The world's population should be at most 50 million.

[The Grand Nudger]

Calling something "pessimism" is an interesting rhetorical tactic.  Yes, Ehrlich and others have been proven wrong.  Do they represent everyone who is concerned about population?  No.

Are scientists predicting the apocalypse?  No, they are pointing out a big problem we need to fix, and soon, or we will suffer the consequences.  I have read over 50 books on climate change and associated issues.  The greatest unknown in climate change research is about how humans will react.  The scientists who model the future therefore create alternative scenarios based on the best available information and project (not predict) various possible outcomes.  Uncertainties are built right into those projections.  So first of all you are arguing against strawmen.

Second, I am not interested in betting the future of the planet on unproven technologies, or on an inductive assessment which depends entirely on the future playing out the same way as the past.  I am interested in more conservative and proven tactics, since our need is immediate to combat climate change.  We only have another 20 or 30 years to make significant progress before all those really nasty feedback mechanisms start kicking in, which may take the future of our climate out of our own hands.

Third, Malthusian assumptions have been proven in evolutionary theory and even historically in certain human societies.  You are more than disingenuous to over-simplify them so you can dismiss them altogether.  That's why I must conclude that you are not arguing in good faith at all, but only as a believer in the dogmas of technological optimism.

Population is, and will continue to be, a multiplier of environmental impact.  I = PAT.  How scientific does one have to be to make a simple and irrefutable point?  You are laughably calling real scientists unscientific.  What kind of scientist are you again?

That's just what the two opposing camps came to be called over the last couple centuries.  I didn't choose the names, and they do seem to fit.  

Moving onward, it's not just that erlich and others have been proven wrong...the fundamental assumptions upon which their estimates where based have been proven wrong.  That's why, ultimately, they were wrong..going all the way back to malthus and into the present day.  

No, climate scientists are not predicting the apocalypse, but population pessimists -are-.  That's the star around which population pessimism orbits - a malthusian catastrophe.  I'm not arguing against this assumption so much as pointing out both that and why it was mistaken.

No one suggests betting the future on unproven tech.  Population optimists point out that the future -relies- on intensification.  We have to make better and more productive use of our land regardless of our population.  This doesn't require unproven tech...and if you're not assuming that tomorrow will be like yesterday then you have no reason to assume that yesterdays yield from conventionals will be the same or smaller than tomorrows or that the population will continue to grow just because it has in thepst...in the first place. We can skip the positioning, lol.   

Malthus was an economist, his theory (1798) has nothing to do with evolutionary theory or evolutionary biology.  Darwin wouldn't write Origin until 1859.  Humanity has never outpaced it's rate of resource collection (the opposite occurred), and the cause of local shortage has been more accurately identified as incompetence, corruption, and global instability.   Malthus was wholly wrong, it is a myth that some science has "proved him right".....on anything. Hey, maybe, tommorrow, it will finally go the other way! That's a possibility!

The assumptions made in the formula provided have known flaws, are fundamentally pessimism biased, and fail to take into account the observations that contradict those assumptions just as much as any other form of population pessimism does.  More than this, there's no requirement that a pop optmist call them into question anyway. They would contend that changes to A and T are more direct and effective than changes to P, and that not all I is meaningfully problematic...is all.

No kind of scientist, by the way...I do fieldwork and build installations for land grant research into alternative ag.  Sort of buzz around people who know better picking up useful things. Which is a fancy way of saying I dig holes for a living....but, should that matter, and even more pedantically why should it matter when population optimism also has advocates well positioned and well regarded in academia? I gave you a factual description of the positions, and their flaws. I gave you a history of notables in the positions. I gave you present day examples that flatly contradict necessary assumptions. The pessimist contends, ultimately, that if we keep doing something some stupid way there is only so far that it can take us. I think we can all agree that this much is true, but in context of a hypothetical global carrying capacity it is a meaningless truth. The potential productivity of "stupid way to do something x" is a different subject than earths hypothetical ability to carry human life. This has always been the mistake central to pessmism, it's always been wrong..not only with respect to the then-future(now past) but even in a present moment. The way we grow our food is not the most productive known use of land, nor is it the most environmentally conscious. It is the most cost effective...itself an economic reality (Hi malthus...perhaps this might help to explain why he was interested), subject to change in the present and blind to future progress.

The productive capacity of pre columbian north american intercropping is no more the carrying capacity of the continent than yesterdays pre industrial fallow system..or today's industrial monoculture is.