Hill Heat

The Lieberman-Warner cap-and-trade bill (S. 2191), which Sen. Boxer said may come to the floor before June, sets a cap of 15% below 2005 emissions levels by 2020 for covered sectors, reducing allowed emissions to the amount last seen in 1990.

Is that near-term target sufficient, in terms of the science?

As Holmes Hummel points out, the IPCC Fourth Assessment Report (AR4) paints a much different picture.

At Bali, all of the Annex I signatories to the Kyoto Protocol (every industrialized country other than the US and Turkey) agreed to this roadmap, which states in convoluted language that the Annex I countries “noted” that the AR4 indicates that global emissions “need to peak in the next 10-15 years” and be reduced “well below half of levels in 2000” by 2050 “in order to stabilize their concentrations in the atmosphere at the lowest levels assessed by the IPCC to date in its scenarios.” The countries also “recognized” that the AR4 indicates that to achieve those levels “would require Annex I Parties as a group to reduce emissions in a range of 25–40 percent below 1990 levels by 2020.”

25-40% below 1990 levels is dramatically below the Lieberman-Warner target. From AR4, these “lowest levels” of concentrations are 350-400ppm CO2.

What’s the value of achieving concentrations “at the lowest levels”? The report says that using the “best estimate” for climate sensitivity (the temperature response to greenhouse gas concentrations), reaching a stable concentration of 350-400ppm CO2 leads to 2.0-2.4 degrees C warming above pre-industrial levels. But Hummel notes that the “best estimate” is just one for which half the estimates are higher and half are lower.

Thus:

To have a 50% chance of making the 2°C stabilization target, global emissions need to peak by 2015 and Annex I countries need to be 25-40% below 1990 by 2020.

As AAAS president John Holdren argued in his speech Meeting the Climate Challenge (at 38:29; see also the slide presentation):

The chance of a tipping point into truly catastrophic change grows rapidly for increases in the global average surface temperature more than about 2°C above the pre-industrial level, and again we’re already committed basically to one and a half. For a better than even chance of not exceeding 2°C above the pre-industrial level, CO2 emissions must peak globally no later than 2025 and they need to be falling steadily after that. That is a great task.

From the UN Scientific Expert Group on Climate Change and Sustainable Development, an international panel of 18 top scientists (including John Holdren):

In our judgment and that of a growing number of other analysts and groups, however, increases beyond 2°C to 2.5°C above the 1750 level will entail sharply rising risks of crossing a climate “tipping point” that could lead to intolerable impacts on human well-being, in spite of all feasible attempts at adaptation.