I quote from An Abrupt Climate Change Scenario and Its Implications for United States Security, by Peter Schwartz and Doug Randall:
With inadequate preparation [there] could be a significant drop in the human carrying capacity of the Earth's environment. … an abrupt climate change scenario could potentially de-stabilize the geo-political environment, leading to skirmishes, battles, and even war due to resource constraints such as:
1) Food shortages due to decreases in net global agricultural production
2) Decreased availability and quality of fresh water in key regions due to shifted precipitation patterns, causing more frequent floods and droughts
3) Disrupted access to energy supplies due to extensive sea ice and storminess
