You can't grow food for a large population when average planetary temps are at +5C. It doesn't mean it's locally always +5C warmer than it used to be; it means you're seeing insane temperature swings in a matter of days, constantly - in both directions, it just so happens that the average is +5C.
Not to mention, at +5C it is all but certain shallow methane hydrate deposits (those stabilized by temperature, not pressure) all over the world are now outgassing CH4; not to mention several other similar tripwires, and likely not all of which we've even identified.
Edit: I can't seem to be able to reply to your comment below, not sure why. You're absolutely wrong about those Siberia figures, and they're not supported by current scientific consensus.
Much of the soil in Russia that will warm is indeed acidic and largely inexploitable, which I suspect is what you were getting at; to their credit though, they picked an area that isn't (IIRC). Not that it changes much.
Most soil can be used for some type of farming with some help from fertilizers. Temperature and access to water is more critical. Higher CO2 also increases crops, in isolation.
We're also talking about a time span of 100-300 years. Keep in mind how much more efficient farming is now compared to even the 1950's.
Not to mention, at +5C it is all but certain shallow methane hydrate deposits (those stabilized by temperature, not pressure) all over the world are now outgassing CH4; not to mention several other similar tripwires, and likely not all of which we've even identified.
Edit: I can't seem to be able to reply to your comment below, not sure why. You're absolutely wrong about those Siberia figures, and they're not supported by current scientific consensus.