Future climate change that turns up the heat could also increase conflicts. Sudden changes in temperature don't directly cause conflict, but they do disrupt water and food supplies. Shortages of such critical resources can lead people to rise against their governments or invade neighboring countries, according to studies led by University of Hong Kong geographer David Zhang and published by the Proceedings of the National Academy of Sciences. The researchers found that there were nearly twice as many wars per year worldwide during cold centuries as there were during the milder eighteenth century. More than 80% of countries around the world experienced more wars in a cold climate, according to Zhang. The researchers reason that the link between climate shock and conflict is the supply of food: Decreases in agricultural production trigger increases in food prices, and when grain prices reach a certain level, wars erupt.