Well, the whole of the Mediterranean (the Greek and the Spanish didn't make their fleets out of the present-day mediterranean vegetation) and the Middle East (the cedars of Libanon, North Africa: the bread basket of Rome). Each time the soil in Mesopotamia became bad enough, the current empire retracted and after a few floodings it was ready to go again.

And farming might be sustainable for millennia in its current form, that's not enough. It must be sustainable practically forever. Even as little as the loss of 1 mm of soil each year is 1 m per millennium. That's unsustainable, the exact period that it goes on depends on the soil supply. How much soil has disappeared in the Dust Bowl area, and how much is left? We can be certain that that kind of agriculture in that area is *not* sustainable.

decline of Sumeria was lowered agricultural productivity due to salt accumulation

Yes! Irrigation and plowing release salts that can otherwise coexist in the soil without a problem. Watch the video:
http://www.permaculture.org.au/greening.htm

Those are hand fulls of salt he is showing you from Jordan. So where did he put it? "It's not supposed to be possible", they told him. With agriculture, it isn't and you get desert.

The process depends on the soil and ecology being damaged. Grasslands are the most vulnerable, but that is what you get when you clear a forest to grow yet more food for a "growing" population. Since the forests are so integral to affecting climate, rain, and increasing soil humus, they take longer, but will become deserts, too. Bring the forest back to the yellow river and it will run clear, again.

Any evidence for this claim?

The problem is that there is no evidence to support the claim that famine happens to Foragers as much as it happens to Agrarians, or that it ever happened to Foragers. You are asking for a proof of a negative. Could you speculate that it happened? Sure, knock yourself out. That's what the article you posted says that they did ("probably" and "may".don't mean they have any basis to state such a thing as fact).

Here's a plausible scenario: Super volcano blocks out the sun for a year.

In this case, for farmers, the crops will surely fail, or be delayed for a year, and surplus must be relied upon. The foragers have a lean year ahead, along with all the animals that they can survive by eating almost exclusively. Many edible bugs will be unaffected, so a change in foraging style will be needed for a time. If you think the Foragers are vulnerable in this sort of crisis, then you don't know much about Foraging. Several animal food sources will be unaffected. For agriculture, no sun for a year would be very bad for the population, as not enough surplus will be available for such a disaster. In fact, famine often coincides with food distribution issues, not as much crop failure. Floods and drought don't affect forests to the extent they affect a field of exposed dirt.

Don't ask for a proof of a negative, it just happens this way, due to the water retention and self-circulation of forests. I already gave you a source for this from Energy Bulletin. Remove the forest and you get drought. Drought cycles translate to famine cycles. The drought eventually becomes semi-permanent.

When a man talks about science as if it's a bad thing, I begin to suspect that he may not be giving me a fully objective appraisal of the situation.

Are you saying that science should be trusted implicitly? Beware of "new scientific understanding" coming your way. Science is merely the best understanding given the facts that we know about. This doesn't mean that science should be trusted as if fact. In the case of human health and nutrition, there appears to be confusion within the scientific and medical establishment. As energy reserves deplete - whether tomorrow or years from now, science will be looking for ways to reduce agriculture's energy foot print. Considering their best guess, thus far, is more in line with economics, I'm skeptical about science when it comes to our food sources.

In the meantime, you can go get your hands dirty and see for yourself what happens when you follow science compared to what happens when you follow Fukuoka and Permaculture paths. Don't believe me and don't believe anything you read, but believe what you see and try for yourself. The worst thing that can happen will be that you spend less time at the grocery store. The upside is that you avoid all those medical bills from such poor food choices, prepare an insurance strategy against disaster, and save some money. The labor costs are primarily upfront: land forming, sheet mulching, some tree planting to get things started, and time observing what works and what doesn't.

Fukuoka started out as a respected scientist. As one scientific theory proved to be bunk after another, he became increasingly disenchanted by modern agricultural science. In fact, he became alarmed enough to write books, travel the world, and be quite noisy regarding his view of science as a result of his work. Considering his success using "do nothing" farming with close to insignificant energy inputs matching harvests in his area using modern techniques (hybrids, fertilizers, deep plowing, pesticides) and huge energy inputs, I'll be trying his methodology for myself. I can't grow rice, as he does, but that isn't the point of his work. I'll be starting with groundnuts (Apios Americana) for my own area.