‘ClimateGate does not affect IPCC-conclusions’

Rik Leemans, professor Environmental System Analysis at Wageningen University:

"The "ClimateGate" affair - the publication of e-mails and documents hacked or leaked from one of the world's leading climate research institutions, The Climate research Unit - is being intensely debated on the web and in the media. Many journalists and politicians saw in these emails allegations of scientific misconduct and fraud. Is this true?

One of the most frequently cited emails by Prof. Phil Jones contained the sentence “I've just completed Mike's Nature trick of adding in the real temps to each series for the last 20 years … to hide the decline". Many state that this decline refers to the relative stabilization or declines of temperatures over the last decade since 1998, which is one of the warmest years on record. Additionally, many indicated that using tricks does not comply with a proper scientific conduct.

However, these interpretations are wrong. The science discussed in these emails involves the reconstruction of historic temperatures over the last two thousand years. This is not straightforward because only for the last 150 years, we have reliable direct measurements with thermometers. Therefore, earlier temperatures have to be estimated from indirect measurements, the so-called proxies. One of these proxies involves annual rings of trees, whose width correlate with temperatures. Ring width and temperature measures of the last century are used to statistically define this relationship. Unfortunately, also other factors influence ring width. One major problem in the northern hemisphere is that the ring width declined while temperature increased since the seventies. One of the possible causes is acid rain. The ‘trick to hide this decline in ring width’ is to only use the period before the seventies to estimate the relationship. This is a very common approach (or trick), which has been presented and discussed in many scientific papers.

Additionally, if one queries the scientific literature for ‘tricks’, over a thousand papers have this word in the title. It is thus a very common scientific term, although somewhat informal.

One of the other emails focus on creating a bias in the peer-review process. In one of the emails, Prof. Phil ones states “will keep them out somehow — even if we have to redefine what the peer-review literature is!”. 'Them' refers to some scientists that deny that global warming is a man-made process. Personally, I strongly disagree with Jones’ statement here. Valid science should be discussed and published in the peer-reviewed literature. But what is the background of this statement? The discussions leading up to this unfortunate statement has a long history and is also related to the reconstruction of historic temperatures.

Many older climate reconstructions for Western Europe show that the period between the16th and 18th century was colder. This was labeled as the little ice age. The middle ages, however were warmer (i.e. the so-called medieval warm period). Some claim much warmer because vineyards seemed abundant in England and the Vikings thrived in Greenland, but the evidence is anecdotical.

In IPCC first assessment report (Houghton et al. 1990), a sketchy graph was presented which showed that the Middle Ages were warmer than the 20th century. This report discussed, however, the large uncertainties involved in these reconstructions and urges for more research. In response, several groups started to collect proxy datasets with a better global coverage. Mann et al. (1998) were the first to publish a more appropriate reconstruction and they concluded that the last decades of the 20th century were warmer than both the little ice age and the medieval warm period. Their graph, nicknamed the ‘Hockeystick’, was persuasively included in the Summary for Policy Makers in IPCC’s third assessment report (Intergovernmental Panel on Climate Change 2001) and attracted much attention and criticism. Especially, McIntyre and McKitrick (2005) used Mann et al. (1998) data and approached and showed that the statistical analysis used always resulted in a ‘Hockeystick’ and that the medieval warm period was indeed warmer than the current period. McIntyre and McKitrick (2005) provided now new science but demanded that underlying data to redo all the analysis and oust mainstream climate reconstruction science. Because of their vicious demands, scientists like Mann and Jones felt harassed, also because US senators made it into a political issue and legal actions were started with reference to the UK freedom of information act. The science became a subject of political controversy.

The debate became even more heated, when Soon and Baliunas (2003) published a review on existing temperature reconstructions, which concluded that the medieval warm period was indeed warmer that the current era. This paper was immediately controversial because it was used as scientific evidence in the US senate hearing that there was no man-made warming, while the scientific community strongly criticized the selected qualitative approach. The paper was published in Climate Research and also created a controversy in its editorial board. The majority of the editorial board argued that this paper did not adhere to scientific standards and resigned.

Both the political and scientific controversy led to an independent investigation by a committee of the US National Academy of Sciences (NAS). The result of this investigation was published in 2006 (Committee on Surface Temperature Reconstructions for the Last 2000 Years 2006) and clearly stated that the Mann et al. (1998)’s  conclusion has subsequently been supported by an array of additional evidence.  They concluded that the mean surface temperature was higher during the last decades of the 20th century than during any comparable period during the last four centuries. But at they same time, they criticized IPCC, which had sent a very misleading message in 2001 about how resolved this part of the scientific research was. In my view this NAS report settles most of the controversy but many keep repeating the hockeystick controversy. This is not an issue anymore! There is not just a hockeystick but a whole hockey team.

Jones email statement on ‘redefining the peer-review literature’ must also be interpreted in the context of this controversy. As contributors to the last IPCC assessment, they did indeed discuss to exclude all papers by McIntyre and McKitrick (2005), and Soon and Baliunas (2003). But statements in personal emails do not count, it is the actual final discussion in the IPCC’s fourth assessment report (Solomon et al. 2007) that defines how the scientific discussion is covered (see Box 1).  The treatment of all the different papers shows that their treatment is balanced and sincere, and that the peer-review process guarantees transparency and thoroughness.

But what do these stolen emails imply for climate science? There can only be one answer to this question: They do not show any problems with the meticulousness of climate science and the peer review process. The findings synthesized in the IPCC reports and its recent updates are convincing. In response to these stolen emails, IPCC (www.ipcc.ch), the American Association for the Advancement of Science (www.aaas.org), the journal Nature and many others have also issues clear statement that the science is sound and the scientific processes are trustworthy.
 
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Box 1. The discussion on the hockeystick controversy in IPCC forth assessment report (Solomon et al. 2007).

The ‘hockey stick’ reconstruction of Mann et al. (1999) has been the subject of several critical studies. Soon and Baliunas (2003) challenged the conclusion that the 20th century was the warmest at a hemispheric average scale. They surveyed regionally diverse proxy climate data, noting evidence for relatively warm (or cold), or alternatively dry (or wet) conditions occurring at any time within pre-defined periods assumed to bracket the so-called ‘Medieval Warm Period’ (and ‘Little Ice Age’). Their qualitative approach precluded any quantitative summary of the evidence at precise times, limiting the value of their review as a basis for comparison of the relative magnitude of mean hemispheric 20th-century warmth (Mann and Jones, 2003; Osborn and Briffa, 2006).

McIntyre and McKitrick (2003) reported that they were unable to replicate the results of Mann et al. (1998). Wahl and Ammann (2007) showed that this was a consequence of differences in the way McIntyre and McKitrick (2003) had implemented the method of Mann et al. (1998) and that the original reconstruction could be closely duplicated using the original proxy data. McIntyre and McKitrick (2005a,b) raised further concerns about the details of the Mann et al. (1998) method, principally relating to the independent verification of the reconstruction against 19th-century instrumental temperature data and to the extraction of the dominant modes of variability present in a network of western North American tree ring chronologies, using Principal Components Analysis. The latter may have some theoretical foundation, but Wahl and Amman (2006) also show that the impact on the amplitude of the final reconstruction is very small (~0.05°C; for further discussion of these issues see also Huybers, 2005; McIntyre and McKitrick, 2005c,d; von Storch and Zorita, 2005).
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References

• Committee on Surface Temperature Reconstructions for the Last 2000 Years and Board on Atmospheric Sciences and Climate, 2006. Surface temperature reconstructions for the last 2,000 years. The National Academies Press, Washington, D.C. 160 pp.

• Houghton, J.T., G.J. Jenkins and J.J. Ephraums, editors. 1990. Climate Change: The IPCC Scientific Assessment. Cambridge University Press, Cambridge. 365 pp.
• Intergovernmental Panel on Climate Change, 2001. Climate Change 2001.The science of climate change: summary for policymakers. Cambridge University Press, Cambridge. 1-18 pp.
• Mann, M.E., R.S. Bradley and M.K. Hughes, 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.
• McIntyre, S. and R. McKitrick, 2005. Hockey sticks, principal components, and spurious significance. Geophysical Research Letters 32: L03710,03710.01029/02004GL021750.
• Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.M.B. Tignor and H.L. Miller, editors. 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. 996 pp.
• Soon, W. and S. Baliunas, 2003. Proxy climatic and environmental changes of the past 1000 years. Climate Research 23: 89-110.