Tailpipes top smokestacks as nation's largest CO2 emitters

Transportation, which runs almost entirely on petroleum fuels, and electricity generation, which had used mainly coal, have long been the largest sources of CO₂ emissions in the United States. Although power plant smokestacks exceeded motor vehicle tailpipes and other mobile sources such as aircraft in terms of CO₂ emissions for nearly forty years, this year brings a crossover of these two emission trends. The CO₂ emitted by the transportation sector has been greater than that from the power sector for seven of the past eight months, and so 2016 is on track to see mobility overtake electricity as the country's biggest contribution to global warming.

Fuel Economy Matters

Pump prices are down and given the outlook of a weak global economy, a strong dollar and a lingering oil glut, they could drop even more as the year goes on. The U.S. average retail gasoline price fell below $2.00 per gallon in January and as of last week it averaged $1.93 per gallon. For over a year now, it's been significantly lower than the roughly $3.50 per gallon average of the previous few years, let alone the brief spike to over $4.00 per gallon in summer 2008. 

Consumers respond to gasoline prices and so it's no surprise that new vehicle sales are at a record high while the vehicle mix has shifted away from compact segments and back to trucks, larger SUVs and more luxurious cars. The amount of driving is back up as well. 

The fuel economy of the vehicle fleet doesn't totally backslide even when the price of fuel does. Most efficiency gains are due to improved technology; once such engineering refinements are made they don't get undone. Corporate Average Fuel Economy (CAFE) standards prop fuel economy up even when consumer interest fades, and that policy is now reinforced with greenhouse gas (GHG) emissions standards that limit the amount of carbon dioxide (CO₂) and other GHGs exhausted from tailpipes. 

Average new car and light truck fuel economy (right-hand axis) 

compared to nominal and inflation-adjusted gasoline prices. 

The adjoining graph compares the average fuel economy of new cars and light trucks to the price of gasoline since 1970, shown as both nominal "dollars of the day" and inflated to 2015 ("real") dollars. It's clear how fuel economy ratchets up as fuel prices rise. We can also see the slow backsliding that happened from the late 1980s until a decade ago. Although fuel economy has been climbing since 2005, we may be in for a serious tug-of-war between the need to keep fuel economy heading up and weakened consumer interest due to lower gasoline prices. 

Bringing biofuels back to earth

After all that's been written about the pros and cons of biofuels over the years, it's fair to ask whether there's anything left to say. It turns out that there is, and a new insight comes from evaluating what actually happens on the earth, that is, on the land where the plants used to make biofuels are grown.

The Liquid Carbon Challenge

In addition to reducing how much fuel gets burned, cutting the climate risk from gasoline and other liquid fuels requires balancing their CO₂ emissions by removing additional CO₂ from the air in locations other than the transportation sector. 

Options for addressing carbon dioxide emissions from liquid transportation fuels

It's easy to take for granted just how important liquid fuels are in everyday life. Gasoline, diesel and jet fuel remain crucial for transportation. Petroleum fuels are the world's most widely used form of energy and rank second to coal as a source of global CO₂ emissions, a situation unlikely to change for at least two decades. Although some worry about peak oil, in reality the world is not about to run out of fossil hydrocarbons that can be turned into convenient liquid fuels at an affordable price.

Cutting carbon while keeping oil -- say what?

In discussions that build on my recent papers, I've been pointing out how trying to "get off of oil" -- that is, replacing petroleum as a source of fuels for transportation in particular -- isn't actually necessary for climate protection. This conclusion challenges a lot of common thinking and especially questions the foundation for many of the public policies being pursued or advocated to address the car part of the climate problem.

Cover of The Atlantic
magazine, May 2013

Such reasoning certainly requires explanation, especially in terms that are easier to grasp than the academic papers that lay out the technical arguments. Those papers include "Factoring the Car-Climate Challenge" in Energy Policy and "Biofuel's Carbon Balance" in Climatic Change, both published last year (2013). A new paper further elaborating on this issue will be published soon.

It's critical to understand the meaning of the word necessary. I'm not saying that petroleum alternatives won't be helpful for cutting carbon; they might well be. But "necessary" is a very strong condition. Is it possible to imagine a world in which net CO2 emissions are reduced to a level that is a small fraction of what it is today -- within the bounds needed to avoid further buildup of CO2 in the atmosphere -- but which still relies extensively on petroleum a source of fuel?  The answer yes, and it just takes one hypothetical counterexample to show that getting off of oil isn't strictly necessary.

The example involves mechanisms that remove carbon from the atmosphere at a rate much faster than is happening now and indeed fast enough to fully counterbalance the amount of CO2 that spews into the atmosphere when petroleum fuels are burned. Those emissions currently amount to about 3 petagrams per year (Pg/yr; a petagram is the same as a billion metric tons) on a carbon mass basis, or about 11 billion metric tons per year of CO2 emissions, representing about one-third of total anthropogenic CO2 emissions (see the Global Carbon Project for the latest detailed estimates).

That amount of CO2 removal is an enormous challenge, but there's no scientific reason to rule it out as a long-term solution. For perspective, plant growth on land plus biological and chemical CO2 uptake in the ocean together remove carbon from the atmosphere at a rate of about 200 Pg/yr. Most of that gets re-emitted annually, but a concerted effort at large-scale carbon management could change the balance enough to offset a large portion of fossil-based CO2 emissions. Such carbon dioxide removal ("CDR") strategies can help to address any form of excess CO2 emissions, but they are particularly important for addressing liquid fuel use because it is not feasible to capture carbon from car, truck or jet exhausts (in contrast to large stationary sources such as power plants).

This post just examines the necessity question. Other questions certainly arise regarding costs and issues of timing (i.e., what can be done to mitigate emissions sooner rather than later, and also what must be done to achieve very deep reductions over the long run). In terms of cost, improving fuel efficiency offers a greater carbon-cutting bang-for-buck than non-petroleum alternatives, at least for the foreseeable future. Nevertheless -- and especially in the face of globally growing transportation activity -- efficiency will not suffice for getting emissions down to the very low levels likely to be needed.

That leaves another question hanging: if getting off of petroleum isn't necessary but merely using it more efficiently isn't sufficient, then what?  Well, that is indeed a tough question, and it's a key question that the research discussed here will both address and pose as a challenge to others.

LCFS: some early history of the concept

The idea of regulating transportation fuels through lifecycle analysis (LCA) has now become widely accepted; it is the basis for California's low-carbon fuel standard (LCFS) as well as provisions of the renewable fuel standard (RFS) and other policies. Although my recent work criticizes the use of LCA to define policy, this disapproval reflects a major change of perspective from 15-20 years ago. Back then I was among the first to propose that LCA -- or "full fuel cycle" (FFC) analysis as we termed it -- would be a great way to regulate motor fuels in terms of GHG emissions. 

Time for realism on renewable fuels

This month, the Environmental Protection Agency upheld its requirement for blending ethanol into gasoline. Though not unexpected given the strength of renewable fuel interests, this decision ignored the pleas of 10 governors, almost 200 members of Congress and many Michigan businesses. With drought destroying much of America's corn crop this summer and Thanksgiving dinners costing significantly more since 2005, the downsides of renewable fuels became all too clear. Responsibility now falls to Congress to roll back the unrealistic renewable fuel goals set in the Energy Independence and Security Act (EISA) of 2007.

To understand how we got to this unhappy place, a bit of history is needed. Renewable fuels such as ethanol and biodiesel have long been hailed as alternatives to America's reliance on petroleum. The fuels bolster crop farmers' incomes and claim to protect the planet by recycling carbon from the air. As prices at the pump climbed over the last decade, biofuel proponents rallied support for a mandate to replace petroleum with home-grown biofuels.