At a wet-bulb temperature of 95F (35C), thermal stress is lethal to humans in a few hours in most cases.
While air temperatures routinely exceed 95F (which is already hot), that's generally at much lower humidities. A wet-bulb 95F means that relative humdity is 100%, and the body effectively cannot cool itself.
Even swimmers have been known (or suspected) to succumb to thermal stress when swimming through warm waters. The international body governing open-water competitions has set an upper bound for competitions of 31C (87.8F):
http://www.swimmingworldmagazine.com/lane9/news/Commentary/3...
Most swimming pools are heated to between 78 and 82F. The difference between the lower and upper ends of that range are pronounced: slightly cool to slightly warm if you're not very active. Many swimmers will find anything over 80F unpleasantly warm, and may prefer cooler temperatures, though lower than about 72F generally feels chilly regardless.
There are a lot of other problems with release of carbon on a massive scale, starting with gross acidification of the oceans and a significant reduction of oxygen in the atmosphere.
A +50 change means the hottest regions, that already reach up to 50 degrees, would hit boiling point for extended periods of the year. Some if those are by the sea, which would continuously boil, filling the atmosphere with steam and cause a new greenhouse effect boost and a wave of heating that would push temperatures even higher.
The problem is there are certain thresholds that cause new greenhouse effects to kick in and push temperatures even higher. For example, if temperatures rise enough to melt the Methane frost layers, they'll melt and probably kick global temperatures up another 5 or 6 degrees all by themselves. If we trigger enough of these knock on effects in a chain, well end up with an unlivable atmosphere. That's the extreme scenario, but it is not impossible.
Look, it is not like adding 6°C on all temperatures and that's it. Weather is getting extreme and the environment changes then. There is a book called 6 degrees by Mark Lynas. It is a good read. He spend some time in the archives and collected what might change by comparing to past times. Rainforests will probably die or at least move, sea level will raise by ~70 meters.
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.
Many areas already deal with extreme temperature variance, the only change will be which areas need to do so.
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.
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.
https://en.wikipedia.org/wiki/Missoula_Floods
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.
+50°C ? The areas of Earth where today the temperature never grows beyond, say, -10°C (40°C sounds like close to an upper limit, to me) are quite small.
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?
I wasn't trying to give a real number, I was saying that 'too hot, planetwide' is rather unrealistic. The initial ass-number was '6', why does your argument rely on numbers an order of magnitude larger?
I think it's perfectly reasonable to postpone worrying about 50+ degrees until it's already shifted at least 5. If it turns out 5 is terrible, we'll have hundreds of years to implement plans to stop 50.
If you think 5 degrees is catastrophic, then sure worry about that in 2013. But it won't make the planet uninhabitable.
The icy tundra areas mostly don't have good soil. Even if they get the temperatures of the American midwest they will not produce nearly the same calories/year/acre. So even if populations could all start migrating north (without somehow triggering world-spanning conflicts as millions of people get forced across national borders) their food source won't be coming with them.
