I think we should run with single cast, permanent effect. I think "Scorch Terrain" would be a good name. (Hammering together a few of those suggestions, lol.) Spending more mana to affect a larger area is a good idea. The costs should be exponential though. In theory you could do the same thing with the terrain changing spells. This one can be used as a weapon that can be hard to counter. The larger the area of the affect the more devastating it can be. Play testing
And, yes, it will need counter spells and it can be defended against. Any spell that affects terrain within a city's border will be subject to that city's spell resistance.