An agrarian economy refers to a system where the primary source of income and labor comes from agriculture. It means that the majority of the population relies on farming and other related activities for their livelihood. Full definition
In agrarian economies, children are an economic asset, though when times are hard, having children can always make it harder. (sciencedaily.com)
As for that cover story in Scientific American, my wording that «we have had agrarian economies for millenia with at most only a modest increase in CO2 concentrations in the atmosphere» was chosen carefully to not contradict the hypothesis presented in that article (which is, at any rate, still quite speculative). (realclimate.org)
So, while your political biases may want it to be scientifically correct that the «backward, stunted anti-capitalist agrarian economies» are to blame and that capitalism is the savior, the science disagrees with you. (realclimate.org)