60%? I'll see that, and raise you 5%

A number, dare I say a plethora, of long-term greenhouse gas (GHG) emissions reductions plans have been proposed lately by politicians in the US, the UK and Canada.

Tony Blair got the ball rolling a couple years ago when he called for the UK to reduce GHG emissions by 60% below 1990 levels by the year 2050. This year, California Gov. Schwarzenegger (I’ll always get a kick out of writing that, seriously, who thought that two of the stars of Predator would become governors?) proposed GHG targets for California of 1990 levels by 2020, and 80% below 1990 levels by 2050. The CA legislature is set to debate a very similar proposal by a rival Democrat. The vote should happen for the Legislature breaks on Aug. 31st.

The proposals just keep coming:
- Oregon and New Mexico: 75% below 1990 levels by 2050.
- John Kerry (US Senate): 65% below 2000 levels by 2050.
- The Safe Climate Act (US House of Representatives): 80% below 1990 levels by 2050.

The Pew Center even put together a map.

The trend of tossing out big numbers has spread across the border into Canada. Michael Ignatieff, the Canadian MP and candidate for the leadership of the Liberal Party, has called to reduce GHG emissions by 50% below 1990 levels by the year 2050. The other leadership contenders, especially former Environment Minister Stéphane Dion and Bob Rae, are also expected to release similar plans in the coming days. It can be comical to hear Canadian politicians propose substantial long-term emissions reductions, when the more modest Kyoto goals have been ignored. Although, one could argue that the mistake by many on both sides of the aisle in Canada was viewing Kyoto as an endpoint, rather than the first step on a longer path to stabilization.

I imagine you’ll see a similar climatic chest-bumping when the battle for the Democratic nomination begins in earnest next year.

There is some real logic behind the various targets. The goal in designing a long-term policy should be an optimal path of emissions – the rate of flow into the bathtub each year – that ensures the level of GHG in the atmosphere – the amount of water in the tub – stays within a limit deemed acceptable. To use the lingo, the goal is to stabilize GHG concentrations at a level that avoids dangerous anthropogenic interference (DAI) in the climate system.

Now there has been plenty of discussion in the climate science and policy community as to what should constitute DAI. The most commonly proposals are (enough residual warming in the climate system to cause) the collapse of an ice sheet, a shift in ocean circulation, extreme or recurrent drought threatening food production, or widespread damage to the world’s coral reefs. The definition of what is ‘dangerous’ allows some back-calculating or modeling of the ‘allowable’ emissions (e.g. to 2050).

Often ignored in the discussion is the fact that, since GHGs like carbon dioxide can remain in the atmosphere for a long time, the path of emissions over time, not just the end result, also matters. Most of the well-conceived policy proposals include a series of targets chosen with the aid of simple climate models.

It appears that the different proposals are aimed at stabilizing CO2 at 500-550 ppm in the atmosphere, roughly a doubling of the pre-industrial level. My guess is the scientific advisors in each case argued that further increases would cause the climate to warm past thresholds for the ice sheets and the slowdown of the ocean’s thermohaline circulation (I doubt corals got much play). An optimist could claim the difference between the endpoints of the various plans – 65%, 80%, 50%, 75%, 60% -- was simply the choice of model in each case.

It is crucial to set the right targets. But, in the end, designing the policy is the easy part. Voters in the Liberal party, in California, and in the 2008 primaries, should not be seduced by the candidate offers to jump the highest, but the candidate that offers the best implementation plan.