Am not sure if your first comment came from the same Patrick as quoted in:
http://books.guardian.co.uk/story/0,,2282286,00.html#article_continue

Holden said that, in light of environmental concerns and what he believes is an impending oil crisis, he was calling for “a national policy of self-sufficiency in staple foods”. Large, centralised distribution systems would become economically unviable, he argued, and food would have to be produced locally. Imports should be limited, though he said there was still an argument for shipping in certain cases.

Like you said 70 years ago agriculture looked very different, but I’m not sure there is real evidence to suggest the ‘carrying capacity’ of the earth has been raised by modern methods.

Like you said now instead of using the sun’s energy, we use fossil fuels. Together with this we have taken such a distant course from where we were 70 years ago, that there is no way to compare what we have now with what we would have had steering the same course.

There is often the assertion that hybrid varieties of crops give higher yields. With only a few exceptions, most knowledgeable people with an independent view on the subject don’t believe this is true.

There is the assertion that if we don’t use nitrogen inputs, we would be able to keep up production levels. This too has been found to not really be true, because like you say crop rotations and nitrogen fixing crops can make external nitrogen inputs unnecessary.

I think other assertions along these lines can also probably be disputed.

Like you say, the world is eating too much meat, and the inefficiencies of this together with the levels of fossil fuel inputs have to be addressed.

It’s a matter of changing priorities and directions, but I don’t think the carrying capacity of the earth is an issue.