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?