Overview of threat
Modern civilisation is far more fragile than people generally think. Supply chains are globalised and events in a far away part of the world can have unanticipated effects in other places. We depend upon a complicated infrastructure powered by dwindling supplies of fossil fuels and other key materials to bring food to the shelves of our supermarkets. Not only do many people not know how to produce their own food but many of us live in areas where we simply would not have this choice.
Despite all our knowledge and technology, we still need to eat. This is a fundamental need to our continued existence and without food modern civilisation would soon be plunged into famine and conflict. Our species is remarkably poor at organising this key resource - a billion people already go hungry every day despite us producing enough food for everyone to eat. Furthermore the affluent are able to divert food crops to fuel for their vehicles and to consume meat that has been fed using crops that could have far more efficiently fed people.
Added to all this our population has swollen on the back of fossil fuel powered industrial agriculture to levels that are simply not sustainable - whether you look at the excess and unreasonable consumption by western societies or the localised land degradation and extinction of species competing with humans in Africa. While the key driver of our collapse is the failure of agriculture at this time there are billions of people ready to fight for their existence if food or other resources become too constrained. It seems that previously stable regimes are already being toppled where high food prices and unemployment put the population under enough stress to demand change as in the "Arab spring".
This is all background information to the immediate problem and the near future loss of our civilisation, which is to do with imminent agricultural failure. The plants that we rely upon for our food have environmental parameters which they are unable to produce a harvest if these are exceeded. Extremes of heat, drought, flood or cold are all capable of destroying a crop and there are specific changes occurring in the earth system which are causing these extreme conditions to occur with increasing frequency.
It should be noted that collapse in itself can become contagious and that once a given society enters a collapsed state the breakdown in the social order will make it much harder or impossible to maintain modern supply chains or agriculture in that region. It is much harder to practice agriculture in a war zone even if the weather is clement. Countries that rely upon food imports are particularly vulnerable as many exporting countries will cut back on exports to try to maintain their own populations.
There are several other notes that should be made. We are committed to decades of further warming by the greenhouse gases already released into the atmosphere and so even to totally stop this pollution now would still leave us with decades of worsening of the current situation. Furthermore about half of the warming effect of greenhouse gases is cancelled out by short lived pollution (particularly sulphate aerosols) and to rapidly shut down industry would in fact rapidly lead to another 1C of average warming. It should be noted that we are within a few years of catastrophe (no minimum bound applied to this statement) and therefore we are entirely comitted to failure in the absence of large scale geoengineering and historically unprecedented rates of policy change.