This is a common claim. People like to expand it into things like, well, more CO2 and warmer climates means a lusher Earth and more food!
That's probably true. Most of the land is in the northern hemisphere, fairly far from the equator. Deserts will expand with warmer temperatures, but likely even more land will become arable in the north. I'm Canadian, and my countrymen often quip that global warming sounds pretty good.
The problem is that cities need a *constant* supply of food. I grew up in a farming community. Farmers admittedly like to bitch about everything, but there's a reason for that. They'll be happy to tell you about how any change from the norm, in any direction, negatively affects their crops. Modern agriculture, especially in the developed world, isn't some old geezer waking up one day, squinting at the sun, letting some dirt trickle through his fingers, standing up and sighing "well, I guess it's plantin' time again." It's incredibly optimized. There was a documentary, unfortunately I can't find it, on all the things the USDA tracks and models in order to make recommendations for pretty much every aspect of agriculture in the US.
Canada is supposed to get (overall) warmer and wetter. Sounds great yeah? Well, a few years ago we had a really wet fall, meaning nobody could dry hay, meaning lots of livestock had to be culled the next year because there wasn't enough food for them. Warmer also means less or no persistent snow pack in the winter, so no spring runoff.
Moving New York, Miami and New Orleans because they're getting regularly flooded is expensive, but doable. Moving Capetown, LA, or a hundred other cities because they're out of water is expensive, doable. Switching Iowa from growing corn to growing wheat is expensive, but doable. All at the same time, and along with a zillion other things starts to get dicey, and definitely more expensive. And the global climate is likely to keep shifting for a long time until the heat budget equalizes and another zillion things all equilibrate.
One of the reasons Australia was sparsely populated historically, never developed large agrarian civilizations, and agriculture is still very difficult, is that its geography makes its climate highly variable. Civilization thrives in stability.