Saturday, November 24, 2007

Has Portland Really Cut its Greenhouse Gases?

Once again Portland, Oregon has claimed to have significantly reduced its greenhouse gas emissions, but once again the claim is subject to a lot of questions.

You might recall that this happened with its last report in 2005, when an error was found nullifying the same claim.

This past week Portland (well, technically Multnomah County, though they are nearly the same place, and I will use them interchangeably here) issued a report on its 2006 greenhouse gas emissions. (The report, titled "Portland Global Warming Overview," November 2006, is available on this page. It's a PowerPoint presentation. There is a written report, though they say it's still a draft.)

The Portland Tribune wrote a news story about the report last week.

Basically, Portland's claim is
Greenhouse-gas emissions... are down to just 0.1 percent above 1990 levels in Multnomah County, according to the city’s Office of Sustainable Development.

That decrease, city officials said, comes despite the area’s 15 percent population growth since 1990.

Here's a graph. Here are some numbers from the report, for Multnomah County:

TOTAL GHG emissions, 1990: 9.78 MMT CO2e
TOTAL GHG emissions, 2006: 9.79 MMT CO2e

(Units are million metric tons of CO2-equivalent). Per resident the numbers are:

per capita TOTAL GHG emissions, 1990: 16.8 MT CO2e
per capita TOTAL GHG emissions, 2006: 14.4 MT CO2e

So while total GHG emissions in the US are up 17% from 1990 levels, Portland has essentially stayed the same, despite a 15% increase in population over that time, and per capita emissions are down 14%.

Is this believable?

The report attributes the decrease to a variety of factors, such as increased use of public transportation (up 90% since 1990), a recycling rate of 63% ("the highest in the nation"), a quadrupling of the bicycle commuting rate to 4.4%, green buildings, planting more than 750,000 trees and shrubs since 1996, and other factors (see the draft report, p. 2).

But there are some questions.

1) diesel: Transportation accounts for about 40% of local greenhouse gas emissions, the largest sector, but Portland's numbers do not account for diesel fuel sold/consumed--that number is only available on the state level, they say. How big an error is this? At least 10%, I think.

Here's my reasoning: 130 B gallons of gasoline are sold in the US in 2006, and 50 B gallons of diesel. Gasoline emits 8.8 kg/gal CO2, while diesel emits 10.1 kg/gal. (Though diesel cars tend to get better gas mileage, so per-mile the emissions are about the same.) So total US CO2 emissions in 2006 were:

gasoline: 1214 MMT
diesel: 505 MMT

Let's assume that, per resident, Portland residents use the same amount of diesel as US residents. (This might not be exactly true, as Portland is high in hybrid car registration, but it seems to me the assumption should be approximately true.) The US population in 2006 was 299,398,484, and Multnomah County's was 681,454. Taking the ratio we get

gasoline emissions, Multnomah County, 2006: 2.8 MMT
diesel emissions, Multnomah County, 2006, 1.1 MMT

Diesel emissions are 42% of gasoline's emissions, and 11% of the claimed GHG emissions.
Including diesel fuel, Portland's emissions would be 11% higher than claimed, or 10.9 MT/resident.
2) Airplane emissions: The report excludes air traffic, though (according to the Tribune) CO2 emissions at Portland International Airport were 0.45 MMT in 2005. That's a additional 5% error.
Including diesel fuel and airplane traffic, Portland's emissions would be 16% higher than claimed, or 11.4 MT/resident.
3) estimation of transportation-related CO2 emissions: Total transportation emissions for Multnomah County given by the report are:

1990: 3.79 MMT CO2e
2000: 3.95 MMT CO2e
2006: 3.89 MMT CO2e

It appears that most, if not all, of the criticisms of the Cascade Policy Institute still stand: the city is using gasoline sales as a proxy for miles driven, even though the manufacturers of the emissions software program used by the city say not to do this. For full details, download the Word document in the 6th paragraph of this Cascade Policy Institute article -- the remainder of this document outlines several reasons why one should be quite skeptical of the claim that 2006 transportation emissions are only 2.6% higher than 1990 emissions, though population is up 15%.

(And if you're wondering, fuel efficiency is essentially unchanged over this time period, nationwide.)

Car registrations are up, gasoline sales in a neighboring county are up sharply since 1990, total commuters in-and-out of Multnomah County are up sharply, total miles driven in the state of Oregon are up...and more.

Has public transportation, bicycling, and compact zoning really made that much of a difference? It seems hard to believe. The Tribune reports:
Armstrong admitted that the gas numbers are not perfect, but said better fuel-efficiency has more than compensated for those increases.
But, as I noted above, fuel efficiency is not any better since 1990, nationwide. Is Portland's higher hybrid registration rate really that much of a factor?


Of course, you might argue that what matters is not the absolute numbers but the trend, and as long as you are consistent in your methodology (consistently excluding diesel fuel sales, for example) you can at least extract the trend from the data. That's not a very scientific or analytic argument, though, given the complexity of the situation. So I don't know. At this point I'm not sure anyone can make any conclusions about the numbers given the significant gaps in the methodology.

People like Nicholas Kristof will probably still buy the claim, though.

1 comment:

John Fleck said...

Here in Albuquerque, we seem to be kicking Portland's ass. According to our mayor, we've cut greenhouse gas emissions 64 percent from their 1990 levels. Our maybe, just maybe, that claim is not believable? Maybe some enterprising Albuquerque reporter should look into this further? :-)