Why is the green revolution unlikely to be the final answer in terms of feeding the billions of people that are, and will be, living on earth?
The Green Revolution, the failings of which are now well known, is the advent of research into technological means by which to increase crop yields through innovations like higher yield bearing crops and through development of inorganic fertilizers and advanced pesticides. The Green Revolution occurred in the latter half of the 20th century, particularly beginning with the 1967 report of the U.S. President's Science Advisory Committee that prompted the Rockefeller and Ford foundations to develop international agricultural research consortiums that emphasized technological advancements in developing countries.
Despite their initial focus and despite the greatly increased yields in rice and wheat for areas where rainfall and irrigation were not limited and were practicable, the technological breakthroughs were not suited to many developing countries because of water insufficiency, restrictive government policy, inadequate or nonexistent infrastructure (roads, bridges, services etc), non-competitiveness of small poor farms with large financially capable farms, and because of environmental degradation from fertilizers, pesticides, irrigation and overuse of ecological capital in water tables.
These same problems are some of the reasons why the Green Revolution has not been and will not be the "final answer," these, in addition to population increases that surpass yield increases along with the deepening divide caused by income inequality and the increase in the extent of absolute poverty (though the number of those in absolute poverty has decreased).
Policy is difficult to implement within countries partly because of lack of governmental budgets. Land degradation extinguishes the arability of the best farmland and causes outcomes like salt build-up, estuary and waterway pollution, death of workers and parallel degradation of biodiversity among species. Inequality of access leads to inequality of income and markets, which leads to increased food inequality and starvation. The second generation of Green Revolution research and innovations turned to focusing on adapting research to produce solutions that are equitably applicable among all types of farmers, amidst all kinds of water availability, through open access and markets and that encourage policy adaptation and infrastructure innovation.
Development practitioners now have a better understanding of the conditions under which the Green Revolution and similar yield-enhancing technologies are [now more] likely to have equitable benefits among farmers. (International Food Policy Institute)