A Tale of Two Climate HockeySticks

Published August 20, 2013

The  false “hockeystick” graph with which (in 2001) the UN climate panel claimed that  current surface temperatures are “unprecedented” in a millennium is at odds with  hundreds of scientific papers and with their own previous position.  There  is nothing unusual about today’s temperatures; the world was warmer in the  Middle Ages.  However, the “hockeystick” graph showing a rapid increase in  20th century CO2 concentration is genuine. 

The  Third Assessment Report (2001) of the UN-sponsored IPCC (Intergovernmental Panel  on Climate Change) espouses a temperature history over the last thousand years  that resembles a ‘hockeystick’ (HS).  The ‘shank’ is the smooth decline of  temperature from 1000 to 1900AD, followed by an apparent sharp rise in the  20th century (forming the ‘blade’ of a hockeystick).  IPCC-AR3  promoted this rise as definitive evidence of human influence on climate;  emission of carbon dioxide was supposed to cause the 20th century  warming.  But this temperature history is fake; it is contradicted by much  other evidence. 



Top figure from IPCC-AR1 (1990)  Bottom  figure from IPCC-AR3 (2001) – does away with MWP and LIA


By  “fake” I mean it is ‘not real.’  Please note that I do not use the term  “faked.”  I prefer to believe that Dr. Michael Mann, creator of the  hockeystick, simply made several scientific errors when he derived this  notorious graph of global surface temperature from his analysis of ‘proxy’ data  (tree rings, corals, lake sediments, etc) of the past millennium.  Canadian  scientists Steven McIntyre and Ross McKitrick, as well as statistics expert  Edward Wegman (of George Mason University in Virginia), demonstrated that Mann’s  data and statistical methods are both faulty; yet he has not withdrawn his HS  paper — though Nature was compelled to publish a belated and  inadequate Corrigendum by Mann.   Quietly however, he has co-published  temperature graphs that show a distinct  Medieval Warm Period (MWP) 1000 years ago and a Little Ice Age (LIA), mostly  around 1400-1800 AD.


1000 years of atmospheric CO2 and isotopic  composition at the South Pole




Note  the Little Ice Age and the Medieval Warm Period in the CO2 record (top graph).  There is a slight peaking (maximum delta C-13) in the isotopic composition at  the Little Ice Age (bottom graph), where presumably CO2 absorption by  ocean would be enhanced.


Carbon  dioxide data, mainly from sampling Antarctic ice cores over the last thousand  years, also show a shape that  (superficially) resembles a hockeystick; but this one is real.  Yet there is no relation between the  two graphs — or between temperature and carbon dioxide — as the following  discussion will show.  I fear that my comparison of the two graphs may  cause great unhappiness for extreme alarmists as well as for extreme skeptics of  global warming.


The  Temperature Record


If  one looks at the best temperature data over the last thousand years, they  overwhelmingly support the existence of an oscillation on a time scale of  1000-1500 years — with a temperature maximum, the Medieval Warm Period (MWP),  followed by the well documented Little Ice Age (LIA).  These temperature  swings are missing from Mann’s HS graph — which the IPCC unwisely featured, six  times, in its 3rd Assessment Report of 2001.  It is telling that  IPCC’s 4th Report of 2007 and the forthcoming 5th Report  no longer display the hockeystick-curve of Michael Mann.


A  more detailed examination of thermometer data of the 20th century  shows a major warming between 1910 and  1940, a slight cooling between 1940 and 1975, and a disputed temperature history  of the last two decades of the century.  It is interesting and worth noting  that Mann’s HS graph stops just before 1980 — even though the use of proxy data  would indicate an absence of warming from 1980 to 2000.  The leaked  ‘Climategate’ e-mails between IPCC scientists, including Mann himself, suggest  that “hiding the decline” in tree-ring temperature reconstructions, which should  have been showing the same warming as the thermometers, was the reason why Mann  stopped his analysis in 1980 — and why his post-1980 proxy temperatures have  never been revealed. 


(An  email from Professor Phil Jones at the University of East Anglia to Mann and his  co-authors said: “I’ve just completed Mike’s [Mann] Nature  trick  [sic] of adding in the real temps to each series for the last 20 years (i.e.  from 1981 onwards) and from 1961 for Keith’s [Briffa] to hide the  decline.”)


The  CO2 Record since 1000AD


Turning  now to the CO2 data (assembled by Tom Quirk in Australia and kindly  transmitted by Prof. Will Happer of Princeton Univ.): They show a remarkable  hockeystick shape — which, however, is real.  The concentrations of  atmospheric CO2 were preserved in air bubbles trapped in snow that  turned into ice; the time record has been obtained from different samplings of  Antarctic ice cores.  It is supplemented and confirmed by a corresponding  decrease in the isotope ratio of C-13/C-12.  The sharp decrease of C-13 in  the 20th century indicates that its source is most likely biogenic;  fossil fuels from ancient plant material and biota would fit this  specification. 


1.   A quick look at the CO2 curve shows a slight increase during the MWP  and a slight decrease during the LIA.  These excursions are readily  interpreted as a (net) reduced uptake of CO2 when the ocean is warm  and increased uptake when the ocean is cold.  [We recall that roughly half  of the CO2 emitted into the atmosphere during fossil-fuel burning and  cement manufacture is absorbed into the ocean and that CO2 solubility  depends inversely on ocean temperature — the well-known ‘champagne  effect’]. 


A  more detailed interpretation of the CO2 curve leads to these  additional conclusions:


2.   Various skeptics have suggested that CO2 levels were higher during  the 19th century than they are today.   There is nothing  wrong per se with these old measurements — though they were performed  by old-fashioned chemical methods rather than current infrared techniques.   It just means that the data obtained were contaminated and were not  representative of global concentrations of free-atmosphere CO2.    Antarctic is reasonably free of contamination.


3.   It is often claimed by skeptics that the human contribution to atmospheric  CO2  (from fossil-fuel burning) is tiny — less than  a percent.  The data clearly show that the contribution is 400 minus 280  parts per million (ppm) — roughly 30% of the current  concentration.


4.   Extreme skeptics have often claimed that George Callender, the British pioneer  of the global-warming story during the early 20th century, was hiding  some higher CO2 values from ice cores that approached present  values.  This does not seem to be the case.


5.   From time to time, skeptics have claimed that the CO2 increase was  mainly due to global warming, which caused the release of dissolved  CO2 from the ocean surface into the atmosphere.  (A recent  adherent of this hypothesis is Prof. Murry Salby in Australia.)  However,  the evidence appears to go against such an inverted causal relation.  While  this process may have been true during the ice ages, the isotope evidence seems  to indicate that the human contribution from fossil-fuel burning clearly  dominates during the last 100 years.


6.   Finally, note that the temperature ‘blade’ starts around 1910, while  CO2 starts its sharp upward climb around 1780AD.




Upon  reflection on these temperature and CO2 data of the last thousand  years, I conclude that the evidence presented here will cause unhappiness for  both extreme alarmists and extreme skeptics.  Maybe that’s a good  thing. 


However,  it still leaves open the question of climate sensitivity (CS), i.e., the actual  influence of CO2 on global temperature.  CS has been steadily  decreasing, from 3oC (for CO2 doubling) in the initial  IPCC report.  I believe confidently that the real CS values may be much  lower — although not quite zero.  I base it on what Dr. Rajendra Pachauri,  chairman of the IPCC, has delicately called the 17-year “pause” in global  warming of the near-surface, and on the near-absence of tropical atmospheric  warming, as seen by weather satellites since 1978. 


The  scientific puzzle is why IPCC climate models predict large values for CS while  the observations only show only small ones.