Many of our most severe ecological threats converge on the stability of the global food supply. The most explosive consequences of food shortages are not population decline from starvation, but civil unrest and conflict (as well as increasing vulnerability to disease/pandemic). During the 2008 food price spikes, there were riots in sixteen countries. And the most visible political consequence of the 2010 food price spike was the Arab Spring (though again there were protests and riots in many other countries). Yes, of course there are other underlying factors in every country affected, but the spike in the price of bread was the initial spark in nearly every country that saw significant instances of civil unrest in the Middle East and North Africa (MENA). The protests that ultimately brought down governments in Tunisia, Libya, Egypt (and possibly Syria) all had the price of bread as their trigger (as did those in Bahrain and elsewhere). And why were prices so high in 2010? Again, all kinds of systematic reasons (biofuels taking an increasing share, changing diets, speculation, government hoarding in response to an initial rise), but the short term trigger was almost simultaneous crop losses from extreme weather events in Argentina, Australia, Pakistan and Russia (which famously stopped exporting wheat after its six sigma heatwave). Food price spikes are inconvenient in the west, where we spend less than 15% of our income on food, but disastrous in the many places with otherwise somewhat stable governments where large parts of the population spend more like 75%+ of their income on food.
The consequences of political unrest are not confined to the poor country. To pick one consequence: our taxes here in the UK recently went towards funding war in Libya, despite an austerity programme of slashing government services in response to the worst recession since the Great Depression. Refugee flows from all the various places involved have also increased. Major protests in the US and elsewhere this time last year questioned the direction of the present economic order. These explicitly drew both inspiration and organisational links from elements of the Arab Spring before being brutally suppressed - perhaps not as brutally as in Syria, but if you had your eyes open last autumn there was plenty of state-sponsored violence happening against protesters in free(r) countries, much of which was never acknowledged or addressed by the justice system.
This is not at all to claim that climate change "caused" the Occupy movement in any straightforward way, simply to chase one strand of causal links as an illustration of the global implications of crises in a single region.
Failed states have all kinds of knock-on effects on their neighbours and the rest of the world. Think about the extra costs to global shipping due to Somalian piracy (leading to many shipping companies eagerly awaiting the further opening up of Arctic shipping lanes to avoid the area entirely), about the seedbed of terrorism that Afghanistan has represented since the US turbo-charged the factions against Soviet invasion, about the effect on global oil prices (and hence the global economy) of war in Libya (or Iran...), about the ongoing repercussions of the Arab/Israeli conflict partially driven by the planned failure/sabotage of the Palestinian state. And so on. The global system can handle a few failed states, but since it does so by distributing the costs across the whole system (UK taxpayers paying for wars in Libya), it does so by increasing the stress on the system as a whole. Electricity grids are a good analogy here, actually - grids can handle the sudden failure of a certain number of elements in the grid, but do so at the cost of placing the entire grid at greater risk of collapse. Globalisation is a super-grid for economic and political stability: failure in one part can be accommodated by increasing stress across the board. But only to a point.
This is why Joseph Tainter says in the final chapter of his intriguing and seminal book, The Collapse of Complex Societies that there can be no local collapses in a global system. The term "catabolic collapse" is sometimes used, which refers to a collapse in one part of a system becomes self-reinforcing and ends up taking down the whole show (see here for a much more detailed and insightful discussion of this concept by John Michael Greer).
So when you read about the coming food price spike of late 2012 as the effects of the US drought kick in, don't just think about poor Indians struggling to put food on the table, but also think about the $700b-odd the US spends on its military (over $1t on "national security" as a whole), about the possible break-up of the EU (troubles in Greece are complex, but one of the causes/manifestations/worsening of their crisis is the fact that they receive per capita more refugees and undocumented immigrants fleeing struggling MENA countries than almost anywhere else in the EU and it has seen a big jump in recent years), about deforestation in Indonesia and elsewhere (which is linked, in complex ways, to food prices), and so on.
Global crises require global (as well as local, provincial, national, regional) responses. We can't simply pull up the drawbridge and hope to weather the storm.