Pass a nationwide and suggest a worldwide law that whenever time travel is invented, an ambassador of that specific time period has to come to our present to share litte, but essential knowledge. Here are two problems for which this ambassador has to come up with a solution:
1. CO² and other gases by cars, planes, cows and other essential necessities that we today cannot live without. So: Bring us clean, fuel-independent engines and some use for the gas: use that wouldn't in succession pollute environment all over again.
2. Bring us useful knowledge we can use to learn and teach, so that ridiculous and embarassingly dumb people won't exist anymore. So instead we could combine intelligence and that knowledge for a better future and a flourishing economy - all over the world.