It is fixed. The absolute limits are either 1) all incoming sunlight is turned into powering humans, which sets a limit of about 10 humans per square meter, (as I recall, solar incidence is 1000 W/sq. meter and each person emits 100 watts), or 2) we use nuclear to exceed that limit, in which case we are limited by are ability to cool that excess heat (Niven's Puppeteers moved their planet further from their sun to help with cooling).

However, the issue here is more "have we approached our current carrying capacity", not "is there an absolute upper-bound."

> There is no particular reason to think that we won’t be able to continue to raise carrying capacity further. Nuclear and solar energy are both clearly capable of providing large quantities of energy for large numbers of people without producing much carbon emissions. Modern, intensive agricultural systems are similarly capable of meeting the dietary needs of many more people.

I have a higher trust in Norman Borlaug, who won the Nobel Peace Prize speech for his work in the "Green Revolution". Quoting is speech:

"It is true that the tide of the battle against hunger has changed for the better during the past three years. But tides have a way of flowing and then ebbing again. We may be at high tide now, but ebb tide could soon set in if we become complacent and relax our efforts. For we are dealing with two opposing forces, the scientific power of food production and the biologic power of human reproduction. ...

"There can be no permanent progress in the battle against hunger until the agencies that fight for increased food production and those that fight for population control unite in a common effort. Fighting alone, they may win temporary skirmishes, but united they can win a decisive and lasting victory to provide food and other amenities of a progressive civilization for the benefit of all mankind

Borlaug believed the carrying capacity was about 10 billion, says https://en.wikipedia.org/wiki/Norman_Borlaug .

(Oddly, this piece didn't directly mention Borlaug's Green Revolution as the barely-in-time method by which Ehrlich's "Population Bomb" didn't go off in the 1970s. Nor does it mention GMOs, preferring only "We selected and bred plants and animals that were more nutritious, fertile and abundant".)

Going back to the essay:

> But they say little, specifically, about how social engineering of such extraordinary scale would be imposed in a democratic or equitable fashion.

One common one - it must be common as I've heard of it, and I know little of the topic - is better education for females, which is often significantly lower than for males. Eg, https://blogs.worldbank.org/health/female-education-and-chil...

"A study (Pradhan and Canning 2013) of education and fertility in Ethiopia estimated that an additional year of schooling in Ethiopia would lead to a 7 percentage point reduction in the probability of teenage birth and a 6 percentage point decrease in the probability of marriage. These are large effects, suggesting that women with eight years of schooling would have a fertility rate 53 percentage points lower than those with no schooling at all, and are consistent with observed data."