Memorandum submitted by
Professor Hans von Storch and Dr. Myles R. Allen (CRU 43)
Scope: We address the first and third questions raised by the enquiry: we have no comment on the terms of reference of the UEA enquiry.
Declaration of Interests: We are both professional statistical climatologists who have made use of the HadCRUT instrumental temperature record and participated in the scientific debate about reconstruction of the climate of the past millennium. One of us (HvS) is criticized in the leaked e-mails.
Note: A substantially abbreviated version of this submission appeared in the scientific journal Nature in January, 20101.
1. The publication
of hacked e-mails from prominent scientists at the Climate Research Unit (CRU)
2. We are both statistical climatologists, specialising inter alia in the detection and attribution of external influences on large-scale climate and regional weather. We have variously worked with the scientists at the centre of this controversy, and have examined, used and at times criticized their data and results just as they, at times, have criticized us (both in scientific forums and in some of the disclosed e-mails). The fact that we disagree with Professors Jones, Mann and others on some matters, such as proxy-based reconstructions, has no bearing on our respect for Professor Jones' analysis of the instrumental temperature record.
3. The hacked e-mails do not prove, or even suggest, that the main product of CRU, namely the HadCRUT record of global surface air temperature based on thermometer readings, has been compromised. Indeed, the thermometer-based temperature record has been verified by results from other groups, and no serious doubts about these products have been raised. Only two substantive criticisms to have emerged of the instrumental record through the intense scrutiny since the e-mails were disclosed:
3.1 An allegation aired on BBC's "Newsnight" that software used in the production of this dataset was unreliable.2 It emerged on investigation that the neither of the two pieces of software produced in support of this allegation was anything to do with the HadCRUT instrumental temperature record. Newsnight have declined to answer the question of whether they were aware of this at the time their allegations were made.
3.2 A problem identified by an amateur computer analyst with estimates of average climate (not climate trends) affecting less than 1% of the HadCRUT data, mostly in Australasia, and some station identifiers being incorrect.3 These, it appears, were genuine issues with some of the input data (not analysis software) of HadCRUT which have been acknowledged by the Met Office and corrected. They do not affect trends estimated from the data, and hence have no bearing on conclusions regarding the detection and attribution of external influence on climate.
4. It is possible, of course, that further scrutiny will reveal more serious problems, but given the intensity of the scrutiny to date, we do not think this is particularly likely. The close correspondence between the HadCRUT data and the other two internationally recognised surface temperature datasets suggests that key conclusions, such as the unequivocal warming over the past century, are not sensitive to the analysis procedure.
5. Regarding the specific question of the independence of these three datasets, they are clearly not fully independent because all three are based on substantially the same input data, but this data (which is largely in the public domain) is not in dispute. The software used by the different groups to compile gridded datasets from these input data are independent, but this does not completely guarantee independence of results, since researchers collaborate and conventions evolve.
6. Hence, if there
are serious concerns about the integrity of these surface temperature datasets
(and we believe there are not), then the logical response would be to
commission an independent analysis from a fully independent group (selected by
a neutral party such as the Institute of Physics, mandated to exclude groups
who have already "taken sides" in the climate debate). An example of good
practice in such disputes would be the Remote Sensing Systems re-analysis of
the Microwave Sounding Unit (MSU) temperature record4 which revealed
previously underappreciated biases and uncertainties in the original
7. The cost of such a complete reanalysis of the surface temperature record should not be underestimated, particularly because, to carry credibility, it would have to be undertaken by a group without an established track record in this kind of work, requiring them to build all analysis software from scratch. While it would be very interesting if such an exercise were to reach substantially different conclusions to HadCRUT or the other two internationally recognised datasets, we believe the chance of such an outcome is relatively small, and the chance of the new analysis actually proving more accurate than HadCRUT, in view of the decades of experience behind the HadCRUT algorithms, would be smaller still.
8. Despite the absence of any evidence of genuine problems in the HadCRUT data, serious doubts are now raised in the public regarding the integrity of this data and conclusions based upon it.6 We do not think that they are warranted. Instead, in spite of some disagreement about technical issues which are normal in the process of science, we are convinced, insofar as is possible in an empirical science, that anthropogenic climate change is taking place and will emerge more strongly in the future. For explanation, a few comments are needed:
8.1 The assessment that elevated greenhouse gas concentration contributes to most of the recent warming since, say 1970, is made up of two steps, a "detection" step and a "attribution" step".
8.2 The detection step reveals that the warming trend extending across the recent few decades is more rapid than warming or cooling trends what would be expected from internal variability alone (from phenomena such as El Nino, the Pacific Decadal Oscillation and so on). The statement is not that the present level of warmth is unprecedented, even though it may very well be, but that the speed of warming is remarkable. The description of the warming in recent decades ("the signal") is based on thermometer data, including the CRU data. Even if this data may not be perfect, its uncertainties have been carefully estimated and the recent warming is robust. The detection is based on a rigorous statistical analysis, but depends on our understanding about the natural variability. The latter, the level of natural variability, is estimated again from the thermometer-based temperature record, and from long climate model simulations. Thus, climate models play only a minor role in this 'detection' step, mostly in helping to determine the magnitude of internal variability.
8.3 Attributing observed temperature variations to specific causes relies more on climate models, as they are needed to discriminate between the response of the climate system to different 'drivers', such as solar activity, greenhouses gases and volcanoes. It turns out that the best, and really the only, satisfactory explanation of the history of surface air temperature change particularly over the last few decades is obtained when the warming influence of anthropogenic greenhouse gases is taken into account. These gases are behind most of the recent decades' warming. The attribution argument relies on a combination of statistical evidence and physical plausibility, and hence relies on the accuracy of large-scale temperature changes simulated by climate models, and also, inevitably, leaves some room for doubt. Studies using records for other aspects of climate, including change in ocean temperatures, change in temperatures in the atmosphere, water vapour, even in precipitation, support the dominant role of human influences in the past few decades, and paint a very consistent picture of a world warming largely due to greenhouse gas increases.
8.4 Importantly, both of these conclusions rely on thermometer-based temperature records such as HadCRUT. They do not rely on reconstructions of temperature over the past millennium. Such reconstructions are not based on thermometer data but on indirect evidence such as tree rings, and are consequently more uncertain and have been the subject of intense debate over the last few years. Because of this uncertainty, coupled with uncertainty in the drivers of climate change prior to the 20th century, reconstructions of past climate have played a more marginal role than the instrumental surface temperature record in the detection and attribution efforts of assessing ongoing climate change. If "tree-ring-thermometers" had never been discovered, our confidence of human influence on climate over the past few decades would not be substantially diminished. It is particularly unfortunate that much of the discussion of the CRU e-mails in the mainstream media has failed to distinguish between the "hockey-stick" and the much more important and less controversial instrumental record.
8.5 One of us (MRA) led a study submitted at the end of 1999 making a prediction of climate change over the early decades of the 21st century, based on a combination of the HadCRUT record and climate model simulations available at that time.7 If the HadCRUT record had been deliberately manipulated away from the true climate, or the attribution of causes of the observed warming were seriously in error, there would be no reason to expect such a prediction to be correct. To date, the prediction has proved remarkably accurate (see figure). While not conclusive, this fully out-of-sample forecast verification provides further evidence that there is nothing substantially wrong with the HadCRUT temperature record or conclusions based upon it.
9. Climate science is clearly a knowledge producer and broker for some of the most important issues of world policy and therefore cannot be conducted behind closed doors. Some commentators have suggested that the e-mails disclose a "team mentality" among climate scientists, as if there is a united front against dissenters. Even we - the two authors of this submission - find it impossible to agree whether or not some people went too far to ensure dominance for particular points of view. We do agree, however, that it is absurd to suggest there is some kind of global conspiracy involving all climate scientists. We welcome a dispassionate discussion on the correct balance between protecting the interests of individual researchers and institutions and the broader interests of open access and transparency. But in the midst of this, the most important conclusions about climate change itself must not be forgotten.
1. H. von Storch and M. R. Allen, Nature, doi:10.1038/463025a (2010)
4. C. A. Mears and F. J. Wentz, Science, DOI: 10.1126/science.1114772 (2005)
5. T. R. Karl et al (eds.), Temperature trends in the lower atmosphere: steps for understanding and reconciling differences: U.S. Climate Change Science Program Synthesis and Assessment Product 1.1, Washington, (2006)
6. See, for example, Rasmussen Reports (2009)
7. M. R. Allen et al, Nature, doi:10.1038/35036559 (2000)
Hans von Storch is director of the GKSS Institute of Coastal Research in
Figure: forecast of global temperature made in 1999 using the HadCRUT data.7 Solid line shows original model projection. Dashed line shows prediction after reconciling climate model simulations with the HadCRUT temperature record, using data to August 1996. Grey band shows 5-95% uncertainty interval. Red diamond shows observed decadal mean surface temperature for the period 01-01-00 to 31-12-09 referenced to the same baseline.
Professor Hans von Storch and Dr. Myles R. Allen