Current IPCC climate estimates end at the year 2100, as do most climate model simulations. This amounts to a roughly the lifetime of a single human being, which I guess is what the IPCC thinks is the limit of most people's imaginations. But in climatological terms, thinking only a single century ahead can be woefully shortsighted. The carbon we emit into the air today stays in the air (and oceans and biosphere) for thousands of years, until the carbon cycle finally retires it back to the lithosphere. But before it goes back to rock, that CO2 – our CO2, the stuff we made and we are responsible for – stays in the air, and continues to warm the planet, during those centuries.
Think about the implications of that.
Our current background CO2 level is 390 parts per million by volume (ppmv), and continuing to rise. The last time CO2 levels were this high, and stayed this high for a few centuries, was during the Middle Miocene, 15 to 20 million years ago. At that time, global temperatures were 3° to 6° C higher than today. And sea levels were about 100 feet higher than today.
So why isn't it that hot now? Because it takes a long frickin' time to warm up the whole ocean, that's why. Anyone who's ever boiled water knows that. Right now, there is a radiation imbalance at the top of the atmosphere that amounts to 1.4 Watts per square meter – or 700 trillion watts for the whole planet. The Earth will continue to get warmer until that imbalance goes down to zero. Which means that even if we were to turn off the fossil-fuel tap entirely and immediately, the Earth would continue to get warmer, until the full effects of all of our past CO2 emissions finally catch up with us -- which would be many decades at least, perhaps a couple of centuries. Based on what happened in the Middle Miocene, we have a pretty good idea of how hot it will get. And how high the sea will rise.
But that's not the half of it, because we're not going to turn off the fossil-fuel tap entirely and immediately. Nobody really expects our fossil fuel addiction to end, or even to decline much. Given the reluctance of pretty much every government in the world to tackle the issue seriously (including ours, I'm sorry to say), it seems likely that the only thing that's going to end the use of fossil fuels is their ultimate depletion, which will probably take another century at least. It took us a long time to go from zero oil to peak oil, and it will take us another long time to get back down to zero. And don't even get me started on coal.
All of which means that we're not really looking at 400 ppmv CO2 like in the Middle Miocene. We're not looking at 500 either, or 560 (the "CO2 doubling" many speak about). No, what we're really looking at is something like 600 to 900 ppmv before economic forces call a halt to carbon. So we're looking at another century to turn off the carbon tap, and another two centuries after that for Mother Earth to re-establish thermal equilibrium. And how hot will it be then?
How about: half the planet uninhabitable for human beings.
There is a limit to how hot it can get before you die, and that limit is set by humidity. Your body's natural processes create heat, and you need to get rid of that heat to live. Much of your body heat is lost through perspiration, the evaporation of water from your skin. But if the humidity and temperature were high enough, water wouldn't evaporate from your skin and you would suffer heat stroke. A recent study found that the evaporation limit occurs when the wet bulb temperature reaches 35°C, or 95° F. In effect, under those conditions even if you were sitting naked in the shade with a stiff breeze, you would still overheat. But don't worry, because currently wet bulb temperatures don't get any higher than 31°C on earth, even in the tropics.
Currently.
While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning.
One would think that a story like this would be reported.
It was reported in Australia.
It was reported in the UK.
It was reported in India.
But in the USA? Land of a thousand networks? USA Today picked it up in their online edition. The New York Times did not.
In a related note, there was no progress on climate change in Congress again today. Again this week. Again this month. Again this century.
What it will be like if we do nothing:
Those pinkish-gray areas will be uninhabitable: India, eastern China, Australia, South America, eastern USA, north Africa, and the Middle East.