So, it seems we have a number of smart people here. It also seems fairly easy to accept that humans have had some affect on the global climate (what exactly that effect will be is difficult to say, but a change in climate patterns is almost guarenteed, and any variation to a complex self-balancing, oscillating system will result in some change)

So, let's assume we will not have enough arrable / irrigated land to produce enough food for 6.7 billion people. It makes sense that people will fight over limited resources. (limited water, as flooding contaminates drinking supplies with fertilisers and other chemicals usually stored safely out of the ground water system, drought also limits the water supply, so it doesn't matter whether you have more or less rainfall in any given area the most likely effect is a reduced supply of drinking water)

In a world of globalised trade, and an internaitonal monetrary system the rich countries will always do better, they can buy whatever they want on the intermarket and even if within those countries the price of food triples, even if the lowest earning 20% of people are hungry and the lowest 2% are starving, the middle classes will have no trouble giving up all the luxuries, i-pods, batteries, expensive new things, they will not starve. The poor countries wil not be so lucky. And as they are seperated by large geographic areas they will not be able to attack directly the rich countries.

Trade will be hit by piracy - as you can see in the gulf of Aden, when you a state fails there is little interest locally in stopping piracy. (Where is our current food supply grown, how is it transported?)

What should we do? In the current day the standard responce of most governments to any destabilising threat is to fight for the status quo (except perhaps the US who is perfectly happy to alter the status of other countries if it is enough to keep their own nation stable) The major activities of the UN are mostly aimed at encouraging stability through food assistance and health programmes ~ the big headline news is always about emergency aid, but they World Food Programme does more than just that (see: http://en.wikipedia.org/wiki/World_Food_Programme)

The UN very specifically is NOT able to prevent militrary conflict, even if that is in it's mandate. Only when China, Russia, France, Britian and America all agree, AND there is enough support in terms of troops and money can the UN do anything about a conflict. At present the troops and money are not sufficient to deal with several minor conflict in africa (especially when they burst into larger events)

The only reason I can see this ever changing is that the powers that be are fighting for their own continued food security. (if they manage to work within the UN system of conflict resolution) At that point you have Rich countries paying for troops to ensure food leaves poor countries where people may be starving by proping up the local governments - in effect those local governments are composed of those rich enough to not personally be starving.

(Please see the 'great famine' in Ireland in the 1840s, as an example when a english government limited relief efforts because "the judgement of God sent the calamity to teach the Irish a lesson") - a demonstration that people don't care that much about those who are 'different' (whatever that means, but i'm sure it includes any simple tribal/national groupings...)

How can we best deal with this global change? The harshest of individuals will encourage a malthusian cathastrophe - which we have managed to avert through the use of tehcnology for many decades now - you might advise a purely capitalistic approach - which includes chariy (as we currently allow via the World food Programme)

I would argue that this will result in a complete breakdown of our current system. (and be used as a proof of the failure of capitalism just as the fall of communism was used as proof of capitalism's superiority.) I know it's really easy to predict the collapse of civilisation - and normaly we humans are more adept than given credit, managing to avoid such a collapse. I could list civilisations which have collapsed - but it is more useful to look at how civilisations have managed to overcome things which people claimed would cause their collapse. Or the biggest challenges which have been faced - like the fall of communism, or the great depression (lessons from which have been used to avert it's repeat in recent years)

So smart people, please contribute your own analysis/suggestions.