Since the 1990s, not all is well in the world of food and here is one big reason why: productivity gains are not coming at the rate they used to.
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The title of the section, "Productivity Decline", seems misleading as, according to the section's video, in fact the agricultural productivity is still growing, although much slower than a few decades ago. A title like "Slowdown in Productivity Growth" would sound more accurate to me. The same applies to the last option of the secong practice question.
Even a basic understanding of chemistry and physics would suggest that there must logically be a maximum productivity point. Where there is full (100%) utilisation of the energy and resources provided to plants. ie Sunlight and nutrients in the soil below. Perhaps we are just reaching that maximum productivity point. That would also explain the SLOWDOWN in productivity GROWTH (ie your "Productivity Decline") as we approach that maximum you would expect a slowing towards the maximum.
What is yield? In this video it sounds a lot like GDP - a measure of the total crop produced. So far I'd been assuming, especially in the GMO video, that it was a measure of efficiency, like say the amount of useful crop produced per acre farmed. Now I'm unsure myself; which is it please?
Your first instinct was right. Average yield is, basically, crop efficiency measured in amount of crop (say in kg) per area farmed. Total yield is like GDP, kind of. So, if efficiency gains (average yield) are decreasing then total yield (GDP) gains will also decrease ... UNLESS the amount of area devoted to farming increases enough to offset the difference (which obviously can't happen for very long).
To use an analogy from evolutionary biology, I would think that agricultural productivity is more often like Stephen Jay Gould's theory of punctuated equilibrium. In other words, rapid changes are followed by relatively flat periods until there is again a rapid change. In agriculture, there have been recent revolutions in technology (e.g., seed drills, reapers), chemistry (e.g., the Haber-Bosch process for the production of nitrogen fertilizer), and biology (e.g., salt-tolerant or dwarf plant strains). Each of these revolutions was followed by a period of rapid gains in productivity (all else -- wars, droughts, etc. -- being equal) and then relatively lower growth until the next revolution came along. Couldn't it be argued that we are in one of those flatter periods? After all, the stats compared a 40-yr period to a 18-yr period. (Perhaps if genetics/biotech fulfills its promise, the productivity numbers for 1990-2029 may end up being similar to the earlier 40-yr period after all.)
what about changes in precipitation and temperature over the years contributing to slower pace of productivity increase?