Saturday, November 7, 2009

Climate change

It's right there at the top of the blog: getting people to shift from cars to transit will reduce pollution. Transit is regularly cited as a potential strategy for reducing greenhouse gas emissions. But how much could we reduce with transit?

It's funny, I haven't been able to find a good answer to that question. Many of the papers and websites are full of nutty things like electric cars and pumping carbon dioxide underground. But I did find a few useful figures around the web. Still, this is a back of the envelope calculation, so make sure you stick lots of grains of salt onto it. Any pointers to better figures would be welcome.

First of all, what kind of decrease are we talking about? Well, no one seems to know. The current concentration of carbon dioxide in the air is about 390 parts per million, and the scientific consensus seems to be that we need to get it down below 350 ppm as soon as we can. More timid people have pushed for 450 ppm or even 550 ppm. Usually it's expressed in terms of reducing greenhouse gases by 20% by a certain date (say, 2030), and 80% by another date (say, 2100).

How much of that is transportation-related? Well, this paper from the EPA (PDF, page ES-15) says that in 2007 the US produced 7,150 million metric tons of CO2 equivalent (which includes things like cow farts and laughing gas), of which 2,000 megatons were from transportation (up from 1,547 in 1990). This includes the emissions involved in generating electricity to power subways. So if we could eliminate all transportation-related greenhouse gas emissions, it would reduce our total by 29%. Well, it's a start!

Okay, seriously, how much of those two gigatons could we eliminate through transit? Again, people were very timid about this. A report by SAIC for the American Public Transit Association (PDF) says that two-car households could reduce their carbon footprint by 30% if they use enough transit to get rid of one of the cars. Lame! I want to know what happens if they get rid of both cars!

Transit advocates seem very keen on telling you how much greenhouse gas would have been emitted if every transit rider had driven instead; the SAIC report gives it as 6.7 megatons, and on page 9 of this PDF from the APTA puts the figure at 4 to 25 megatons. Sounds impressive until you remember that the total annual transportation emissions are 2,000 megatons. You can kind of understand why they don't put it that way.

Still, I'm happy for transit that it's sparing us from all those gases, but even though it's labeled in the SAIC report as "Potential Role of Public Transportation in Reducing CO2 Emissions," it's not. It's the current role. What is the potential role? Well, two University of South Florida researchers interpret the National Household Travel Survey to indicate that the mode share of transit is currently "1.59% of person-trips." In other words, one sixty-third of the total person-trips. So if we multiply that figure of 25 Mt by 63, we get 1,575 Mt.

So shifting the entire population of the US to transit would eliminate three-quarters of our transportation-related greenhouse gas emissions, and 22% of the total US emissions. How do we manage that? This APTA report (PDF) argues that a 10% annual compound growth in transit ridership would reduce annual carbon emissions by 142 Mt in 2020 and 910 Mt (13% of the total 2007 US emissions) in 2040. I personally think their model is limited and would like to see a logistic model that takes into account the increasing difficulty of shifting people to transit, but maybe that will appear later. It probably doesn't affect the estimates for 2040 significantly.

Since the transportation sector only accounts for 29% of greenhouse gas emissions, other contributions will probably have to come from the commercial, industrial and residential sectors (agriculture is relatively small). But we can do a lot with transportation, if we can find the will.


Alon Levy said...

Transportation is the hardest in the sense that there's no good alternative to gas-powered cars yet for suburb-to-suburb transportation and family vacations. Industry and residences can be powered by renewables (and are when it's cheap enough, as in Iceland) - though some industrial emissions are not related to power generation and would remain.

But the biggest obstacle is converting the grid to zero-emissions. The main problem there is cost rather than convenience or battery capacity, but it's a large cost. Unlike with transportation, there isn't a decades-long history of good planning telling people how to do it right, beyond "Don't build nuclear reactors with positive feedback loops."

W. K. Lis said...

One problem is the road designs of many suburbs. Cul-de-sacs looping away from shops, schools, and transit encourage the use of the automobile. There is also a lack of sidewalks, forcing many to walk on the roads.
First step is to build sidewalks and walkways cutting through the cul-de-sacs.

nathan_h said...

Consider also that most of the possible futures where all people take transit also involve people living in smaller houses, benefiting from shared heating in multi-family buildings, and so on. By reducing consumption in one dimension we end up reducing it in others. It's going to be very hard to put a number on that overall reduction, but I think if you look at actual differences in carbon output per capita in cities vs. sub/ex-urbs, you could get an idea of what it would look like if the rest of the country were re-organized back into small towns with decent transit.

Alon Levy said...

W. K. Lis, there are many suburbs with street grids, which despite being denser than the cul-de-sac suburbs are still auto-dominated. For example, White Plains: it has a street grid, but the streets are so wide and designed for so high a speed that they discourage pedestrianism.

Scott said...

It's a Land Use Issue.....
I like reading transit blogs on the internet and one that I always enjoy is Cap'n Transit's blog. In a particular post he is discussing the benefits of transit, notably the amount of carbon that is not burped out of our tailpipes, by taking transit. He goes through some calculations and derives the savings if everyone in the U.S. gave up personal motor vehicles and switched to transit:

(you are quoted here)

As much as I want transit to be a part of the solution to anthropogenic induced climate change, myself not wanting to see the South Carolina Low Country inundated by rising seas and all, for me transit represents how we value our land and what we end up doing with it. Although I didn't go through Cap'n Transit's sources, I wasn't sure if the net carbon savings took into account how poorly planned our cities are in their current auto-centric form and how much more we would save if we included density into his equation matrix?

When I think about benefits to investing in transit, I see cities you can walk or bike in, farms on the outskirts of town that can provide a decent percent of an area's nutritional requirements and wilderness areas that are preserved environments, managed with many uses (recreation, preservation, biodiversity, sustainable resource extraction, water quality, seedbank, etc.) in mind. I see highway urbanite (concrete & asphalt) ground up for use as railroad track ballast, returning previously fragmented environments to be stitched back together.

I think that transit, if properly implemented in a more holistic fashion, could make cities and towns whole again, eliminating wasteful life patterns, such as the twenty mile trip to the grocery store, the 2000 mile Caesar Salad or the 2 hour gridlocked commute.

.....maybe it's a quality of life issue as well?

Cap'n Transit said...

Thanks for the write-up, Scott! The APTA does address this on pages 46-51 of this PDF.

I agree with W.K., Nathan and Alon on the sprawl issue: Cul-de-sacs are bad for walkability and transit use, but so are traffic sewers. We can work on improving transit at the same time as we deal with this issue. Also, the more popular transit gets, the less people will want to live in unwalkable sprawl. But it is definitely a "virtuous cycle" feedback loop.

arcady said...

Having higher density of land use not only makes transit more viable, it causes some trips to just vanish entirely, as people become able to walk to more places. In the "carbon reduction" scheme of things, transit should be viewed as a necessary evil for commuting, with the only truly zero-impact form of transportation being walking.

fpteditors said...

These comments are correctly pointing out that the problem is not just the auto, but autosprawl. The spreading out of buildings and parking lots waste energy and misuse land. The fix should be applied in the city and town centers with free public transit. The suburb subsidy advantage will then gradually wither. Great blog and excellent discussion!

Jarrett at said...

I think what they mean is not "potential" as in "possible future" but as in "hypothetical extreme interpretation of the present", i.e. assuming "if everyone now on transit drove a car." Problem is, we know that's not the case. Many of them don't have licenses, or cars. Plenty would walk or cycle.

Real assessments of climate benefits are hard for some of the reasons you capture. If you insist on direct impact the benefit can look tiny. Indirectly, though, there's a huge climate benefit to broad adoption of sustainable urbanism, and transit is a key part of that package. So that's where we have to put our money, in my view.

Scott said...

Thanks for the link Cap'n.

BruceMcF said...

What arcady said ... on the one hand, not all of the 2,000 megatons of CO2 emitted by transport is local transport ... but on the other hand, getting 40% of local transport onto transit could well have secondary CO2 emissions impacts that are greater than the primary impact, both in terms of CO2 impacts of residences and commercial property, and in terms of "need" for travel miles removed from the system by clustering of destinations.

A better whole systems approach would be to identify the CO2 emissions that go with the whole settlement system - powering transport and built structure alike - for various levels of clustering, and use that to identify the CO2 impact that goes along with the whole package that would be both implied by and supported by getting 20%, 40% and 60% of local transport on transit (I stop there since I'd expect 60% local transport on transit would then mean 20% on foot or bike and that only leaves 20% for private cars, shared cars and taxis).

Alon Levy said...

On the third hand, Singapore has per capita emissions above the average of most developed countries, despite Manhattan-level car ownership. The reasons? It has a lot of heavy industrial pollution, and generates its electricity from methane trapped in waste.