Firstly, 5 degrees C is a much larger degree of warming than anyone is predicting for any of their worst case scenarios in the next 200 years. So 200 years from now, the problem will likely sound laughable.
But to answer your question anyway, yes I think that we could solve that problem today with current technology.
There are a couple reasons why. For 1, increasing temperatures also opens up unused land in cold places, that werent bring used before. Potentially, it might produce MORE arable land than it removes, once we get all the infrastructure up, of course.
And 2, it's not about "technology". There are tons of land that we "could" be using for food right now. We aren't using that land, because it is mountainous or has a bad climate, and would therefore produce lower yields.
But just because a mountainous area or a bad climate produces low yeilds doesn't mean that it would produce literally 0 food. It would produce SOME food, and the only reason we aren't using it now is because it is more expensive.
So the answer is the tons and tons of land that we aren't using for farming right now, but we could be using.
