My 5 major improvements:

1. Support renewable energy. If we could move our energy consumption to all (or mostly) renewable, it would result in a huge difference. This includes supplying developing areas access to renewable energy.
2. Enhance public transportation to make it inexpensive, convenient, and available almost anywhere. This might include driverless conveyances.
3. Universal healthcare. Healthy people cost less, and work more.
4. Require planting a tree for every tree you cut down.
5. Reduce consumption of meat animals, which would free up much more food production for people, and reduce methane production at the same time.
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