blind said:
Come now "destroy agriculture..." NO. We simply change or move it. Its not like it happens over night.
If it happens in only a couple of hundred years, it might as well be overnight.
Suppose the warmup is rapid, as feared and predicted:
The rice fields in the river deltas of Asia, that currently feed a high proportion of the large populations there, will flood with salt water and grow no more rice. New deltas will take thousands of years to form, upstream on smaller areas.
The temperate zone corn belt will dry out in the south, move north - but there is no rich mollosoil under the pine trees and peat bogs of the north, and it will take hundreds, maybe thousands, of years to form any. The sunlight regime is not the same, either.
The glaciers currently irrigating much farmland will vanish. Rivers will shrink and water tables recede, while the rain that does fall will fall in harder storms, increasing erosion and runoff and decreasing the percentage recharging the aquifers. Meanwhile the evapotranspiration deficit experienced by most crops will increase, sharply increasing the demand for irrigation and soil water even as the supply shrinks.
The growing seasons and best farming practices in msot places will change, and it will take time to adapt to new crops, new weather patterns. Adding to the difficulty, the changes will be unstable in the transition, leading to wild swings in the weather and crop failures scattered. Both deserts and bogs will expand, as the absent rains of some places fall all together on others.
The higher demand for irrigation with faster evaporation will cause more salting of irrigated fields. Diseases and pests currently restricted by cold weather will spread. Diseases and pests currently adapted to cold weather will find easier prey in the struggling plants of the northern farms. Trees and perennials will suffer in place.
And so forth.