In the previous chapter we looked at the concept of increasing the efficiency with which fossil fuels were used to produce work. However, this increase in efficiency will need to be taken up at an unprecedented rate under most of the scenarios discussed in Chapter 1 in order to stabilise greenhouse gas concentrations in the atmosphere. As an alternative, we could deploy technologies that emit near-zero greenhouse gas. This would also have the effect of reducing the carbon dioxide intensity.

About 20% of the world's primary energy at present comes from sources that emit no net carbon dioxide. Firewood is the largest component. Here carbon dioxide is extracted from the atmosphere by photosynthesis as the tree grows and is returned to the atmosphere during combustion. The primary source of energy for firewood is the sun, which radiates energy to the earth. The next most important low-emissions energy sources are nuclear energy and hydro-energy, which are about equal contributors to energy supply.

We need to recall the magnitude of the reduction in net carbon dioxide emissions required to stabilise the atmospheric concentration at some value. The emissions are shown in Figure 1.8. For example, if we assume the scenario IS92a is a business as usual scenario for the world without concern for greenhouse gas, then to achieve stabilisation of concentration at 550 ppm after 100years as per Figure 1.8, the world net emissions need to be held to about 9GtCyr−1 in 2030 and have a lower value thereafter.