If there was one global issue i could fix, it would be stop pollution.
If we stopped pollution it would be an amazing miracle to the world. In several places pollution is caused by air pollution from cars and factories. There is also water pollution by mixing waste into water. If we stopped pollution people would be healthier because do to the stop of air pollution, people would breath fresher air which can actually make you healthier with the excessive amount of oxygen. If we fixed water pollution we would have more water to drink even in other countries like Africa where its needed.
This would also lead to more trees being grown because less tractors and other vehicles in the forest to kill the tress with over carbon dioxide. with this more plants and animals would be born and grow faster. This why i think if we stopped pollution would be the greatest issue to fix.