Thomas Malthus predicted a time when there would be too many people to feed on the planet. In the 70s the Club of Rome with people like Paul Elrich said similar but were mocked and shown to be wrong. Fossil fuels, chemical fertilisers etc saw an agricultural boom, but dead zones in our oceans, toxic algal blooms and depleted soils should point to this being a mixed blessing. Likewise, nitrogen ends up as NO2, a greenhouse gas. Humans fix more nitrogen for agriculture than all other sources.
And now as if all these impacts of intensive agriculture were not enough, a new study reported in Scientific American shows that global warming is producing a decline in cereal yields. It seems while increased CO2 increases yields, increasing temperatures adversely affect yields, particularly wheat. Apparently, this is already evident in increased prices as yields decline.