The “25-40/2020” scenario was published in the IPCC’s 2007 Fourth Assessment Report Working Group III report, in Box 13.7 on page 776, “The range of the difference between emissions in 1990 and emissions allowances in 2020/2050 for various GHG [greenhouse gas] concentrations for Annex I and non-Annex I [Kyoto] countries as a group”, where targets were given for stabilisation at 450, 550 and 650 ppm CO2e.The targets for 450 CO2e were:
Region: Annex 1 countries
2020: –25% to –40%
2050: –80% to –95%
Region: non-Annex 1 countries
2020: Substantial deviation from baseline in Latin America, Middle East, East Asia and Centrally-Planned Asia
2050: Substantial deviation from baseline in all regions
25-40/2020 subsequently became a focus of debate at the COP 13 (Conference of the Parties to the UNFCCC) meeting in Bali in December 2007, and then the principal advocacy target in 2008 in Australia for organisations such as the Climate Institute, the World Wildlife Fund, the Australian Conservation Foundation and the Climate Action Network Australia, and for a number of climate scientists who entered the public policy debate. Garnaut talked about a 450ppm target requiring Australian emissions to be reduced to 25% below 1990 by 2020 (noticeably dropping the upper range of 40% range, a sleight of hand he was not alone in undertaking). So how might we assess such an advocacy proposal?
The temperature increase: Analysis for the 2006 Stern report (p. 195) shows that, taking uncertainty about climate sensitivity into account, a 450ppm CO2e target has:
• A 26–78% probability of exceeding 2 degrees
Celsius (˚C) relative to pre-industrial
• A 4–50% probability of exceeding 3˚C
• A 0–34% probability of exceeding 4˚C
• A 0–21% probability of exceeding 5˚C
Using a risk-management approach, it cannot be said that this is even a 2˚C target. And 2˚C is far, far too high, given the now clear evidence that at less than 1˚C of warming we are already on the precipice of climate catastrophe, from the Arctic to the Great Barrier Reef, from the Himalayas to Siberia.
Climate sensitivity: The work of the IPCC generally assumes a climate sensitivity (CS: that is, how much temperatures would increase with a doubling of GHG levels) of 3˚C. Whilst “short-term” CS is well established at 3˚C ± 0.5˚C, there is now a very convincing case that long-term climate sensitivity (including “slow” carbon feedbacks such as ice-sheet albedo, loss of ocean carbon-sink efficiency, loss of permafrost and other soil carbon, carbon release from tropical rainforests drying/ wild-fire, and so on) is closer to 6˚C. There is strong evidence in climate history of the last million years to support this view. This is recognised in the IPCC 2007 synthesis report which notes that “emissions reductions... might be underestimated due to missing carbon cycle feedbacks” (page 67) and this may require the cumulative emissions budget for the 21st century (the total amount of GHGs than can be emitted for a stabilisation level) to be “about 27% less” than is assumed. But the 25-40/2020 target and other IPCC emission reduction scenarios do not include this consideration!
On carbon cycle feedbacks, there is already evidence that the strength of ocean, and especially some land, carbon sinks are weakening and becoming less efficient, and that this will persist into the future. Thus the predictions from climate-carbon- cycle models may be too conservative and CO2 in the atmosphere will probably increase more rapidly than the models suggest, which has implications for the development of policies that seek to stabilise atmospheric CO2 at a given level, including those of the IPCC (Cox & Jones).
The IPCC gives a CS range of 1.5–4.5˚C and warns that “policymakers may want to use the highest values of climate sensitivity... to guide decisions” (and virtually admits it is higher than the 3C the IPCC chooses to assume). This precautionary warning has apparently been ignored by those who advocated the 25-40/2020 target during 2008.
If long-term CS is 6˚C, then 450ppm would produce a temperature increase of 4.1˚C, enough to melt all ice sheets and produce a 70-metre sea-level rise, amongst many impacts that would end life on this planet for most people and most species.
After a careful reassessment of climate sensitivity and climate history data, James Hansen and his co-authors concluded in a 2008 research paper that the tipping point for the presence, or absence, of any substantial ice-sheets on Earth is around 450 ppm (plus or minus 100 ppm) of CO2. This means that the CO2 levels often associated with a 2˚C rise may just be the tipping point for the total loss of all ice sheets on the planet and a huge sea-level rise. Yet this is what the principal advocacy was by “our” side in 2008!
[For a more detailed discussion of climate sensitivity, see the discussion on pages 15-16 of the
Climate Safety report published in late 2008 in the UK]The impacts: 450ppm CO2e is roughly the current GHG level, and in 2008 Ramanathan and Feng found that if greenhouse gases were fixed at their 2005 levels (and assuming the IPCC CS of 3˚C), the inferred warming is 2.4˚C (range 1.4˚C to 4.3˚C) and that 2.4˚C would be sufficient to result in the loss of Arctic summer sea-ice, the Himalayan–Tibetan glaciers and the Greenland ice-sheet (based on Lenton et al). The loss of Greenland ice sheet produces about a 7-metre global sea-level rise. One conclusion is that advocacy of the 20-40/2020 target, for example by the ACF in its 2008 Special Places campaign, will result in the destruction of many of Australia’s “special places” ACF wants to protect; Kakadu, for example, will salinate with a sea-level rise of less than a metre.
The scenario is out of date. The scenarios in IPCC 2007 WGIII Box 13.7 were prepared in 2005 or earlier.
• The 450ppm scenario relies heavily of the work of Del Elzen and Meisnhausen, presented at a 2005 UK Met conference (and then published as chapters 28 and 31 in Avoiding dangerous climate change, Schellnhuber (ed.), CUP). Since then, emissions from non-Annex I nations have grown beyond all expectation, which means that emission reductions scenarios need to be re-visited.
• Thus in recent powerpoints, Meinshausen has crossed out the words “Substantial deviation from baseline in Latin America, Middle East, East Asia and Centrally-Planned Asia” and pasted over “–15% to –30% by 2020” for non-Annex 1 nations, a recognition that the 2007 propositions need re-working.
• In addition, the IPCC methodology (and emissions reduction scenarios generally) implies that climate change is a linear event, where predictable changes in emission levels will have predictable outcomes and impacts. But events such as the “big melt” in the Arctic summer of 2007 are non-linear and unpredicted events that can turn climate science knowledge on its head and demand that the whole question of what needs to be done and what are appropriate targets be urgently re-assessed in light of new data, including evidence that carbon cycle feedbacks are kicking in sooner than expected. The Arctic shows that tipping points for dangerous climate change and large sea-level rises have already been passed, a fact that the IPCC did not recognise in failing to include any emission scenarios for less than 2–2.4˚C in its 2007 report.
• It is noteworthy that IPCC author Bill Hare, in the climate chapter for the 2009 State of the World report, includes detailed modeling by the Potsdam Institute for Research on Global Warming Effects of a 1-degree stabilisation scenario (the first I have seen), and sketches the actions for a 300 ppm target. It will be interesting to see if Hare’s views, which in the past have been influential in groups such as CANA and Greenpeace, will be embraced.
• Potsdam Institute head Hans Joachim Schellnhuber says previous predictions about climate change and its catastrophic effects were too cautious and optimistic. “In nearly all areas, the developments are occurring more quickly than it has been assumed up until now,” Schellnhuber told the Saarbruecker Zeitung newspaper in an interview published on 29 December. “We are on our way to a destabilization of the world climate that has advanced much further than most people or their governments realize.”
A safe-climate target
James Hansen told scientists at an American Geophysical Union conference in December 2007 that: “We either begin to roll back not only the emissions [of CO2] but also the absolute amount in the atmosphere, or else we’re going to get big impacts ... We should set a target of CO2 that’s low enough to avoid the point of no return.” In order to achieve the return of the Arctic sea-ice, we have seen above that Hansen and his co-authors have identified the target as in the range 300–325ppm CO2, well below the current level. Given the key role the Arctic plays in the global climate system, a precautionary approach would therefore suggest a long-term target of 300ppm CO2e.
This would rule out a domino effect of sea-ice loss, albedo flip, a warmer Arctic, a disintegrating Greenland ice sheet, more melting permafrost, and further knock-on effects of massively increased greenhouse gas emissions, rising atmospheric concentrations and accelerated global warming.
Any proposal for a target higher than 300ppm would imply confidence that it is safe to leave the Arctic sea ice melted, and an assumption that this would not bring about the train of consequences just described. This is, implicitly, the view of all the major nations and organisations involved in setting climate policy. Accordingly, they must be challenged to provide a reasoned argument as to why leaving the Arctic Ocean free of ice in summer is safe. If they cannot, the only acceptable course of action is clear.
David Spratt
March 2009
