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A useful metric for vehicle efficiency is miles per gallon, kilometers per liter, or liters per 100 km. The amount of gasoline used by a given vehicle can easily be understood and compared.

Energy use intensity is a similarly useful and intelligible method for understanding and comparing energy usage of buildings.

However, metrics for setting greenhouse gas emissions reduction targets are varied and confusing, and based on the inherently context-dependent percentage:

  • National Grid announced a target today to reduce emissions by 20% (compared to 1990 levels) by 2050.
  • In 2015 the U.S. pledged to reduce emissions by 26 to 28% (compared to 2005 levels) by 2025.
  • The E.U. has a target of a 40% reduction (from 1990 levels) by 2030.
  • This article in Energy Live News discusses an increase in the U.K.'s target to 43% reduction by 2030, but doesn't mention the baseline year (or what the old target was, for that matter -- this could simply be an example of shoddy journalism).
  • Oil company ConocoPhillips has a (not so) long-term target to reduce "emissions intensity" by 5 to 15% (compared to 2017 levels) by 2030.

With different target years, baseline years, and definitions of "emissions," these targets cannot be compared in a straight-forward way. National Grid's 20% reduction sounds better than ConocoPhillips' 5 to 15%, but the latter is targeted for 20 years sooner. Additionally, the choice of baseline year seems arbitrary and a convenient way to game the system.

Imagine trying to buy a car when one manufacturer tells you the gallons per 100 miles, one gives average commuting time per tank, and another lists wheel revolutions per liter.

So, does there exist a metric for greenhouse gas emissions target that is robust and easy to understand and compare? Could the examples given above be translated into this metric in order to compare them?

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Great question! I'm not sure if there is a globally agreed-upon standard, but here is what I would propose.

A Shared Baseline

The first problem is that all countries and cities are approaching the task of reducing emissions from different starting points. Some countries have been very wealthy with high emissions, and others have been very poor with low emissions. It doesn't seem fair for both countries to reduce their emissions by 50%, because the initial imbalance persists.

Therefore, we have the concept of one-planet living **, which is defined as the level of material consumption and waste production per capita that could be enjoyed indefinitely by everyone on Earth without jeopardizing global life-support. It doesn't matter exactly where we draw this baseline, as long as it is used consistently between countries and cities.

The Sustainability Gap

The difference between what we use and what is available to us on an equitable basis (one-planet living) is the sustainability gap. Using a globally shared baseline means it is not possible to game the system by selecting a year in which national or citywide emissions were particularly high or low. This prevents gaming the system through choice of baseline.

The magnitude of the sustainability gap provides perspective on how much change is needed for a particular city or country.

Gap Points Per Annum (GPA)

Then in order to factor out the effect of target years, we should also report targets in terms of annual rate of change. For this, I suggest "gap points" per annum

For example, consider a country where the citizens are using twice their share of resources. The country is using 200% of resources compared to one-planet living, which means the sustainability gap is 100%. If they set a target to completely close that sustainability gap in 20 years, that could be presented as 5 gap points per annum (5 gpa).

This makes it much easier to compare annual progress! For example, a short-term target of 50 gap points in 10 years would be the same rate: 5 gpa.

One thing this measure would not represent is the relative difficulty of implementing measures to achieve reduction targets. It's going to be much, much easier for a city with 400 gap points to reduce 5 gpa than a city which is exactly at the baseline.

** (This wording was borrowed from Worldwatch International.)

  • I like this idea... I wonder how other factors could be included -- for instance, living near the coast is inherently more sustainable than living in the desert. Could a coastal city and a desert city somehow "trade" gap points? – LShaver Oct 25 '18 at 14:54

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