As the nations represented at Copenhagen debate proposals for reducing their absolute emissions or carbon intensity, I was reminded that we shouldn't just focus on the percentages being offered, but also on the reference year emissions on which these proposals are based, since these aren't necessarily comparable. That's particularly true for the US and EU, which have consistently referred to 2005 and 1990 base years, respectively. This distinction is crucial in understanding how much of what each party might commit to has already been achieved and how much remains to be accomplished.

Start with the EU, which has proposed a reduction of 20% versus its 1990 emissions levels. Based on a recently-issued report by the European Environment Agency, the 27 countries constituting the European Union today have already reduced their emissions by 10.7% as of last year, compared to 1990. According to this report their collective emissions in 2005 were 9.3% below 1990, so that 20% figure for 2020 really translates to roughly 12% below 2005--and only a bit more than 10% below where they are now--which is substantially less than the 17% reduction that the US has put on the table. However, the US proposal should also be put into context, because of the divergent emissions trends of the two regions since 1990, which was the base year for the Kyoto Protocol that has guided EU policy in the intervening years, but which the US never ratified.

Between 1990 and 2005 total US greenhouse gas emissions increased by 16.5%. After counting net emissions including all sources and sinks--mostly natural processes such as forestry that absorb these gases--the increase was about 14%. So either way, that 17% cut that US negotiators were authorized to take to Copenhagen is really 1990 less about 3-5%. At the same time, 17% vs. 2005 is no slam dunk, even if it appears that the recession helped ax 2% of our emissions as of last year, and this year could be down a bit more. A recovering economy will use more energy and emit more GHGs, even if only from the work commutes of millions of re-employed people.

While I don't expect nearly as much controversy over the choice of baseline years as surrounded the Kyoto negotiations, 1990 and 2005 represent very different worlds, with the former largely pre-dating the collapse of the high-emission Soviet bloc economies, giving rise to all those Russian "hot air" allowances and a major portion of Germany's cuts post-reunification, along with a big shift in UK power generation away from coal and toward natural gas. Although the EU has certainly instituted comprehensive policies to reduce its emissions, including a cap on industrial emissions and a union-wide Emissions Trading System, a large chunk of the reductions for the current membership were achieved before any of these policies went into effect, through the rationalization of the inefficient economies of formerly-communist Central and Eastern Europe. 1990 looks even less relevant to the current economies of large developing countries like Brazil, China and India.

On the other hand, while 2005 has much to recommend it as probably the most recent year for which fully-audited emissions data are available globally, it also represents nearly the high-water mark of a world of easy money and massively-globalized supply chains that may never return in quite the same form. Choosing it might also appear to let the US off the hook for a decade of relative inactivity on climate change, though that ignores the fact that our actual emissions have grown by much less than the 33% business-as-usual increase that was expected in the late 1990s, presumably because of the discipline imposed by the steadily-rising energy prices that accompanied our bubble economy of recent years.

Whatever emerges as the baseline for an agreement or framework coming out of Copenhagen, it ought to be a single year that provides both ease of comparison and a reasonable congruence with the realistic starting point for any actions that countries will commit to undertaking in the years ahead.

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