ABSTRACT
We will make the case that the ultimate constraint to human sustainability will be determined by the amount of available ‘solar income’ held by a group of people. Solar income translates into the land's productive capacity, and we contend that to be sustainable we need to shift towards managing and using mostly current solar income (Box 9.1). This would move us to the idea of living within our ‘solar income’ rather than the ‘solar capital’ (Box 9.1). Our current approach to assessing sustainability of resource uses mainly measures our solar capital – the area of a natural resource or the quantity of fossil fuel supply that exists. Using this approach, it is easy to calculate that we have hundreds of years of resource supplies and reserves, so there is less concern about consuming resources. If the focus centred on identifying the productive capacity thresholds or the ‘income’ available from lands being used to consume resources, it would become easier to make decisionsthat were more sustainable, because climateand soils would be factored into decisions. This approach also provides us a way to measure thresholds so that we do not continue toover-consume our solar capital unsustainably.
Solar capital is defined as the total amount of fixed carbon available for human use. In energy terms this is coal, oil and their relatives (tar sands, etc.), and natural gas. Forests contain a lot of fixed carbon too, and their current mass is also solar capital. Solar income is the annual increment of fixed carbon captured in agriculture and forestry (or other solar energy captured such as PV, wind or hydro). The analogy with financial capital is that capital should only be invested (used) to enhance income, and only income should be used for expenses. Thus we should use fossil fuels to enhance our ability to capture solar energy. We do this when we use fossil fuels to enhance forestry, agriculture, and the non-carbon sources above. We don't do it when we use fossil carbon for transportation fuel, direct home heating, and for other reasons. To fully accept this analogy, we must assume that the goal is to move to using only current solar income at some future time. That is probably unrealistic, but defines an upper limit.
