Measuring Environmental Performance: One Number Is Not Enough

In his article ‘Saving the Planet by Numbers’, David MacKay offers the quantitatively minded enviro-skeptic a new energy standard for comparing the relative performance of green actions and technologies to each other and the status quo. (MacKay, David. (April 23, 2009). “Saving the planet by numbers” BBC News website).

Unfortunately, a single measure of environmental performance usually is not enough to tell the whole story. MacKay proposes a new standard for putting, “. . . green lifestyle ideas under the spotlight.” This standard would measure energies in Kilowatt-hours and how fast activities use or produce energy in Kilowatt-hours per day, or some other unit of time or distance. He provides an overview of how this standard might be used to compare activities and technologies for four categories: ‘Simple Individual Actions’ like you might find on green top-ten lists; ‘Transport’; ‘Heating’; and ‘Decentralized Power Generation’.

For example, under the ‘Transport’ category, MacKay reports that the average fossil-fuelled British car outperforms hydrogen vehicles at 80 KWh/100 km versus 254 KWh/100 km, respectively. He also includes the performance of electric vehicles (6-20 KWh/100 Km) noting that “. . . even if electricity comes from traditional fossil-fuel power stations, electric cars are still more efficient than petrol cars.”

Multiple Measures of Environmental Performance Help Us to Assess Trade-offs Between Alternatives

Unfortunately, a single measure of environmental performance doesn’t tell the whole story. For example, MacKay’s single-number energy standard can’t be used to compare the relative performance of the transport options in terms of reduced emissions of carbon dioxide.

Since we are usually interested in more than one thing when we address environmental concerns, looking at multiple performance measures can provide a clearer picture of the relative tradeoffs inherent to alternative options, transportation-related or otherwise.

Comparing PHEV Impacts Using Ground-Level Ozone Production and Carbon Dioxide Emissions

This is illustrated by the multiple performance measure approach taken in a paper I recently reviewed that used simulation modeling to explore the impact of large-scale use of plug-in hybrid electric vehicles (PHEVs) on air quality: it compared both ground-level ozone production and carbon dioxide emissions across a range of alternative PHEV-use scenarios (see here).

Interestingly, not only did the researchers find that the large-scale use of PHEVs reduced ground-level ozone production, the main focus of their study, but also that carbon dioxide emissions were lower too.

The latter result was somewhat counterintuitive since the virtual PHEVs were charged using electricity from coal-fired generating stations, increasing the greenhouse gas emissions of these facilities above baseline conditions.

The reason was that the large-scale replacement of gasoline-powered vehicles with PHEVs drastically reduced total car-related emissions, more than compensating for the increased emissions from the coal-fired stations.

Multiple Performance Measures Can Also Increase Confidence in Results

This result would have been missed by focusing on ground-level ozone alone as a performance measure. Thus, in this case at least, using multiple performance measures helped build a stronger-case for the expanded use of PHEVs in areas with power-grids fed with electricity from fossil-fuelled generating stations.

My intent here is not to detract from MacKay’s proposed energy standard. It looks like a useful tool for comparing the energy performance of green actions and technologies – in conjunction with other measures. But I think it’s important to remember that environmental choices usually consider, explicitly or implicitly, multiple objectives and values. Focusing too narrowly on a single measure of performance leaves us open to wrongly interpreting results and can also preclude the opportunity to learn. Cheers.

References for this post:
MacKay, David. (April 23, 2009). “Saving the planet by numbers” BBC News website.

“The copyright of the article ‘Do Measuring Environmental Performance: One Number Is Not Enough’ on ‘What’s Your Ecotype?’ is owned by Ian Parnell. Permission to republish ‘Measuring Environmental Performance: One Number Is Not Enough’ in print or online must be granted by the author in writing.”