1. Dam The Mediterranean
> German architect Herman Sörgel devised a massive plan, also known as Atlantropa, that would establish a gigantic hydroelectric dam built across the Strait of Gibraltar. This would generate huge amounts of emission-free electricity while lowering the surface of the Mediterranean Sea by 200 meters. This would open up parts of the Adriatic Sea for settlement and farming lands. The plan was widely discussed in the 1920s and 30s, but was discarded for obvious practical reasons.
However in 1997, Robert Johnson, a retired professor at the University of Minnesota noted that the Mediterranean Sea is being starved of fresh water, due to many rivers such as the Nile being diverted for industry purposes. He pointed out that as a result the Mediterranean evaporates and grows saltier (aided by global warming). As more seawater pours in from the Atlantic through the Strait of Gibraltar, the Mediterranean salty water is pushed out into the deep waters of the Atlantic where it causes a massive ecological problem.
As the Mediterranean water mixes with very cold water on the Atlantic floor, it is pushed by currents to the north. Normally Europe's warm current isn't affected, but if the Mediterranean outflow is altered by a dam the warm water would instead be pushed west into the seas of Canada where it would create increased snowfall expand the country's ice fields.
So there you go. Want to stop the Northern hemisphere freezing AND have more land for an ever-growing population? Dam the Straits of Gibraltar!