Every time I post about falling fertility, someone replies: “Great for the planet.” I understand the intuition, but it gets the economics almost exactly backwards. To be clear: I am not arguing for explosive population growth. A gentle decline or stabilization would be my first choice. The problem is that we are not heading toward a gentle decline. We are heading toward a collapse. And a collapse changes everything. Environmental protection behaves like a luxury good. As countries become richer, citizens demand cleaner air, cleaner water, and stronger climate policy. Prosperity creates both the willingness and the fiscal capacity to pay for these goods. This is not a theoretical curiosity. The modern environmental movement was born in California in the 1960s, when the state was among the richest in the richest country on earth. That was no coincidence. You need to be prosperous before you start worrying about the spotted owl. A sustained fertility collapse works in the opposite direction. As populations age, pension and healthcare costs rise while the tax base shrinks. Governments under that kind of fiscal pressure protect mandatory spending first because that is what voters scream about (I am from Europe, and I can tell you this is the case with 100% certainty). Environmental investment, which is largely discretionary, is the easiest to postpone. And it will be postponed, particularly in middle- and low-income countries. Environmental policy is not a costless virtue. It requires administrative capacity, long planning horizons, and resources. Lots of resources. Decarbonization alone demands trillions in public and private investment over the coming decades. Where will that money come from if the working-age population is shrinking and the dependency ratio is exploding? If demographic collapse erodes prosperity and fiscal space, and the evidence strongly suggests it will, it will not increase environmental investment. It will make it harder to sustain. So, if you care about the environment, I am sorry, but what is happening with fertility right now is terrible news.