Creating an Emissions Scenario
There are many ways to meet our energy demands for the future, and each way could include different choices about how much of each energy source we will need. We’re going to refer to these “ways” as scenarios — hypothetical descriptions of our energy future. Each scenario could also include assumptions about how the population will change, how the economy will grow, how much effort we put into developing new technologies and conservation strategies. Each scenario can be used to generate a history of emissions of CO2, and then we can plug that into a climate model to see the consequences of each scenario.
Emissions per unit energy for different sources
The global emission of carbon into the atmosphere due to human activities is dominated by the combustion of fossil fuels in the generation of energy, but the various energy sources — coal, oil, and gas — emit different amounts of CO2 per unit of energy generated. Coal releases the most CO2 per unit of energy generated during combustion — about 103.7 g CO2 per MJ (106 J) of energy. Oil follows with 65.7 g CO2/MJ, and gas is the “cleanest” or most efficient of these, releasing about 62.2 g CO2/MJ.
At first, you might think that renewable or non-fossil fuel sources of energy will not generate any carbon emissions, but in reality, there are some emissions related to obtaining our energy from these means. For example, a nuclear power plant requires huge quantities of cement, the production of which releases CO2 into the atmosphere. The manufacture of solar panels requires energy as well and so there are emissions related to that process because our current industrial world gets most of its energy from fossil fuels. For these energy sources, the emissions per unit of energy are generally estimated using a lifetime approach — if you emitted 1000 g of CO2 to make a solar panel and over its lifetime, it generated 500 MJ, then it’s emission rate is 2 g CO2/MJ. If we average these non-fossil fuel sources together, they release about 5 g CO2/MJ — far cleaner than the other energy sources, but not perfectly clean.
So, to sum it up, here is a ranking of the emissions related to different energy sources:
|Energy Source||g CO2 per MJ|
*Hydro, Nuclear, Wind, Solar
**This will decrease as the non-fossil fuel fraction increases