Yesterday, I highlighted the tail risk of climate change; that is, low probability but high impact outcomes that could devastate the planet.
Nonetheless, while climate change is already showing up as biodiversity stress, it is not yet appearing as an aggregated agricultural impact across the globe. True, we can produce regional examples of likely climate change-induced distress. For example, climate change probably (but not certainly) helped tip the odds toward the strong western United States drought that we have been witnessing. This, in turn, has affected certain types of crop. But climate change has yet to have a material impact on global food production in its entirety.
Below is the composite Food Price Index from the Food and Agriculture Organisation (FAO). Data source here (click for larger image).
Real inflation-adjusted food prices are currently not far off their 1960s/70s levels. True, they are up about 40% from 20 years of so ago, but the world has got a lot richer since then. Indeed, Chinese real GDP per capita has more than doubled over the last 10 years and India’s same statistic is up about 60%.
Moreover, despite new global temperature records being made over the last few years, cereal production continues to trend upward (click for larger image):
Finally, with production up and the relative price of food to income down, malnourishment has also been on the decline (click for larger image):
The 1996 World Food Summit target of halving the number of undernourished people by 2015 has not been met, but the trend has definitely been down. And given that the global population has been growing, the percentage of undernourished has nearly halved (Source: State of Food Insecurity in the World 2014).
So the fingerprints of climate change on food security are still barely visible. But given current CO2 emission projections, the day will come when this all changes.
Risk and Well-Being 
At about 25%, cereal stocks are such a small percentage of annual production that continuous year around production has to be maintained. This has always seemed amazingly precarious to me. One would think that a whole year of reserves would be normal.
But perhaps these stock levels only include commercial stores, not including those reserves held for their own use by small farmers around the world. Even so, one really bad harvest in a major grain producing region and supplies could get very tight very fast.
I guess the diversification here is both regional and between different crops. Bad harvest years are basically ones where we see a marginal decline in production, rather than a slump. Therefore, you can get away with a relatively small stock. Probably, we will know when climate change starts to bite when we see large spikes in prices rather than large outright declines in production.
Demand is likely to be relatively inelastic, so when there is a shortfall in supply, even by only a little, we will need a large price spike to get people to reduce consumption or substitute into other foods.