Without yet reading it, my Spidey sense is already tingling at equating "dramatic improvements have been seen since 1961" with sustainability. Any practice of comparing itself with itself is "relative sustainability" at best (we can be confident that we won't see a life-cycle analysis of beef vs. broccoli).
Big, not broken: American farmers as environmental stewards
This blog post, by Caroline Grunewald and Dan Blaustein-Rejto of the Breakthrough Institute (a US think tank), argues that the large scale of much American farming does not mean it is necessarily unsustainable - rather, when looking at the intensity of greenhouse gas emissions, land use and water use per unit of output, dramatic improvements have been seen since 1961.
North America is the northern subcontinent of the Americas covering about 16.5% of the Earth's land area. This large continent has a range of climates spanning Greenland’s permanent ice sheet and the dry deserts of Arizona. Both Canada and the USA are major food producers and some of the largest food exporters in the world. Industrial farms are the norm in North America, with high yields relative to other regions and only 2% of the population involved in agriculture.