topic of fertility and environmentalism can be tricky to navigate; it's prone to being misunderstood or carrying negative connotations.

"We really want to bring population back into the environmental conversation. It's kind of been forgotten and ignored because it's a taboo. People don't want to talk about it," Moyer said. "But at the same time, in the US more than 50 percent of pregnancies are unplanned. So when we start there, we can talk about population in a way that's really supportive of human and reproductive rights."

Here's how quickly the world population has ballooned: It took all of human history, up until about 1800, for us to make our first billion. The Industrial Revolution hit, and the number of humans exploded exponentially. We added our next billion people by around 1930, hit three billion thirty years later, and topped four billion just 15 years after that.

Currently, there are more than 7.1 billion of us roaming the earth. By 2050, according to the latest United Nations projections, we'll be at 9.6 billion, and by 2100 the human population is expected to exceed 11 billion.

The problem, of course, is feeding and housing a growing number of people on a planet that's predicted to become more difficult to live on — as the effects of climate change become more and more apparent — when we can't even seem to get those things right today.

The rise of human beings has been especially tough on the rest of the natural world, Moyer pointed out.

"We are currently in the sixth mass extinction, and this one scientists agree is caused by human activities," Moyer said. "This is self-explanatory: Human activities don't happen without humans. We've doubled our population in the last 50 years, and at the same time wildlife numbers have been cut in half."

Anne Hendrixson, director of Hampshire College's Population and Development Program, however, argues that there are more pressing issues worthy of attention.

"They're locating environmental problems in fertility, rather than looking at larger issues of consumption and really holding the worst polluters accountable for their actions," Hendrixson said. "Instead it's looking at environmental problems as an issue of an equalized carbon footprint that is shared globally. They're arguing that each person has the same impact on the world. That really takes away so much complexity of how population does impact the environment."

It's important to focus on specifically who is consuming the most, and to target the wealthy nations and the biggest corporations that are disproportionately responsible for greenhouse gas emissions, Hendrixson said.

Other CBD programs, Moyer said, do focus on limiting fossil fuels, on supporting renewable energy, and reducing meat consumption. 'Get Whacked,' in other words, is just one slice of their overall strategy.

"We try to hit all of the issues, because they really are all connected. It's really hard to say, 'Well, if there are fewer people, everything would be fixed' or 'If we didn't waste, everything would be fixed,'" Moyer said. "It's all tied together."