The Economist Screws Up on the Draft IPCC AR5 Report and Climate Sensitivity

Posted on 19 July 2013 by dana1981

Earlier today, The Economist published a piece of irresponsible journalism regarding information in the draft Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). The Economist saved us some effort by explaining the problems with their own article:

“There are several caveats. The table comes from a draft version of the report, and could thus change. It was put together by the IPCC working group on mitigating climate change, rather than the group looking at physical sciences. It derives from a relatively simple model of the climate, rather than the big complex ones usually used by the IPCC. And the literature to back it up has not yet been published.”

So folks at The Economist, please explain to us, why are you reporting on climate sensitivity information in this draft report about climate mitigation that uses a simple climate model and is based on unpublished literature?

Readers may recall that climate contrarian blogs behaved in a similar fashion when the IPCC AR5 draft report on the physical science was “leaked” last December. The contrarians made a huge to-do about a figure that seemed to show global surface temperature measurements at the very low end of the IPCC model projections. As we discussed at the time, in reality the IPCC temperature projections have been very accurate. As Tamino noted, the draft IPCC graph itself was flawed, using a single year as the baseline (1990) rather than aligning the data and models based on the existing trend in 1990. Fast forward a few months later, and we hear from IPCC reviewers that this graph has been revised accordingly, now correctly reflecting the accuracy of the IPCC surface temperature projections. The lesson to be learned is that you shouldn’t report on draft documents that are subject to change!

Thus problem #1 is that this article never should have been written, and doing so was a great example of irresponsible journalism, as climate scientist Kevin Trenberth told Climate Progress. Problem #2 is that The Economist’s interpretation of the information from the draft IPCC report is wrong. A similar table is shown in the 2007 IPCC Fourth Assessment Report (AR4) (Table SPM.5):

This is the table referenced in The Economist article showing a 2.0–2.4°C global mean surface temperature warming in response to a CO2-equivalent (the warming from all greenhouse gases, not just CO2) concentration of 445–490 parts per million (ppm). Note that this corresponds to a CO2 concentration of about 350–400 ppm – in other words, we’re already there. Right now cooling from human aerosol emissions is roughly offsetting the warming from non-CO2 greenhouse gases; the problem is that greenhouse gases have a long life in the atmosphere, and aerosols do not. As we transition away from coal energy and reduce pollutant emissions, their associated cooling effect will also decline, and will no longer mask the warming from non-CO2 greenhouse gases. Thus they must be accounted for in this CO2-equivalent calculation.

The Economist article makes some more mistakes when discussing this issue (emphasis added):

“[according to the draft AR5 table], at CO 2 concentrations of between 425 parts per million and 485 ppm, temperatures in 2100 would be 1.3-1.7°C above their pre-industrial levels. That seems lower than the IPCC’s previous assessment, made in 2007. Then, it thought concentrations of 445-490 ppm were likely to result in a rise in temperature of 2.0-2.4°C. The two findings are not strictly comparable. The 2007 report talks about equilibrium temperatures in the very long term (over centuries); the forthcoming one talks about them in 2100. But the practical distinction would not be great so long as concentrations of CO 2 and other greenhouse-gas emissions were stable or falling by 2100.”

The bolded text decribes the difference between the AR4 and AR5 tables. One describes the warming as of 2100 (AR5), the other describes the eventual warming once the planet reaches a new equilibrium energy state (AR4). The Economist staff seem to think the difference is also in part due to a change in climate sensitivity values - that is not the case. According to an IPCC reviewer we talked to, both tables appear to be based on an equilibrium climate sensitivity of 3°C for doubled CO2. It's important to bear in mind that the world won't end in 2100 (we hope!) – there's nothing magical about that date, and the planet will continue warming until we get our greenhouse gas emissions down near zero.

The Economist is also wrong to say the practical difference between short-term transient warming and long-term equilibrium warming is ‘not great’ if greenhouse gas concentrations are stable or falling. In order to achieve no further warming, net greenhouse gas emissions must be zero or negative. Only in that scenario can atmospheric greenhouse gas concentrations fall, offsetting the ‘warming in the pipeline’ to be achieved once the planet reaches a new equilibrium energy state.

The rest of the article is based on the fact that at the moment, the draft IPCC AR5 physical science report has reduced the lower end of the “likely” equilibrium sensitivity range from 2°C in AR4 to 1.5°C. Again, the report is still in draft form, and that is subject to change. Based on this, The Economist speculates,

“That seems to reflect a growing sense that climate sensitivity may have been overestimated in the past and that the science is too uncertain to justify a single estimate of future rises. If this does turn out to be the case, it would have significant implications for policy. Many countries’ climate policies are guided by the IPCC’s findings. They are usually based on the idea (deriving in part from the IPCC) that global temperatures must not be allowed to increase by more than 2°C above pre-industrial levels, and that in order to ensure this CO 2 concentrations should not rise above 450 ppm. The draft table casts doubt on how solid the link really is between 450 ppm and a 2°C rise.”

This builds on an earlier article by The Economist discussing a few recent studies that estimated climate sensitivity a bit lower than the IPCC AR4 best estimate. Michael Mann and I published an article in ABC explaining the problems with that piece. Long story short – the full body of evidence suggests that climate sensitivity is approximately in the range estimated in the IPCC AR4 report. Some recent studies have arrived at lower estimates, but may contain flaws, and other recent studies have arrived at higher estimates.

More importantly, even if equilibrium climate sensitivity were as low as this best case scenario of 1.5°C global surface warming in response to a doubling of atmospheric CO2, we’re still not doing enough to reduce emissions if we want to avoid dangerous climate change. The Economist misses the big picture. The figure below illustrates the amount of warming we can expect if we continue on a business-as-usual path with continued reliance on fossil fuels and a slow transition to low-carbon energy sources (IPCC scenario RCP 6) for equilibrium climate sensitivities of 1.5°C (best case), 3°C (most likely), and 4.5°C (worst case), compared to the climate experienced during the history of human civilization.

Image created by John Cook and Dana Nuccitelli, added to the SkS graphics page.

Even that “best case” is a dangerous level of climate change if we continue with business as usual rather than taking serious steps to reduce our greenhouse gas emissions. In any case, The Economist’s claim that “The draft table casts doubt on how solid the link really is between 450 ppm and a 2°C rise” is simply a misinterpretation of the draft IPCC report. The table simply indicates that 450 ppm CO2-equivalent (which we’ve already reached) would not lead to 2°C global surface warming until sometime after the year 2100. But since we’re already at that level and our greenhouse gas emissions continue to rise fast, that’s kind of a moot point anyway.

In fact, we’ve learned from reviewers of the draft IPCC AR5 report that the scenario in question (1.3–1.7°C global surface warming by 2100) applies to IPCC emissions scenario RCP 3-PD. In this scenario, after peaking in 2020, our annual CO2 emissions decline at a rate of around 3.5% per year. Human greenhouse gas emissions actually become negative after about 2070, meaning we remove more CO2 from the atmosphere than we add. The scenario they’re considering involves extremely aggressive greenhouse gas emissions reductions.

We recommend that everyone write this article off as a case of terrible judgment by The Economist that led to a factually wrong article. We hope their staff will learn from the many mistakes made in today’s piece and think twice before publishing articles based on draft reports next time.