I can only speak for what I've seen of Bernie Sanders, not Buttigeg, and the reason I brought up the Dilbert strip is because you remind me of the guy Dilbert is talking to.
The "surplus population", as you called it, in this specific case refers to babies born to parent whom had no other choice but to have the baby. By providing easy and safe access to abortion and birth control (so methods that include not even getting pregnant in the first place) that surplus population goes down over time because now the people who don't want children no longer have to have children. This will predominantly affect poorer countries where the average woman does not have access to birth control but, as Bernie says in the clip itself, it's the right of women in the USA too and they are affected as well.
And you've managed to take that and turn it into "Democrats want to kill black and brown babies". You've ignored all context, especially the specific inclusion of providing women with access to abortions and birth control , meaning they get a choice in the matter, just to call Bernie a racist.
So what you are saying is that Bernie Sanders Climate Change plan is not about eugenics but infanticide., or least killing the most innocent and helpless of human life. OK, that's a fair distinction. Just so you are aware, I always discuss abortion using black babies. I find that lefties don't see the unborn as victims unless you label her as "black". Sad, but very true. I don't see Sanders driven by racism but depravity.
1) It is unrealistic to think that developing nations contribution to CO2 will not increase. As these countries' economies grow their energy, CO2 output will grow exponentially regardless of how many innocent human lives die. Poor countries are too focused on survival to be overly concerned about climate change (that's why China and India are exempted by the Paris Climate Accords, even though China's energy production now has more coal power plants than the US with a 25% increase on the way within years and 100 plants that were supposed to be closing - actually engaged in new construction). Caring about global warming is the domain of rich countries. That's reality. How do you deny developing countries the right to grow their economies as the rich countries have done? Answer: You can't.
2) Bernie's inclusion of population control in his Climate change plan is ridiculous. Again, even if you assume that you will kill half of all the black and brown unborn babies in developing countries over the next 100 years. The CO2 output will actually be going up, not down in those countries. Sanders has to know that. His commitment to population control is more about his obeisance to planned parenthood (founded by a supporter of eugenics).
3) Abortion has become a sacrament to the Democrat party. The dismembered body and pooled blood of the unborn has religious like meaning to the Democrat party. Not a single presidential candidate doesn't support abortion through at least the 3rd trimester (most even tacitly allow for post birth abortion scenarios in cases of botched abortions). That's an extreme position which ignores science. 95% of all biologists affirmed the biological view that a human's life begins at fertilization (5212 out of 5502). When a preemie can survive from 22 weeks (there are a few instances of surviving earlier), it is hard to rationalize how a baby girl born at 22 weeks gestation has value but a child at much older gestational age does not. due to her physical location.