So why is it that many so-proclaimed Catholics who support abortion, homosexuality, and euthanasia also champion the false science of climate change, which ultimately seeks to reduce human population?  

Notice the pattern?  That methodical denial of objective reality -- life does not begin at conception, gender is not fixed, taking your own life is not a sin -- culminates in the fanatical belief that a complex non-linear chaotic system can be controlled by toggling the amount of carbon dioxide which accounts for less 2% of all greenhouse gases on a medium-sized planet, 71% of which consists of water.