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.