Arable land means, in current high tech enveronment, much less than 100 years ago. Firstly - with some efforts non-arable land can become arable. Israel and Spain are prime examples here. Second, food can be relative easely purchased on international market. And third, there are VERY large areas in China still almost undeveloped.
On the first point, there's limits to what non-arable land can bring. For example, if one wants to irrigate a desert that water has to come from somewhere, whether groundwater, distilled or some river etc. Deserts do not have such fertile soil etc. There's a reason why non-arable land is non-arable. At this stage of world development further extensive development is very short-sight solution. As for the second, sure, food can be bought from international market now, and with enough money in the future, but rising food prices will most certainly hit the world economies. Additionally, climate change will devastatively hit many food production areas.
In case of Russia, discussed in this thread, the climate change, if it goes like predicted, will naturally warm up many arctic areas but the effects on agricultural production are smaller what they seem, as most productive areas will head for a fall in production and areas newly opened for agriculture will produce a smaller yield due to very shallow soil. (My english is stretched to the limits trying to explain this.)
While situation for, say, next 15-20 years, looks quite normal with just minor colonial wars (Afghanistan etc.), it's what happens after, say, 2020, what scares me the most.