system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What will happen when we run out of petroleum?
gpt
There's no lack of ways to generate energy.  However most of them cost more than pumping petrochemicals out of the ground and burning them.

When supply of petrochemicals becomes constrained, there will be huge spikes in the price of those commodities.  Those price spikes will change the economic calculus, making other technologies cheaper to use than burning fossil fuels.  Industry and commerce will switch to those alternative sources.

The challenge is to make those changeovers without massive widespread global chaos and disruption.  It takes years to build power plants and dams and solar power farms and nuclear reactors.  As we approach the time when fossil fuels will become uneconomical the places that prepared in advance and built infrastructure to transition to alternative energy sources will have a smoother path than those who waited until the crisis to start such construction.

Think about the fact that 99.99% of the vehicles on the roads in North America and Europe all run on oil-derived fuel.  Every one of those vehicles will probably run on electricity, hydrogen, or something else in 100 to 200 years.  All the gas stations have to be replaced with charging stations or hydrogen cells.  The infrastructure upgrades carry astronomical costs.  The disruption implied by making that change is enormous.

In any time of sudden, disruptive change, you get social unrest (countries fall apart, political parties rise and fall), war (some people will want the last of the oil and will do anything to get it), and massive widespread economic disruption (what will the people who own Exxon and BP Stock think those companies are worth when there's no more oil to pull out of the ground?)

The next 100 years are going to see tremendous realignments of national boundaries, national priorities, and the rise and fall of fortunes and multinational companies, all driven by the end of cheap petrochemicals.