There's a depression in the middle of Australia that's lower than sea level. If we just built the right canal, we could change the climate and ecology of what is currently Central Australia by a wild degree.
With catastrophic effects on climate elsewhere, of course. Like how Greening the Sahara would kill the Amazon because the amazon is fertilised by saharan dust.
The Sahara winds whip up fine desert dust into the upper atmosphere, it floats on the jet stream to the Amazon and fertilizes it with nutrients the rainforest would otherwise deplete.
I posit turning Lake Eyre into a sea would have similar effects.