If the Earth's sea level rose several meters in an instant, yes, it would be a worldwide catastrophe with massive loss of life. But I fail to understand how that's physically possible; such change would take years or decades.
This would give people time to adapt. If the ocean's risen and there's seawater in your house up to your ankles, you move; you don't stick around for three more years until it's over your head and you drown.
Agriculture would certainly be affected by climate patterns. But how do we know that the outflow of farmland that stops being usable for farming due to climate change won't be balanced by an inflow of farmland that becomes suitable for farming due to climate change?
And even if agricultural output goes down: (1) New technology is invented all the time which improves agricultural output. Maybe by the time significant amounts of farmland become unfarmable, we'll be able to make due with less.
(2) Livestock takes a lot of land, because fundamentally, growing plants and eating them is much more efficient than growing plants, feeding them to animals, and then eating the animals. Vegetarians have been around a long time, and we know that it's possible for humans to live on plants alone. So if there's less farmland, why won't we just eat less meat as food becomes more expensive?
(3) Everyone needs to eat. iPod's are a luxury; food is a necessity. As food gets more expensive due to a scarcity of land and people start spending more of their income on food and less on iPod's, Apple will just close a couple factories and their land will be converted to farmland (funding for this will be available, because, again, if food is expensive, owning farmland will be very profitable.)
I fundamentally fail to understand why global warming deserves the same level of threat as (say) a bioengineered plague, a large nuclear war or an enormous asteroid on a collision course with Earth would.