Setting targets for greenhouse gas reductions has turned into a house of mirrors. It’s hard to know what anyone means when they talk about an “80% reduction” in emissions. Reader Steve Bloom raised this point in response to my January 15 post. It’s an important one.

Most of California’s targets are based on 1990 levels (also 80% by 2050). On the other hand, The USCAP plan announced last month by a national coalition of business & environmental groups, also aims for an 80% reduction by 2050–but from 2005 levels. That 15 years between 1990 and 2005 is hardly trivial. Much of the explosive development in China and India occurred during this time, as U.S. emissions were also rising.

The number that will matter the most is the one that comes out in the federal legislation, which is still being drafted. In his video address to the Governor’s Climate Summit in November, President (then-elect) Obama appeared to be using California’s aggressive goal as a benchmark when he promised to “set us on a course to reduce emissions to their 1990 levels by 2020, and reduce them an additional 80% by 2050.”

We’re hearing more voices saying that California’s 2020 target (about 15% from today’s level) is unobtainable, Stanford researcher Steve Schneider being a recent example (see Gretchen Weber’s post from 1/30). As a practical matter, this would mean cutting California’s per capita carbon footprint from 14 tons per year, down to about ten.

Lately more people seem to be looking toward the 2050 target of an 80% reduction. But for national policy, the question is still sort of hanging out there: 80% of what? It’s one that will have to be answered soon, as congressional leaders press to have a climate bill ready by Memorial Day.

Photo by Reed Galin