by Graham starts a short series on the state of the art in appraising climate-change impacts of transport schemes. First up: getting to grips with embodied carbon
Prompted by last week’s useful and well-attended DfT webinar on carbon appraisal, I want to take a look at the state of the art and some current debates in this area. There’s a lot to cover, so I’ll break it down into a series of posts.
This first part covers the recent introduction of embodied carbon to appraisal practice in England, and some practicalities of estimating it in the early stages of a project. Subsequent posts will cover more of those practicalities and some wider issues.
Beginners start here
First, a quick recap on how it all works. ‘Carbon appraisal’ is our shorthand phrase for economic appraisal of a transport scheme’s greenhouse gas impacts and hence its climate change impacts. It’s actually a range of greenhouse gases, predominantly carbon dioxide (CO2), so it’s measured in terms of CO2 equivalent (CO2e). To appraise an intervention (which could be a policy or financial decision as wells as an infrastructure scheme), you estimate the amount of CO2e it will lead to, and then apply a standard set of £-per-tonne values. This produces the monetised value of the emissions impact, which goes into the ‘benefits’ side of the appraisal calculations (irrespective of whether it’s a positive or a negative number).
Now featuring embodied carbon
It’s now a year since the DfT’s Transport Appraisal Guidance (TAG) was changed to include embodied carbon (also known as capital carbon – the emissions from construction activities and materials) in appraisals. Previously, we only counted user carbon (typically the net change in tailpipe emissions) and ideally also some other categories such as operational carbon (from operations and maintenance). The changes also included a new ‘carbon summary table’ (CST), which does what it says on the tin, and a set of carbon metrics to be calculated. For the background and more on what these changes involved, see my earlier explainer here.
There’s always a transition period where appraisals already in progress continue under the old guidance. But by now we should expect most appraisals to be following the new guidance, including embodied carbon and reporting it all in a CST.
Early-stage estimates of embodied carbon
Methods and datasets for estimating user carbon are often well-established. For example, on road schemes of any size, even in the early stages, it’s normal to run a traffic model and feed the results into the TUBA software, which spits out not just the journey time and vehicle operating cost impacts (the main reason for using it) but also a figure for the carbon impacts. For many other schemes, the Marginal External Costs (MECs) approach provides standard methods and parameters for valuing the carbon impacts of mode shift.
Methods for embodied carbon, however, are newer to the transport planning profession. It’s been easiest for the later stages of the larger construction projects, where we can often borrow work that’s being done anyway for a carbon management plan (CMP) and/or an environmental statement (ES). Both of those activities have been a little ahead of us in calculating embodied carbon. With a bill of quantities (a parts and materials list) and an idea of the construction activities, they look up the emissions for each one, with a range of published or proprietary databases and ‘carbon calculator’ tools to choose from, and add it all up. For appraisal, we can borrow that figure and convert it into an appraisal value. (The ES will often borrow our user carbon calculations in return.)
It’s harder for smaller schemes that don’t have an ES, or in the early stages of those larger schemes. The issue is that we need a carbon estimate before the CMP or ES really get going – not just for appraisal, but even earlier in the optioneering stages where decision-makers are increasingly looking at carbon impacts of options when deciding how to tackle a problem. For example, on one recent project that I worked on, aimed at addressing congestion and growth issues, the range of options included a major road option, a public transport infrastructure option, and a more operationally-focused, low-infrastructure option: the relative carbon impacts of each was an important factor for the client team.
So what we need is ‘roughly right’ generic benchmarks or comparators that don’t rely on knowing quantities of specific materials or construction activities. There are already some benchmarks, such as those produced by Decarbon8 for road and rail infrastructure. And with a bit of digging you can look up comparators from other schemes that are further along the process and have done the detailed estimates. But it does need some digging – and to know what’s out there to use.
So for now, it’s a bit tricky. But people are doing it, and as we get more familiar with it as a profession and more benchmarks and tools come through, it will gradually become more normal and less scary.
A handy-looking new tool
At last week’s webinar, DfT briefed attendees on a promising new tool, the Local Transport Infrastructure Carbon Benchmark Tool (LTICBT). It does what it says on the tin, aiming to be a proportionate and standardised method for early-stage infrastructure carbon assessment. It covers 55 benchmarks across a range of local transport infrastructure interventions, ranging from a new footway to a new bus station, and including some highway treatments. It deliberately allows you to reflect the level of ‘carbon intensity’ in an intervention – for example the carbon in a cycle lane will vary depending on the level of segregation. It’s not online, but DfT say it’s available on request, free of charge. It looks handy, and I’m looking forward to trying it out.
Next in this series: part 2 – consistency and moving targets
