The Union of Concerned Scientists has issued a new report that takes a high-level look at three key transportation fuels: gasoline, biofuels, and electricity.

It compares their greenhouse gas emissions, taking into account their full life cycle, from production to end use. And it looks at how those life cycle emissions are evolving over time.

Here's the key graph:

(These are all based on a car that gets 25 mpg or the equivalent, driving 12,000 miles a year.)

Like the graph says, gasoline is getting dirtier while biofuels and electricity are getting cleaner. Why is that?

Oil is getting dirtier, and it's unstoppable

It's not intrinsic to the fuels. It is possible to make extremely carbon-intensive biofuels and electricity. The reason they're getting cleaner is that they are being held to account. Their emissions are tracked and required by regulation to decline.

Emissions from gasoline aren't rising because cars are polluting more. They aren't. Thanks to fuel economy standards, per-gallon emissions from gasoline combustion are holding roughly steady in the US.

What's rising is emissions from oil extraction and refining.

Emissions from oil extraction are not carefully tracked. Reporting is haphazard, as is regulation. Such emissions can vary by a factor of five, depending on the place and methods used.

But overall, the number is rising. UCS has a nice single-paragraph explanation for why:

Conventional, easy-to-access oil is running out. But the result is not gasoline shortages. Rather, the oil industry has shifted its focus to unconventional fossil resources and extraction methods. The prototypical "gusher" that marked the discovery of the Spindletop oil field in Texas in 1901 is no longer an accurate representation of where oil comes from or what oil production looks like. As oil fields age and their output declines, oil companies are turning to oil resources once thought to be too risky or too expensive to exploit, establishing and rapidly scaling up production of unconventional sources of oil that are costly, physically difficult, and energy-intensive to extract and refine compared to "easy" oil. So long as continued demand for oil exists, oil companies will find technical means to develop these increasingly challenging and risky fossil fuel resources

There are all sorts of ways extraction and refining could be better regulated to reduce their carbon impact — the report goes into great detail about it — but the long-term trend toward riskier, dirtier, more energy-intensive oil is inevitable. The big, easy gushers are gone.

Thus, no matter how fuel-efficient vehicles get, the life cycle emissions of gasoline face tidal pressure upward. And given the number of gasoline cars in circulation, each increase in life cycle emissions has enormous, lasting effects.

It's all about electricity, he said again

The switch to electric transportation could take some time, and it would be nice to have cleaner fuels in the interim. The report also goes into great detail about how biofuels can be improved, including through more efficient production, a shift to corn husks or other agricultural wastes (rather than food), or a shift to more abundant non-food sources like switchgrass.

But advanced biofuels have hovered on the edge of commercial viability for as long as I can remember, forever disappointing people's high expectations.

With some smart policies, they can likely scale up to be more of a complement to gasoline than they are now, blunting some of its carbon impact, but there's little prospect of them taking over as a primary fuel.

Here's how UCS puts it:

We anticipate the role of electricity in a clean transportation sector steadily growing, the use of gasoline steadily falling, and biofuels evolving from having a complementary role in gasoline blends to a complementary role with electricity in certain parts of the transportation sector that are more difficult to electrify, such as aviation.

Eventually electricity will be the primary transportation fuel, and (cleaner) liquid fuels will be a complement.

In short, it's all about electricity.

Already electric passenger vehicles are cleaner than gas vehicles across almost all of the US. And in some regions they are much, much cleaner:

Importantly, every single electric car on the road is capable of being a zero-emissions vehicle. All it takes is an emissions-free electric grid to draw from. Every little investment in cleaning up the grid also cleans up every single electric vehicle.

And the greening of the grid is happening more quickly than almost anyone anticipated. A zero-carbon electricity system is, if not exactly in reach, at least on the horizon. The path from here to there is clear, at least enough to get started.

Liquid fuels, however, remain an intractable puzzle. Everyone agrees corn ethanol is a nightmare, and no more benign alternative has been able to scale up. Natural gas is only an incremental gain and would require lots of new fossil-focused infrastructure. Hydrogen is just a way of storing electricity, which would also require tons of new infrastructure.

Electrifying transportation serves several goals at once

The infrastructure for electric transportation, on the other hand, is already mostly in place. It is the infrastructure that serves more and more of Americans' varied energy needs: the grid.

Yoking transportation — perhaps the most challenging source of emissions to reduce — to the grid would accomplish several things simultaneously.

It would draw huge new investments into greening the grid. It would create a large source of new demand for power, boosting investment in renewables.

It would provide, through millions of electric-vehicle batteries, a form of distributed energy storage that could ease fluctuations on the grid and held integrate more variable renewable energy (solar and wind).

It would radically reduce air and water pollutants like particulates and mercury, saving millions of lives.

And it has the potential, like no other transportation fuel we know of, to eventually reach zero carbon, which is the ultimate goal.

(When a technology shift is important and inevitable, there's an argument for accelerating it with public policy, and UCS makes several sensible recommendations on that score.)

More and more it has become evident that a decarbonized economy is an electrified economy. The faster the transition happens, the more benefits will accrue to future generations.