We can actually play around with greenhouse gas emissions scenarios ourselves. To do so, we will take advantage of something known as the Kaya Identity. Technically, the identity is just a definition, relating the quantity of annual carbon emissions to a factor of terms that reflect (1) population, (2) relative (i.e., per capita) economic production, measured by annual GDP in dollars/person, (3) energy intensity, measured in terawatts of energy consumed per dollar added to GDP, and (4) carbon efficiency, measured in gigatons of carbon emitted per terawatt of energy used. Multiply these out, and you get gigatons of carbon emitted. If the other quantities are expressed as a percentage change per year, then the carbon emissions, too, are expressed as a percentage change per year, which, in turn, defines a future trajectory of carbon emissions and concentrations.
Mathematically, the Kaya identity is expressed in the form:
- is global emissions from human sources
- is global population
- is world GDP
- is global energy consumption
By projecting the future changes in population (P), economic production , energy intensity , and carbon efficiency , it is possible to make an informed projection of future carbon emissions . Obviously, population is important as, in the absence of anything else, more people means more energy use. Moreover, economic production measured by GDP per capita plays an important role, as a bigger economy means greater use of energy. The energy intensity term is where technology comes in. As we develop new energy technologies or improve the efficiency of existing energy technology, we expect that it will take less energy to increase our GDP by and additional dollar, i.e., we should see a decline in energy intensity. Last, but certainly not least, is the carbon efficiency. As we develop and increasingly switch over to renewable energy sources and non-fossil fuel based energy alternatives and improve the carbon efficiency of existing fossil fuel sources (e.g., by finding a way to extract and sequester ), we can expect a decline in this quantity as well, i.e., less carbon emitted per unit of energy production.
Fortunately, we do not have to start from scratch. There is a convenient online calculator here, provided courtesy of David Archer of the University of Chicago (and a RealClimate blogger ). Below a brief demonstration of how the tool can be used. After you watch the demonstration, use the link provided above to play around with the calculator yourself.