Published online 2 April 2008 | Nature 452, 508-509 (2008) | doi:10.1038/452508a

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Are the IPCC scenarios 'unachievable'?

Claim that the challenge of cutting emissions has been underestimated is debated.

From left, Roger Pielke, Tom Wigley and Christopher Green.C. CALVIN; MCGILL UNIV.

The Intergovernmental Panel on Climate Change (IPCC) has grossly underestimated the challenges of reducing and stabilizing greenhouse-gas emissions, according to an influential group of climate-policy experts.

The scenarios produced by the IPCC assume that very substantial technological advances — leading to greater energy efficiency and reduced carbon dioxide intensity — will happen spontaneously, without extra policy measures (see page 531). Roger Pielke, Tom Wigley and Christopher Green argue that this is a “dangerously optimistic” assumption. To show its effects, the trio offer a contrasting 'frozen-technology' scenario, which assumes that future energy needs are met with technology available at the baseline year. They say that this demonstrates a need for new energy technologies as much as four times greater than that which seems to be required looking at some IPCC scenarios. Nature gets some reactions.

S. KHAN/AFP/GETTY IMAGES

Bert Metz, co-chair of IPCC Working Group III

The claim that the IPCC has underestimated the technological challenge of stabilization is unwarranted and must be rejected. The fact that technological change is already significantly included in the reference scenarios is clearly stated in our report and its Summary for Policymakers.

The embedded technological change in the reference scenarios included in the IPCC's Special Report on Emission Scenarios was based on historical information. The assumptions about the rate of technological change in these scenarios have been thoroughly reviewed and are accepted by the community of technological-change experts. They confirm well-known facts about, for instance, the enormous improvements in computers over much shorter time-frames than expected. The assumptions also reflect that high economic growth normally goes hand in hand with high rates of technological change.

Fears that actual economic growth and energy use may develop in different ways than assumed in our scenarios are more justified. If so, it could be more difficult to reach low stabilization levels. The IPCC in 2006 initiated the development of new long-term scenarios. New scenarios will be developed over the coming years.

Richard Tol, energy economist, Economic and Social Research Institute, Dublin

This is a very valuable piece of education. Indisputably, in the IPCC's reference scenarios there is a lot of spontaneous technological progress going on already. Experts know this, but many people who write about climate change, or have to make policy decisions, don't.

Improving energy efficiency is not enough. Engineers could do it, but would need to work at least four times as hard to stabilize emissions.

The IPCC scenarios developed in 2000 don't match historical observations. For instance, nobody foresaw the rapid growth in recent years of China's economy, or the launch of the US$2,500 people's car in India. There has also been less technological change in past decades than the scenarios would have us believe will happen in the future, and hence a downward bias in the cost of emission reductions. We need new scenarios — not just any, but scenarios that are in line with the real development of global energy systems.

John Reilly, Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change, Cambridge

No question, these reference scenarios are dated and underestimate the amount of warming that will occur without new climate policies.

We at MIT have argued for probabalistic projections, but the IPCC scenarios followed a storyline approach that they explicitly stated should not be interpreted in terms of likelihood. This subtlety is often lost when the scenarios are rolled out.

Clearly, emissions have grown much more rapidly than projected. In our business-as-usual projection, technological advances that improve efficiency are more than offset by growing demand, especially in developing nations. An accurate picture requires careful assessment of various economies rather than a broad-brush numerical exercise at the global level.

Detlef van Vuuren, emission scenario developer, Netherlands Environmental Assessment Agency, Bilthoven

Emission scenarios are built around historical observations. Energy efficiency has in the past improved without climate policy, and the same is very likely to happen in the future. Including unprompted technological change in the baseline is thus logical. It is not very helpful to discredit emission scenarios on the sole basis of their being at odds with the most recent economic trends in China. Chinese statistics are not always reliable. Moreover, the period in question is too short to signify a global trend-break. Rapid economic growth, in combination with the high price of oil, might spur long-term developments in renewable-energy technologies, for instance.

Ottmar Edenhofer, IPCC Fourth Assessment Report lead author, Potsdam Institute of Climate Impact research, Germany

The dramatic conclusions grossly overshoot the mark. It is wise to ask whether the IPCC's assumptions on spontaneous versus policy-induced technological change are sensible. But freezing technological change is a mere thought experiment. Given the past, a frozen-technology baseline is extremely unlikely.

Global energy efficiency and intensity have improved over the past 30 years. China in the first half of the decade is a special case owing to structural economic changes, although it is indeed worrying that neither carbon nor energy intensity in China has decreased since 2001.

It is a misapprehension that the costs of mitigating climate change will be much higher than assumed. Our studies show that the transformation towards a low-carbon energy system is possible if appropriate policy instruments are implemented. As the price of fossil energy increases, energy-efficiency gains are likely to materialize, even without extra policy measures.

Robert Socolow, Carbon Mitigation Initiative, Princeton University, New Jersey

The case for aggressive action to stabilize atmospheric CO2 concentrations at less than double pre-industrial values is far more robust than this commentary suggests. Politicians rightly associate such targets with deep cuts in annual CO2 emissions in industrialized countries relative to today. The commentary seems to argue that the case for aggressive action is not strong unless the world in the absence of attention to carbon would emit far more CO2 than the IPCC predicts.

The authors demonstrate remarkable self-confidence in dismissing the IPCC's consensus estimates of future carbon intensity. The premise of the IPCC's econometric models is that the past is a guide to the future. If I were constructing a revisionist case for very high future emissions, I would instead argue that the IPCC underestimates world economic growth.

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Current attempts to mitigate climate change are based on evolutionary but unprecedented expansion of tested strategies, including renewables and nuclear power, CO2 capture and storage, methane emissions reduction, and forest protection. No one can be certain these efforts will be sufficient. The revolutionary should be pursued in parallel with the evolutionary. It is foolhardy to undermine indispensable commitments to evolutionary measures.

To the authors, the IPCC report is a lullaby. To most of us, it's a bugle call.

Marty Hoffert, former chair of the Department of Applied Science, New York University

This analysis is long overdue. We're under a delusion that we will solve the problem of climate change casually. But what we have — cap-and-trade systems and the like — is plainly insufficient. We need a massive engineering effort, the size of the Manhattan Project. 

Quirin Schiermeier with additional reporting by Jeff Tollefson. Read the commentary, the Nature editorial, and further news, listen to the podiumextracted.mp3">podcast interview, and find further discussion on climate feedback.

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