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, 2010.1
1. The publication of hacked e-mails from
prominent scientists at the Climate Research Unit (CRU) at the
University of East Anglia has initiated an intense debate about
the credibility of climate science. Unfortunately, this debate
sometimes goes so far as to question a key result of climate science:
that the climate system has unequivocally warmed over the past
century and most of the recent warming is very likely caused by
human activity. We welcome debate about the ethics of science,
about the interaction of climate, policy and politics. The language
used in some of these e-mails has created concern, among both
scientists and the public, about the openness and integrity of
the scientific process. But at the same time it is critical to
point out that no grounds have arisen to doubt the validity of
the thermometer-based temperature record since 1850, nor any results
based upon it. The mainstream media has confused discussions about
uncertain climate reconstructions built on tree-ring data, such
as the so-called "hockey stick", with the more secure
thermometer record. While these proxy-based reconstructions remain
a controversial area of active research, the thermometer record
alone shows unequivocally that the Earth is warming. Moreover,
it is the temperature record, not proxy-based reconstructions,
that provides the principle evidence that most of the recent warming
is very likely attributable to human activity.
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 University
of Alabama in Huntsville analysis. While the question of which
MSU analysis is more accurate remains hotly disputed, it is generally
agreed that the reanalysis exercise has vastly improved our understanding
of the MSU record.5
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.
REFERENCES1 H.
von Storch and M. R. Allen, Nature, doi:10.1038/463025a
(2010).
2 http://news.bbc.co.uk/1/hi/programmes/newsnight/8395514.stm
3 http://hadobs.metoffice.com/crutem3/jan_2010_update.html
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 Geesthacht, Germany, and a member of the
KlimaCampus CLISAP in Hamburg. Myles Allen is Head of the Climate
Dynamics Group, Department of Physics, University of Oxford, UK.
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.
February 2010
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