A lot depends on how gradual the onset is, esp in highly agrarian countries - like India. A gradual reduction of crop yields will lead to smaller numbers of peo

A lot depends on how gradual the onset is, esp in highly agrarian countries - like India. A gradual reduction of crop yields will lead to smaller numbers of people supportable on a given acreage = slow exodus, smaller families, like the decline of the Anasazi in the US Southwest. Perhaps DNA analysis of residual bones along with the Amerindian gene pool will reveal this? There is a water current called the Gulf Stream - it is part of what is called the Atlantic Conveyor - the Thermohaline circulation driven by thermal AND concentration gradients. https://en.wikipedia.org/wiki/Thermohaline_circulation https://en.wikipedia.org/wiki/Thermohaline_circulation This transfers huge amounts of heat from the Gulf of Mexico to Northern Europe. This makes Northern Europe quite arable, compared to Northern Saskatchewan in Canada which is about the same latitude - with a far colder climate and shorter growing season. This warm water wends it's way North, cools and eventually falls to the deeps as a cold counter current - see the Wiki. Some climate scientists fear this current pattern could be getting unstable. If this is disrupted, Europe would see a new ice age, crop yields would drop, many plants would die as the ground freezing zone moved 1000 miles South. This would leave Europe unable to feed itself, save by imports of food. There may be other effects?