Over the years, IPCC has issued numerous scenarios describing the trajectory of civilization and what they may mean for CO2 emissions and the like. The most famous of these is the “Business-as-Usual” scenario, also called IS92A, although this has been supplanted somewhat by the SRES familiy of storylines that have been discussed here often.
While the different storylines and assumptions can be a little confusing, the ingredients for making such a forecast can be fairly simple, and I have coded them up into an interactive web site which can be used to explore the world of possibilities. The prediction is based on an idea called the Kaya identity, using numbers published by Hoffert et al. in Nature 1998 [Hoffert et al., 1998]. You could just read the excellent Hoffert et al. paper, but you might also enjoy playing with your own “live” forecasting model, located here.
The Kaya identity states that
C emission = population * GDP/capita * watts/GDP * C emission/watt.
Population is easy to forecast a few years in advance, and impossible to forecast a century in advance. In general, the rate of population growth on earth is slowing, and the hope is that population will level off at some figure in the coming century. The first parameter in the Kaya model is then the asymptotic leveling-off population.
The second term in the equation is the GDP per capita. Adjusted to 1990 U.S. dollars, this term has risen by about 1.6% per year over the past century. You may adjust the growth rate to whatever you like, and you will see how well your growth rate hindcasts the past as well as what it predicts for the future.
The third term is called the energy intensity. It is the number of watts of energy required to produce a dollar of GDP. The energy intensity reflects energy efficiency, and as such has been declining at a historical rate of about 1% per year. The energy intensity also reflects the character of industry, light versus heavy and the balance between manufacturing and services in making up the GDP.
The fourth term is the carbon efficiency of energy production. Coal releases more carbon per energy yield than oil or gas do, and nuclear or renewable energy sources release almost no carbon at all. Historically the carbon efficiency has been decreasing by 0.3% per year.
Change those parameters and push the button. The top two plots in the resulting output will be the predicted carbon emissions to 2100, and the resulting pCOs atmosphere using the ISAM carbon cycle intermediate complexity model. (Yes, the ISAM model was run just for you. Change the inputs and you’ll get a different answer.) What if we decide that this climate trajectory is not acceptable, and we decide to replace some of the carbon-based energy with some new carbon-free energy source? Let’s assume we just replace coal, because oil and gas will be gone soon anyway. The third plot then shows how much carbon-free energy we will require in the coming century if we are to stabilize CO2 at some level. The lower the level, the more carbon-free energy. The third plot shows curves for the IPCC 350, 450, 550, 650, and 750 ppm stabilization scenarios. The result is typically several tens of terawatts of carbon-free energy will be required by the end of the century. For comparison, global energy production today is about 13 terawatts.
Where could this energy come from? Pacala and Socolow  propose a solution (only good to 2050, not 2100) based on a combination of technologies and conservation strategies. There are also a few radical ideas such as solar cells on the moon , beaming energy back to space by microwaves, and high-altitude windmills , flying like kites in the jet stream, tethered on conducting cables. Carbon sequestration is also an option; saline aquifers within the earth are permeable enough for injected CO2 to spread out away from the injection point, yet large enough to flush the bulk of the fossil fuel carbon into.
The bottom line is that the change in the world’s energy infrastructure that would be required, to limit the CO2 concentration in the atmosphere, is not small. A few Toyota Priuses are not going to do it, nor is the Kyoto Protocol by itself even close to solving the problem. Conservation helps, but the historical rate of improvement in energy efficiency is already built into the forecast. There is some scope for trade-offs over time, cuts in emissions now versus cuts later. Ultimately, however, tens of terawatts is a lot of carbon-free energy.
Hoffert, M.I., K. Caldeira, A.K. Jain, and E.F. Haites, Energy implications of future stabilization of atmospheric CO2 content, Nature, 395, 881-884, 1998.
Pacala, S. and R. Socolow, Stabilization wedges: Solving the climate problem for the next 50 years with current technologies. Science 305: 968-972.