As we reported yesterday, Harlan Watson advised the media to be careful of accepting at face value the IPCC's work on climate stabilisation.
For industrialized countries to target a 25-40% reduction in greenhouse gas emissions by 2020 was premature, he suggested. Too few scenarios had been examined the make the figure reliable. Further analysis was needed before the figure could become a basis for negotiation.
So what lies behind his remarks?
The IPCC has set out six scenarios for stabilizing the global climate. Most attention has focused on scenario 1. This scenario would:
- Stabilize greenhouses gases at 445-490 ppm (expressed as a CO2 equivalent and including a correction for the cooling effect of aerosols - the level for 2005 was 375 ppm).
- Lead to an average global temperature rise of 2-2.4 degrees Celsius from pre-industrial levels - it's not really known how long it would take to reach this temperature.
Achieving this scenario requires:
- Emissions of carbon dioxide to peak by 2000-2015.
- Emissions to fall by 50-85% by 2050.
It is on this basis that the developed countries (barring the US and, until now at least, Australia) have agreed that, as a group, they will need to achieve cuts in the range of 25-40% by 2020, based on 1990 levels.
Harlan Watson's principal complaint about the science is that the IPCC's "analysis involved six scenarios out of 177" and would need further study before being accepted as a basis for negotiation.
A few points are worth bearing in mind about this claim:
First, if you go back to the full report of IPCC Working Group 3 from which these findings are drawn, you find that even if we stabilized at today's CO2e level, we'd have only an 80% chance of avoiding dangerous climate change (table 3.9).
Second, all the IPCC's estimates explicitly exclude any changes in the ability of sinks to soak up carbon dioxide. Include this factor and temperature rises are likely to be higher. According to the IPCC, "the emission reductions to meet a particular stabilization level...might be underestimated due to missing carbon cycle feedbacks."
Third, scenario 1 - the lowest level scenario - is based on only six studies, but the other studies inform progressively higher level stabilization targets. Eighteen studies for 490-535 ppm; 21 studies for 535-590 ppm; 118 studies for 590-710 ppm; etc etc.
And here's the important bit: each higher scenario supports the lower ones:
- For 490-535 ppm, emissions have to peak in 2000-2020 and fall by 20-60% by 2050.
- For 535-590 ppm, emissions have to peak in 2010-2030 and by 2050 need to be between 30% lower and 5% higher.
- Etc.
So in conclusion:
- All the signs from the IPCC's review of current knowledge are that these scenarios are more likely to underplay than overplay the cuts needed to avoid a 2 degree temperature rise.
- The toughest emission reduction scenario has more support than Watson lets on - and is consistent with the other scenarios. Maybe we don't know exactly how deep cuts will need to be by 2050, but the IPCC's conclusion that they need to peak as soon as possible seems to be supported by the evidence.
A few minutes ago, I asked the UN's Yvo de Boer what he thought of Harlan Watson's remarks. The IPCC had narrowed down a huge amount of information to something that is representative. "I think the IPCC has done its job," he said.