Frankly, the world as we know it is going away, soon.

[kurt_cagle]

123. I'm guessing 150 years

CO2 cycle has a lifespan of about 100 years. Methane cycle has a lifespan of about 30 years. Biggest danger to humanity is not from heat (at least not at first) it's from ever more vicious weather. Global oil production has peaked already (2007), and we will probably reach a point where oil is out of the economy by 2035. NatGas may last another ten years after that. Coal will become the dominant power source by 2050, which will increase carbon densities.

Clathrates are beginning to sublimate now, by 2050, methane will be the dominant greenhouse gas in the atmosphere. Sea level rises, Category 10 hurricanes, Class 8 tornadoes and extreme droughts and flooding will be the norm. At that point, the weather is too intensive to sustain anything beyond an iron age economy. This will have an interesting consequence however. Clathrate sublimation will cascade in a positive feedback loop that will likely last for 30-40 years, but it's worth noting that the Earth is naturally in a very long term cooling cycle. Human contributions to GW will have ceased by 2060, and the amount of clathrates is limited. Without that additional contribution, the methane cycle washes most of the methane out of the atmosphere by 2100. By 2160, the carbon cycle will have also washed out the oversaturated CO2 out of the atmosphere. By that time, humankind has been reduced to small bands of stone-age wanderers, perhaps as little as 10,000 people on the whole planet. Of course, this will likely have also tipped the planet back into an ice age, for which those 10,000 people, used to 140 degree summers, will be ill prepared for.