If all the peoples of the world got together tomorrow and agreed to never build another fossil fuel power plant or gasoline-powered automobile, all the carbon-emitting structures already built would still produce 496 gigatonnes of carbon between now and 2060.
According to Steven Davis and his team at the Carnegie Institution, our energy infrastructure is already so hugely built up that producing another 500 billion tons of carbon is almost a certainty, even if we had zero growth of carbon-emitting devices. Here's the (slightly) good news - if only that amount of carbon is emitted, then carbon dioxide levels would stabilize at about 430 parts per million in the atmosphere, and that would only raise temperature levels to about 1.3 degrees above pre-industrial times.
That figure is somewhat heartening, because most climatologists agree we will start to see the more dire effects of global warming once temperatures rise two degrees. That means we have to figure out a way to utilize lots more alternative energy sources over the next fifty years without using that final 0.7 degrees worth of carbon emissions. Davis estimates we will need to produce 30 terawatts of power from alternative energy sources if we want to maintain current growth while avoiding the worst effects of climate change.
Here's the (decidedly) bad news - 30 terawatts is about double the world's total energy output in 2006, and fifteen times the amount of renewable energy we managed to produce, so we've got our work cut out for us.