And even people in poverty can migrate. I really don't understand your point.
On top millions or billions of people start moving around because their homes and their jobs no longer exist. Food will get a problem because the oceans go acid. Did you ever chose jellyfish as a diet? Social friction will increase. After Sandy NY run short on gas supply, remember the guarded gas stations? The western economy relies on weather behaving within a reasonable range, imagine cities without food, because the just in time transport system stucks in 2m snow for weeks. I can continue the list even more, but the point is basically everything will change: culture, economy, cities, agriculture, food, friends, borders, countries, and so on.
The last thing I can imagine is 7 billion people moving orderly to Russia or Canada to begin a new live in peace.
And exactly how fast do you think the overall temperature is going to change? All the predictions I've seen have been single digit degrees per century.
That's not even wrong.
What's changing is the total energy within the biosphere. That's going to have profound effects throughout the ecosystem. Modeling just what those will be is very difficult.
One trend that's already emerged has been an increasing variability and range to the jet stream, especially in how it increasingly "wanders" north to south. This means that regions might see very rapid and wide temperature swings from summer to winter or back over the course of a few hours to a day. For crops and ecosystems which rely on more predictable and stable conditions, this could prove deadly.
Glacial melting and rising seawaters don't just mean floods, but salt-water intrusion, disturbances of ocean current systems (themselves responsible for transporting vast quantities of heat around the globe, etc.
Some breakdowns in systems seem to have been very rapid. During and toward the end of the last glaciation period, vast ice dams and lakes would form, some over what's now Utah (historic Lake Bonneville, of which the Great Salt Lake is among the last remnants), as well as in eastern Canada. The formation and bursting of ice dams in the pacific northwest lead to the formation of what are now known as the Washington Badlands by way of the Missoula Floods -- as many as 25 major inundation events which created waterfalls, gravel banks, sandbars, and other features, over what's now dry land. Flow speeds exceeded 80 MPH and consisted of cubic _miles_ of water.
In what's now Eastern Canada, bursting of an ice dam shut is thought to have shut down the Gulf Stream at least once, in a period of one year or less:
Both circumstances involved warming, though from lower temperatures than today's baseline. The point is that secondary and tertiary climate change effects can be difficult to predict, but also exhibit very great nonlinearity. That is: we don't know what might happen, and it could happen very quickly, even in hours, or days. Certainly massive changes in less than a year's time are possible.
But most importantly... +50°C is survivable ? OK. Then when the temperature reaches +75°C, what do we do ? +100°C ?
I'm surprised at how many people seem to think we can simply patch problems from day to day and only do anything short term. It seems extraordinarily short-sighted to me.
Building dikes? Sure, building dams for sealevel three meters higher sounds reasonable, right? Oh well, that's what the Netherlands already have? Let's just build a new six-meter dam then, where's the problem? Well, six meters might be doable, but what do we do when 12 meters are needed? 50? 100?
In fact those two would essentially climax hilariously, because at 100 degrees the sea will be in the atmosphere, thousands of metres high.
The point is: there is no predictable upper limit, period.
If you think 5 degrees is catastrophic, then sure worry about that in 2013. But it won't make the planet uninhabitable.