//if global average surface temperatures reach 4 degrees of warming above pre-industrial levels then huge parts of the earth will be unfit for human habitation (though some areas will likely become habitable) and agriculture on the scale that we understand it will be impossible...//
If this is true (and I've some doubts that it is, but won't argue) then people will have to move and agriculture as we understand it will have to change to agriculture as we don't understand it. Because one thing is for sure - mankind has no more chance of controlling global average surface temperatures than it has of preventing the tides ebb and flow. Attempts have been ongoing in earnest for around forty years and have spectacularly failed, despite vast sums being expended. Average global temperatures are now about 0.8 degrees C above the 20th Century average. As a result of these failed efforts, European economies have suffered from energy insecurity and vast sums of what is essentially worthless money have been printed and used to pay citizens' energy bills. This will only worsen as it is not possible or sustainable to produce enough energy from so-called "renewables" (which include about 14m tons annually of mature timber, freshly felled on the other side of the Atlantic, processed and shipped to be burnt in Yorkshire).
This is a ludicrous state of affairs which is costing huge sums for no substantial benefit. Governments need to wise up and accept that the climate cannot be controlled. Producing "league tables" such as the one cited above, is simply window dressing. To believe otherwise is delusional - with far more damaging consequences than if they were to accept the truth.