Climate change is leading to ever-increasing drought, crop failure, death of livestock, flooding and sea-level rise which can all cause migrations of people. Migrants take with them their language, ethnicity, culture and religion any of which may differ from those of the populations into which they move. The new arrivals compete with the existing inhabitants for jobs, housing and other resources, fuelling resentment that can lead to violence which, if it builds on top of previous community discontent, can lead to armed conflict.
It is probably no coincidence that, just as the climate is getting warmer, so civil wars – that is wars between groups within a single country – are becoming more common.
Civil wars are probably more frequent today than during any previous period of history. Historically it has been rare for people to organise and fight their governments, but in the past 70 years a new conflict has started somewhere in the world on average every 2 months, most of them civil wars . Syria is an example of how this can happen.
In her new book “How Civil Wars Start and How to Stop Them” , acclaimed expert Barbara F. Walter explains that:
Syria experienced a devastating drought that, combined with the government’s discriminatory agricultural and water-use policies, resulted in significant crop failures. In search of opportunity, roughly 1.5 million people—mostly Sunni—migrated from the countryside to Syria’s cities. In the capital city of Damascus, the center of Christian Alawite power, these Sunnis were viewed as enemies of President Bashar al-Assad, and before long felt discriminated against because of their religion, creating resentment. When the government began to award water well-drilling rights on a sectarian basis, that anger intensified, accelerating the march to war.
A World Bank report  published in 2018 predicts that over 140 million people could be internally displaced because of climate change by 2050. It is very likely that this will lead to even greater conflict.
And we should note that climate change is not new and nor is the conflict that accompanies it.
When the last glacial period ended about 11,500 years ago and most of the ice on land melted, an interglacial warm period began and still continues today.
As the climate warmed, the horse-riding warrior nomads were forced off the Eurasian Steppe  south towards the areas inhabited by farmers in the near east, India and China. After much pillaging and killing, these nomads eventually became the kings of these regions.
In October 2021, Mike Duncan wrote in Time magazine  that:
Long term changes in the earth’s climatic conditions lead flourishing societies like the Roman Empire to wither and fade. But perhaps there is no greater example of the explosive intersection of climate disruption and political upheaval than the period surrounding the French Revolution of 1789.
The warmest period of the interglacial was about 5000 years ago and temperatures have been slowly declining since then. In the middle ages there were a series of mini-ice ages which led to lower crop yields and higher prices, causing economic chaos and famine. This was the spark that led to the French Revolution.
But we should note that climate change alone does not cause war. Only if it comes on top of other injustices will a society decline into violence. A well-run state would find more peaceful ways of coping with it. Let us hope that our current leaders are intelligent enough and visionary enough to help their populations adapt to the huge changes necessary for states and civilizations to survive.
It might also be interesting to note that, had the industrial revolution not happened, we would now probably be heading into the next glaciation. The climate is an inherently unstable thing and, if we manage to save ourselves from war and chaos, we will need to learn how to control it instead of letting climate control us.
 Wars since UN was founded article by Philip Brown: https://unacov.uk/wars-since-un-was-founded/
 How Civil Wars Start and How to Stop Them by Barbara F. Walter: https://www.amazon.co.uk/How-Civil-Wars-Start-Stop-ebook/dp/B095XSQL5K/
 Eurasian Steppe article in Wikipedia: https://en.wikipedia.org/wiki/Eurasian_Steppe
 Climate Chaos Helped Spark the French Revolution—and Holds a Dire Warning for Today: https://time.com/6107671/french-revolution-history-climate/