It is all a question of intensity. If I have a leisurely stroll around the park, spending an hour on the 2-mile loop looking at leaves and the lake, I will have brought peace to my soul but not burned enough calories to justify eating a pint of ice cream. If I had run a few laps around the same park during that hour, I might have missed a few of the trees but would have earned the right to relish the ice cream. Same amount of time off the couch, same lake, but very different experiences in terms of net energy use. Both cases can be called exercise but both are not equal.
Different intensity means a very different outcome in terms of energy burned. The same can be said for types of fuel and sources of power. Not all fuels and not all energy are created equal. They differ in terms of how much power they generate and how much carbon is released to get that power. This is true for both fossil based fuels and renewable energy. Lately (read in California) this is being officially recognized with the Low Carbon Fuel Standard.  Hopefully other states will soon follow suit.
Carbon intensity is both a function of the source of the energy and the path from source to tank or outlet. Carbon intensity means that turning on the lights in Washington State (hydroelectric power) burns a whole lot less carbon than turning on the lights in Indiana (coal combustion). In other words, not all electricity is created equal and not all fuel is created equal. Some is cause for celebrating with ice cream and some just makes it melt.
The big elephant in the room here is fuel from fossil fuel deposits. That means gas for your car or diesel for the truck. It also means coal and natural gas for your lights and power. These fuels pack a punch because they come from fossil reserves — the long ago leaves and trees that got buried and squished and morphed into long term carbon deposits. Releasing these deposits is the well understood and accepted culprit behind climate change.

Intensity Comparisons

For the regular unleaded that you pump and the electricity that lights up the service station, it is not just a question of releasing those reserves into the atmosphere as carbon dioxide (CO₂). It also takes a lot of energy to get them from where they are buried to your local service station or power outlet. Energy is required to bring them to the surface, transform them into a form that can be used, then get them
to the pump or power generating point. Each step of that journey, for each type of fuel or energy source, increases the carbon intensity of an already carbon intense fuel source. How much it is increased depends on the individual fuel or energy source.
Take, for example, natural gas versus coal. Currently, natural gas is putting coal out of business, primarily due to its lower price rather than any major desire to burn clean. Natural gas, although typically derived from fossil deposits, has a lower carbon intensity than coal — emitting about half of the CO₂ per unit of energy. When you burn natural gas for electricity at a new and efficient plant you can save over 50 percent in emissions in comparison to burning coal in a new plant.
When you consider the process though, natural gas is not always so good. Any leaks in the transport of the natural gas from the ground to the power plant can quickly reduce the benefits of burning the gas. Natural gas is methane (CH₄), a gas that is many times more potent than CO₂. Accidental CH₄ emissions can take away the carbon benefits of burning natural gas when this leakage starts getting higher than 3 percent of the total gas transported.
The energy intensity of fossil fuels varies as does the process to make them into a form that we can use them. Table 1 shows both the inherent energy per unit of fuel (BTUs per pound, gallon or cubic feet (ft³), as well as the lbs of CO₂ emitted per million BTUs of energy created.
Alternative fuel sources like wind and water for electricity or biodiesel and ethanol for cars and trucks typically have a head up because they do not start out with something that has embedded CO₂ from fossil deposits. Even with that advantage, it doesn’t mean that these alternative fuels are carbon free. Each will have its own energy intensity and each will require different amounts of energy to be transformed into a usable form and transported to an end use site. For example, a liter of switchgrass-based ethanol will have less power than the same amount of fuel derived from corn.

Life Cycle Assessment

Understanding the full carbon impact of a fuel/energy source is a convoluted and often confusing process. Life cycle assessment (LCA) is the tool of choice. This type of analysis tries to evaluate the full impact of the fuel/energy source by considering potential carbon emissions during each stage of each process. This is a laudable goal that is not always fully realized or successful. There are even computer models that have been developed to make your calculations easier.
The GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) is the model of choice for vehicle fuels. It was developed by the Department of Energy and is public property.  The model covers fuel costs/emissions from “well to pump” and also covers emissions associated with vehicles. In other words, you can find out what the impact of corn grain-based ethanol really is and whether you should pump it into a hybrid.
The model includes the carbon and environmental impacts of growing the corn. This means that fertilizer use and other energy inputs for farming are part of the cost of burning ethanol. Corn is very energy and fertilizer intensive to produce, and that is why many have argued that corn-based ethanol is not a terrific alternative to petroleum. The costs of producing the nitrogen fertilizers and the nitrous oxide emissions when those fertilizers are applied to the cornfields are taken into consideration in the accounting. No fossil CO₂ in the corn kernels themselves but plenty to get the corn to grow. One reason that cellulosic ethanol is seen as so much better than corn grain-based ethanol is the lower impact associated with growing the grasses. Typical crops for cellulosic ethanol such as switchgrass require less fertilizer to grow and are grown as perennial crops, and so store more carbon in soils (Wang et al., 2012).
There is no single model for a comparable analysis of electricity generation but there are a very large number of studies on this topic. Life cycle assessment is the tool of choice for these as well. A review of the studies suggests that with hydropower and wind, you can eat all of the ice cream that you want (Turconi et al., 2013). Concerns about birds and fish have not yet factored into these calculations. I would expect that even considering those of you are still running a marathon in comparison to sitting on the couch with coal combustion. Biomass and solar are a little worse but both give you about equal carbon intensity and both allow for eating plenty of ice cream.
And then there is the dream case: Divert food scraps from a landfill and bring them to a digester where the methane produced was sold for electricity. Pipes to transport that gas and the digester itself were both airtight meaning no debits for fugitive methane. The resulting solids were then land applied to a cellulosic ethanol crop where they both increased soil organic matter and replaced synthetic fertilizers. With that system you could drive to a brightly lit ice cream parlor and eat all you want, all guilt free. That is assuming that the life cycle model was advanced enough and the data solid enough that you could get all the credit deserved.
The take home for this column is that all power and fuel are not created equal. That goes for those derived directly from fossil fuels as well as those that are generally classified as renewables. The technique for determining the true cost of each of these fuels/sources of energy is life cycle assessment, a good but not perfect tool. More data and more time will make it better. In the meantime, read the fine print and whether you run or walk, getting off the couch is a good idea.
Sally Brown is a Research Associate Professor at the University of Washington in Seattle (slb@uw.edu) and a member of BioCycle’s Editorial Board.