Friday 4 June 2021

Bridge Maths

My excellent colleague Matt Burgess has done some fun back-of-the-envelope work on the carbon benefits of the proposed $685,000,000 bicycle bridge across Auckland Harbour. Some people have strongly favoured the bridge, noting its potential effects on reducing greenhouse gas emissions. 

Here's Matt's back-of-the-envelope reckoning. The strikeouts are because a first iteration of the work hadn't adjusted car emissions for average vehicle loadings; the updated version is so-corrected. But still back-of-the-envelope, order of magnitude stuff. 

The new Auckland crossing costs $685M. That is a daily cost of $112,500 at 6%. The minister justifies the bridge based partly on emissions.

Assume 10,000 one-way crossings/day and an average commute profile between Takapuna and Sky City = 8.7km.

Depending on whether each bridge user would otherwise have done their commute by bus or car, the bridge will avoid 8-23 8-14 tonnes of emissions per day.

So that is $4,800$7,800-$14,700/tonne abated.

To be competitive with the ETS at $37 would require 1.3 2.1-4.0 million crossings per day.

If bridge users would otherwise use the ferry, which may be the most likely alternative, then the bridge avoids just 0.5 tonnes of emissions per day at a cost of $237,000 per tonne.

He also notes that these kinds of cost-per-tonne figures would have serious consequences for New Zealand's ability to get to net zero, if we did it all via MAMIL-approved bike bridges (and the like) rather than by relying on the ETS.

The cost of getting long-lived emissions down to net zero is:

  • At $7,800/tonne: 94% of GDP
  • At $14,700/tonne: 176% of GDP
  • At $237,000/tonne: 2600% of GDP
The bike bridge is (roughly) 200-6300 times more expensive, as carbon mitigation initiative, than just buying credits in the ETS and running them through the shredder.

And so I fully expect that Rod Carr will strongly endorse the bike bridge in next week's Climate Change Commission report. 

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