Last spring we provided an update of an analysis of Hoffert et al. (1998) which projected the need for carbon-free energy at various points in time, consistent with the stabilization of atmospheric concentrations of carbon dioxide at a level of 550 ppm. Here is my annotation of their figure (click for larger version):

And here is how I described it:

The figure shows carbon free energy required to achieve stabilization at 550 ppm carbon dioxide as a function of the rate of average energy intensity decline. The figure also shows 1990 total energy consumption (about 11 terawatts, TW) and the share of this value from carbon-free sources (about 1.2 TW). I have updated both of these values to 2008 using data from the EIA, which I extrapolated to 2008 values, for which I arrive at 17.4 TW of total energy consumption of which 2.4 TW are carbon-free.

Hoffert et al. estimated that we’d need 10-30 TW of carbon free primary energy production by 2050, assuming energy intensity declines of 1.0-2.0% over the first 5 decades of the 21st century. So far at least, that assumption has proved optimistic, as actual energy intensity has increased, as indicated by the blue dot on the leftward-extended horizontal axis. If energy intensity does not improve beyond this value then the world will need 22 TW of carbon-free energy by 2025, and if this value works out to a net 0.5% decline through 2025, then this figure would be halved to 11 TW. For 2050 the values are 51 and 25 TW respectively.

A few weeks ago Nature provided an analysis of the prospects for generating carbon-free energy from a range of technologies. Here is a brief summary of their verdicts for each technology for “coming decades” (and where they did not provide numbers I erred on the high side):

Hydropower = 1 TW
Nuclear = 1 TW
Biomass = 0 – ??
Wind = 1.5 TW
Geothermal = 1 TW
Solar = Lots
Ocean = 0

Total = ~4.5 TW + solar

So if the world will need between 11-22 TW of carbon free energy by 2025 to be consistent with a 550 stabilization path, where will that energy come from? This question is rarely addressed. Here are some other questions:

Can solar provide between 6.5-17.5 TW?

Are there other technologies to fill the gap not discussed by Nature, e.g., coal with CCS?

How can any cap and trade program ever hope to work if there are not suitable alternatives under a cap?

These are the central questions of mitigation.