Human Impact on the environment equals Population times Affluence times Technology: I = P x A x T, for short. The equation starts with population for a reason. Seven billion of us have a larger footprint than one. So why, if you care about the planet, are environmentalists often so hesitant to tackle population head on?

The population question is a loaded topic—a loaded topic that perhaps comes down to one simple question: Why are you an environmentalist?

If the answer is because you like birds and bees, population may need to be on the table.

If the answer is because you like people and want to protect the planet “for future generations,” or “maintain a livable planet for all,” population becomes tricky.

To maximize the collective well-being of humans on the planet, one answer surely is to add more humans.

All of this has its limits, and most global population projections do show a flattening out at around nine billion by 2050. There are also clear and present problems, especially in already overcrowded and overtaxes local environments. But the question of how and even whether to tackle the P in IPAT is anything but clear-cut.