World agricultural productivity could decline between 3-16% by 2080. Developing countries will experience the biggest drop.
"World agriculture faces a serious decline within this century due to global warming unless emissions of carbon dioxide and other greenhouse gases are substantially reduced from their rising path, and developing countries will suffer much steeper declines than high-income countries, according to a new study by a senior fellow at the Center for Global Development and the Peterson Institute."
"Developing countries, many of which have average temperatures that are already near or above crop tolerance levels, are predicted to suffer an average 10 to 25 percent decline in agricultural productivity by the 2080s, assuming a so-called "business as usual" scenario in which greenhouse gas emissions continue to increase, according to the study. Rich countries, which typically have lower average temperatures, will experience a much milder or even positive average effect, ranging from an 8 percent increase in productivity to a 6 percent decline."
Thanks to Jon Cecil