April 24, 2018

Is taking a seaplane from Seattle to Vancouver greener than driving?

This week Seattle’s Kenmore Air announced that they will be partnering with Vancouver’s Harbour Air to start a scheduled seaplane service directly between downtown Seattle and downtown Vancouver, BC. I was asked whether taking a seaplane for this trip would be more environmentally friendly than driving. As much as I love floatplanes, the answer is almost certainly not.

A couple of years ago I looked at the GHG emissions from a scenic flight around Seattle in one of Kenmore Air’s DHC-3 Otter seaplanes. Applying the same basic methodology, but assuming a one hour cruise time for the flight from Seattle to Vancouver, I estimate 300 – 350 lbs of fuel burn for a one-way flight. If we take the low end of this range and assume that the planes are always full (10 passengers), the greenhouse gas (GHG) emissions per person could be as low as 50 kg CO₂-equivalent for the one-way trip. Using the higher fuel burn estimate and assuming a load factor of 0.5 (5 passengers) means emissions of 117 kg CO₂-equivalent per person.

We can also estimate emissions of criteria air pollutants for the trip. I do this here for nitrogen oxide (NO_x) emissions, as an example. The International Civil Aviation Organization (ICAO) documents emission factors for larger turbojet and turbofan engines used on commercial aircraft, but this does not include the smaller Pratt & Whitney Canada PT-6A turboprop used on the Otter. Extrapolating from the average emissions indices from these larger engines, we might assume that the Otter generates somewhere in the range of 10-30 grams of NO_x per kilogram of fuel burned. This suggests emissions of something like 140 – 950 g of NO_x per person per trip, again depending on fuel burn and load factor, as well as the actual NO_x emission factor.

What about driving?

We can contrast this with the emissions from driving. If we assume a car that gets 30 mpg on the highway, with a solo traveler, the emissions per person for the 143-mile trip from downtown to downtown would be 51 kg CO₂-equivalent. For a 45-mpg car (e.g. a Prius) with 4 occupants, the GHG emissions drop to just 8 kg CO₂-equivalent per person for the one-way trip. According to the Transportation Energy Data Book (36th edition), light-duty vehicles in the US released 2.29 million short tons of NO_x while traveling 2.71 trillion miles in 2014, for an average emissions rate of 0.77 g/mile. This translates to 110 g of NO_x for the trip, or 27 – 110 g / person depending on occupancy. In reality, the people who can afford a float plane flight on this route probably own newer (and therefore cleaner) cars than average, so the actual NO_x emissions from driving would probably be substantially lower in most cases.

Or the train?

We can also compare against the emissions from taking the train. According to the Transportation Energy Data Book (36th edition), the energy intensity of Amtrak travel averages 1,663 BTU / passenger mile. Amtrak’s Cascades line, which serves the Seattle – Vancouver corridor, has a load factor of 54%, slightly higher than Amtrak’s overall average of 51% (see p. 7 of Amtrak’s Monthly Performance Scorecard), so we can adjust the energy intensity for the Cascades line to 1,663 x 51/54 = 1,571 BTU / passenger mile. Multiplying by 0.001055 MJ / BTU and 86 g CO₂-equiv / MJ, and assuming the rail distance is also 143 miles, yields an estimated 20 kg CO₂-equivalent per person for the one-way trip.

We can check these numbers against an analysis by MJ Bradley & Associates (see p. 27). They estimated that for the 187 mile trip between Seattle and Portland, Amtrak generates 22 kg of CO₂ and 350 g of NO_x. We can multiply these by 143 / 187 to estimate emissions of 17 kg of CO₂ and 270 g of NO_x per person making the Seattle – Vancouver trip.

Or the bus?

The MJ Bradley report also estimated emissions for bus (motor coach) travel. Adjusting their results for Seattle – Portland for the Seattle – Vancouver distance, we can estimate emissions of 7.5 kg of CO₂ and 62 g of NO_x. per passenger for the one-way trip.

Summary

In short, it appears that emissions of GHGs and NO_x from flying on a seaplane from Seattle to Vancouver are almost certainly going to be higher than those from driving. If the seaplane service is running at full occupancy, then the emissions per passenger trip for a seaplane might be only slightly higher than driving alone. But if the aircraft are running less than full, and if a car trip can be shared between multiple travelers, then driving will produce much lower emissions per passenger trip than flying. Among the shared modes, the bus appears to be the greenest option, with the lowest per-passenger emissions of both CO₂ and NO_x. Taking the train would probably produce a similar amount of NO_x as flying, but considerably less GHG emissions.