This article from the BBC describes some of the conclusions of a report by the US intelligence community. It seems to me that the US intelligence community has been a little slow in catching up.

My point is that there are already tensions between nations due to climate change and the over-use of resources, which are only going to get worse:

• There are tensions between the USA and Mexico because the US is taking too much water from the Colorado river, because there are water shortages in the Western USA, so that most of the time there is no water flowing across the border, as reported here by The Guardian;
• There are similar tensions between Israel, the Palestinians, and Jordan because Israel is abstracting so much water from the Jordan river, as reported here by The Conversation;
• Ethiopia is having a huge row with Egypt and Sudan about the huge dam that Ethiopia has built, and is currently filling, on the Blue Nile, as reported here by Nature;
• An increasing proportion of domestic water supplies are contaminated in the USA, the UK, Canada, parts of Africa and Asia, for a number of different reasons (probably the best known example was the scandal in Flint, Michigan, described here on Circle of Blue) - an overview of groundwater pollution causes and locations can be found here on Wikipedia;
• Deforestation is one thing that is making climate change worse, and Brazil is under increasing international pressure to act against illegal logging in the Amazon forest, as described here by Reuters (it is not inconceivable that military intervention or foreign support for a coup could be used to stop Brazil from cutting down more of the Amazon);
• Some nations continue to build coal-fired power stations, and the export coal and oil, despite the climate change that it causes (India and China are the worst offenders - see this BBC report on India's plans to continue to rely of coal for electricity generation), and international pressure is mounting; China, in particular, is notoriously unresponsive to diplomatic pressure, so a likely outcome is the imposition of carbon tax tariffs on the import of goods manufactured in countries using large amounts of coal for electricity generation, which could lead to another tariff war;
• Overfishing of commercial fish stocks, due to illegal or unregulated fishing, is happening all around the world, especially off the coasts of Africa and South America, with the main culprits being China and the European Union (specifically Greece, Italy and Portugal), according to this report on Seafood Source, and pressure is mounting, and will get stronger, on the countries engaged in illegal fishing;
• There is a slow but inexorable move away from fossil fuels, in an attempt to limit global warming, which runs the risk of bankrupting most of the major Middle-Eastern economies, in a region where there is already major international tension and many nations are heavily militarised, so the risk of conflict is high;
• Some nations are likely to sink completely beneath the waves, as global temperatures rise, leading to waves of climate refugees, and other nations like the USA, Britain and Bangladesh can expect to have large displaced populations from their lower lying regions, also leading to climate refugees (probably not only internally in the effected countries); refugees always lead to international tensions.

This is only a sample of the existing international tensions, caused by or exacerbated by global warming or resource over-exploitation. Also remember that global tensions are a less grim scenario than the societal collapse being predicted by some experts (see here).