People were brought together by the need to produce food cooperatively, as the use of food surpluses in one place to relieve shortages in another ended due to a change in climate.
Question 1: Does as mean in the similar way of "Do exactly as I say."?
Question 2: Is the second subject of the above sentence "the use of food surpluses in one place to relieve shortages in another"? (In this way ended is the main verb of the second sentence.)
Question 3: Has one been omitted after in another?
Some more related information:
Sharing and cooperation were almost certainly part of early Puebloan life. Scholars believe that this cooperation allowed people to contend with a patchy environment in which precipitation and other resources varied across the landscape: if you produce a lot of food one year, you might trade it for pottery made by a distant ally who is having difficulty with crops — and the next year, the flow of goods might go in the opposite direction. But all of this appears to have changed thirteenth century. Although the climate remained as unpredictable as ever between one year and the next, it became much less locally diverse. In a bad year for farming, everyone was equally affected. No longer was it helpful to share widely. Instead, the most sensible thing would be for neighbors to combine efforts to produce as much food as possible, and thus aggregated towns were a sensible arrangement.