A note on the "hockey stick"
20. While we have not entered into the scientific
debate in any detail, we received a significant amount of evidence
on the so-called "hockey stick" debate and hence feel
we should comment on this issue.
21. The hockey stick refers to the shape of the
long-run time series curve of temperature changesee Box
6. This appears to show gently declining global (actually Northern
Hemisphere) temperature from at least 1000 until about 1700, with
a rise from then until the present. Most importantly, the recent
past shows a sharp upturn such that the later part of the 20th
century is warmer than any previous period. Thus the series resembles
a hockey stick with the blade facing upwardssee Box 6.
We noted earlier that a similar graph is suggested for carbon
dioxide concentrations. The importance of the hockey stick shape
is that the upturns in both temperature and CO2 coincide
and both are relatively recent phenomena, i.e. in the last 150
years or so. The hockey stick thus appears to be persuasive visual
evidence that the recent temperature change is human-induced.
The "hockey stick"
According to the hockey stick literature, the time-profile
of temperature in the Northern Hemisphere has the shape shown
in the stylised diagram below. Temperatures before the mid-19th
century tend to be inferred from ice-core and tree-ring data ("proxy"
data). The implication is that natural climate variability
has not generated temperature variations that compare with the
temperature change witnessed since the onset of industrial times.
Hence the temperature change of the last few centuries must be
due to human-induced factors. The historical evidence is debated.
Several studies have found fairly long periods in the last millennium
in which variations of up to +1oC may have occurred.
In their evidence to us, the Royal Society drew attention to these
papers but argued that natural variation alone cannot explain
recent warming. In
a separate critique, Dr McIntyre and Professor McKitrick of Canada
argue that one of the prominent hockey stick series is consistent
with marked increases in temperatures between 1400 and 1500.
Source: P. Jones and M. Mann, Climate over past millennia.
Reviews of Geophysics, 2004.
22. Some critics argue that the experience of
the last few hundred years is too short a period for the climate
models to determine the balance of natural and man-made factors
in temperature change. This is why considerable attention has
been paid to the longer run temperatures and the "hockey
stick". One attempt to reconstruct a long-term temperature
record is that of Professor Michael Mann of the University of
Virginia. The picture
that emerged for the period 1000-1980 is very much the hockey
stick shape. The Mann hockey stick appeared in the IPCC Climate
Change Assessment of 2001, thus achieving, as one journalist put
it, "iconic status".
In an analysis of Mann's et al data, Dr Stephen McIntyre and Professor
Ross McKitrick of the University of Guelph in Canada claim that
the analysis involves "collation errors, unjustifiable truncation
or extrapolation of source data, obsolete data, geographical location
errors" and other defects.
Their reconstructed series shows close correlation with Mann's
series from 1550 to 1980 but shows temperatures higher between
1400 and 1500 than any of the 20th century temperatures.
If correct, the late 20th century is no longer historically
unprecedented and the "hockey stick" does not exist.
We sought evidence that refuted the claims of McIntyre and McKitrick,
but have not come across any detailed rebuttal. One curious feature
of the debate over Professor Mann's time series is that the critics
appear to ignore other studies which secure similar hockey stick
23. We are in no position to determine who is
right and who is wrong in the growing debate on the hockey stick.
If there are historical periods of marked temperature increase,
it seems to us it is important to know why these occurred. Overall,
we can only urge that the issue is pursued in the next IPCC Assessment.
On past scares
24. Some of our witnesses drew attention to previous
environmental and resource exhaustion scares. The implication
is that since these scares did not materialise, neither might
accelerated global warming. While forecasters do seem to indulge
periodically in "end of the world" stories, there is
no guarantee that if they were wrong before they will be wrong
again. More importantly, the science of global warming has advanced
following substantial expenditures on research. Previous alarms,
such as the 1970's Limits to Growth debates (which have
not, in any event, gone away), earlier fears of global cooling
(rather than warming), and even the fear in the 19th
century over exhaustion of coal supplies, were based on more limited
scientific investigation. We do not believe that today's scientists
are "crying wolf": they may turn out to have been wrong
in some respects, but the arguments on which they base their case
are better researched than in earlier cases. That said, this Chapter
has sought to highlight some pressing issues which we believe
deserve a further response from the scientific community in order
to enhance understanding and resolve current controversies.
4 The effect is actually to warm the Earth by around
35oC, i.e. to +15oC rather than the approximate
-20oC that would otherwise prevail. Back
Sir John Houghton, Global Warming: the Complete Briefing.
Cambridge University Press. 3rd Edition. 2005, p32.
Ice cores can also be used to construct temperature and CO2
records going back over 400,000 years (and, most recently, cores
have been extracted that go back 900,000 years). As snow fell,
the air in the snow became trapped in the ice that subsequently
formed, so that greenhouse gas concentrations in the trapped air
bubbles can be measured. This gives the CO2 record
for the whole period. Examination of the oxygen and hydrogen isotopes
in the ice core also permits temperature readings. The two time-series-temperature
and CO2-appear to show a very close correlation, suggesting
that the two are closely linked. Ice ages had low levels of CO2
(about 210 ppm) and warm periods had high levels of CO2
(around 270 ppm). See J. Petit et al. Climate and atmospheric
history in the past 420,000 years from the Vostok ice core in
Antarctica. Nature, 399, June 3, 429-436, 1999. While correlation
is not causation, and there remains some dispute over the nature
of the linkage, there is also evidence that CO2 concentrations
"lead" temperature rather than the other way round.
See M. Maslin, Global Warming: A Very Short Introduction.
Oxford University Press, 2004, p.60. Back
R. Watson et al., op.cit. p.47. Back
J. Weier, Global Warming, Earth Observatory, NASA, Washington
DC. 8 April 2002. Back
"Radiative forcing" refers to the amount of energy
trapped by the atmosphere and is measured in watts per metre squared
A unique series exists for Central England from 1659 and can
be accessed at www.met-office.gov.uk/research/hadleycentre/CR_data/Annual/cet.gif
It is important to understand how temperature changes are computed
and portrayed. Temperature can obviously be measured daily and
even hourly, so there are huge numbers of observations from the
instrumental record. These are made more manageable by a process
of averaging through time. A "moving average" of, say,
5 years, would take the average over the first 5 years 1 to 5,
then the average of the 5 year period from years 2 to 6, and so
on. The larger the averaging period, say 50 years instead of 5
years, the "smoother" the resulting trend line becomes.
Turning points in this moving average therefore tend to change
with the averaging period. In the climate science literature the
difference between this moving average trend line and the actual
temperature is known as an "anomaly". To test whether
temperature and a greenhouse gas like CO2 are correlated,
it is the anomalies in temperature that are compared to CO2
concentrations. This allows the correlation not to be unduly influenced
by the time trends in the series. Back
Evidence from R. Lindzen (Vol II, pp 44-55) Back
On (a) see Sir John Houghton, Global Warming: The Complete
Briefing. Cambridge University Press, 2005. p.103. On (b)
see R. Watson et al., op.cit., p.198. Back
Evidence from the Royal Society (Vol II, pp 293-306) Back
R. Watson et al., op.cit. p.198 Back
Evidence from Sir D. King (Vol II, pp 96-106) Back
An excellent description of most of these debates is to be found
in M. Maslin, Global Warming: A Very Short Introduction.
Oxford: Oxford University Press, 2004. Back
H. von Storch et al. Reconstructing past climate from noisy data.
Science. 2004.306:679-682; A. Moberg et al. Highly variable
Northern Hemisphere temperatures reconstructed from low-and-high
resolution proxy data. Nature. 2005. 433: 613-7. Back
Evidence from the Royal Society (Vol II, pp 293-306). Back
M. Mann, R. Bradley and M. Hughes. Global-scale temperature
patterns and climate forcing over the past six centuries. Nature.
392, 1998. 779-787. 1999. M. Mann, R. Bradley and M. Hughes.
Northern hemisphere temperatures during the past millennium: inferences,
uncertainties and limitations. Geophysical Research Letters.
26. 1999. 759-762. M. Mann, R. Bradley and M. Hughes. Global-scale
temperature patterns and climate forcing over the past six centuries:
Corrigendum. Nature. 430. 2004. 105. The 1998 paper by
Mann et al. is for the period 1400-1980. The 1999 paper expands
the historical coverage back to 1000. Back
D. Appell. Behind the hockey stick. Scientific American.
March 2005. Back
Evidence from R. McKitrick (Vol II, pp 262-266). See also S.
McIntyre and R. McKitrick. Corrections to the Manne et al. (1998)
proxy data base and Northern Hemisphere average temperature series.
Energy and Environment. 14. 6.2003. 751-771. S. McIntyre
and R. McKitrick. The IPCC, the Hockey Stick Curve and the
Illusion of Experience. Washington DC: The George C Marshall
Institute. S. McIntyre and R. McKitrick. Verification of multi-proxy
paleoclimatic studies: a case study. Accepted Abstract. American
Geophysical Union Meetings, Paper PP53A-1580, December 2004. Back
K. Briffa et al. Low frequency temperature variations from a
northern tree ring density network. Journal of Geophysical
Research, 106, (D3), 2001, 2929-41. Back