Agriculture was a development that fundamentally altered the course of human history. By enabling large groups of people to settle in concentrated groups it is very much the foundation stone of civilisation. It should be noted that in the event of the widespread loss of agriculture we have largely destroyed the natural ecosystems that supported our distant human ancestors in a hunter gatherer lifestyle by cutting down the forests and displacing native wildlife to use the land for growing crops.
For ten thousand years we have enjoyed a reasonably stable climate within which we have adapted our plants and agricultural behaviour to suit each region of the planet that we cultivate. With reasonable certainty until now, we would know what crops would work well in which regions and when to plant them. Today there are 7 billion people who largely depend upon agriculture for their existence and we fail to adequately feed a billion of these despite theoretically producing enough. In many places around the world we are using farmland as a consumable resource and allowing it to become degraded. We rely upon finite fossil fuels to provide the chemicals and fuels that enable modern agriculture and are unsustainably stripping the ocean of fish.
Therefore it is reasonable to say that our reliance on agriculture to sustain civilisation is in trouble even without throwing abrupt climate change events into the mixture. We are in no way behaving in a manner that suggests we care about our descendants - including our children and grandchildren.
The specific immediate problem to agriculture is very simple - extreme weather events.
As with all living things, plants (and to some extent animals) have a range of conditions to which they are well adapted. Within this range of conditions they will perform optimally, providing a good harvest. Temperature and moisture are two examples of key conditions that must be adequate for plants to perform well but it should be noted there are other factors that can be fundamentally affected by the weather too. For example some plants can be devastated by outbreaks of pests that are favoured by changing conditions or have pollination adversely affected by a shortage of insect pollinators. Life is a complex thing and as well as the effects that are obvious and easy to predict there can be many unexpected surprises too.
If the range of conditions is exceeded by too much, plants will die or a harvest will be entirely destroyed. If you are a gardener you know that some plants can not tolerate frost and an unexpected late frost can cause major damage. Plants absolutely depend upon water but most plants of agricultural interest are not able to withstanding being flooded. Temperatures that are too low may inhibit growth while those that are too high may cause too much thermal stress or destroy the viability of pollen. Some plants are unable to effectly form their tubers above a certain temperature.
It should be fairly obvious that only one sufficiently exteme event within a growing season is required to destroy or largely compromise a harvest.
It should also be rather obvious that as agricultural output starts to rapidly diminish as the ice melts and the jet stream alters that within a few years at most civilisation will be presented with an existential crisis.
The outlook is grim. Analagous conditions to extreme agricultural failure were imposed on people during the Holomodor. People attempted to eat all sorts of things to survive - earthworms, old boots, each other and even in some cases their own children.
This is the legacy that my parents and grandparents have bestowed upon me, which my generation should (and is largely failing to) face up to.
This is the key reason civilisation is on the brink of total collapse.