Mike passed me a recent report from the Intergovernmental Panel on Climate Change (IPCC) Working Group III (Mitigation of Climate Change). The IPCC is quite famous for its reports summarising the scientific consensus on climate science, so I was curious to see what the process and results of their follow-up reports looked like. I only read the transport chapter, since it’s the part I understand.
Overall, it’s a decent summary of current understanding of transportation trends, which is difficult to do at an international level with a wide spectrum of urban forms and demographics. The report includes a summary, and working group members vote on each paragraph to establish the level of knowledge and agreement on the report’s conclusions. One paragraph in particular struck me:

Providing public transports systems and their related infrastructure and promoting non-motorised transport can contribute to GHG mitigation. However, local conditions determine how much transport can be shifted to less energy intensive modes. Occupancy rates and primary energy sources of the transport mode further determine the mitigation impact. The energy requirements for urban transport are strongly influenced by the density and spatial structure of the built environment, as well as by location, extent and nature of transport infrastructure. If the share of buses in passenger transport in typical Latin American cities would increase by 5–10%, then CO2 emissions could go down by 4–9% at costs of the order of 60–70 US$/tCO₂ (low agreement, limited evidence).
[page 326, emphasis added]

I’m a little shocked that this paragraph garners low agreement and is considered to be backed by limited evidence. (I’ll exclude the final sentence, since I don’t know anything about that particular study.) The paragraph is already weakened by many words indicating uncertainty – “can contribute” … “local conditions” … “could go down” etc. But there isn’t even consensus with the weakened wording. I emphasised the one sentence on density and spatial structure – my personal research interest. Again, I’m astounded that there is still wavering about this subject.
That sentence represents one of the report’s only discussions of urban form. It appears occasionally elsewhere in the chapter, but the framing unfortunately focuses on transportation, and treats land use as fixed – a massive oversight. While there are occasional mentions of the value of “co-ordinating” transportation and land use, these are not quantified and do not make it into the conclusions of the report. There is a separate chapter on housing, but it focuses on building construction and energy consumption, again omitting urban form. As so often in the past, urban structure is forgotten and falls into the cracks between disciplines.
The report sensibly treats the US as a “special case” in the international context, since it’s so low density. (e.g., increasing transit service in many US cities could plausibly increase GHG emissions – if no new riders are attracted but more buses are on the road). But it’s a double-edged sword – it suggests that US cities can continue to follow an auto-dependent path, since the report doesn’t contemplate changing land use.
At the end of the day, though, the main problem is the inconclusiveness of the research – the focus on the exceptional context of the USA has too often limited researchers from observing the clearer trends in other parts of the world. Integrated land use/transport models are still too immature for this type of policy analysis, and international comparison studies remain plagued by data incompatibilities. Finally, the field rarely presents its results in a policy-relevant manner – I have never seen a transportation/land use report that estimated the cost of a policy in terms of US$/tCO₂-equivalent. It’s unfortunate – researchers in fields like biofuels are doing a lot of work to estimate greenhouse gas reductions, and their results are immediately relevant to policymakers.