Global Warming

[This is part 2 of the Global Warming Summary series at www.appinsys.com/GlobalWarming]

Part 2: The IPCC Hockey Stick and the Medieval Warm Period

The “hockey stick” graph produced by Michael Mann was influential in creating the current global warming crisis. The hockey stick graph (so-called because of its shape) appears to show that the Earth’s climate was very stable from AD1000 to 1900, and then suddenly began to change, with temperatures in the Northern Hemisphere rising dramatically. It was central to the 2001 Third Assessment Report (TAR) from the Intergovernmental Panel on Climate Change (IPCC). It appears several times within the reports that are part of the TAR. This was a major impetus in creating the current notion that a crisis is looming and that it is caused by CO2 released by the burning of fossil fuels.

Figure 2-1 shows pages 28-29 of the IPCC 2001 TAR technical Summary. The right-hand page shows the hockey-stick graph, while the left-hand page shows two other relevant climate data series.

Figure 2-1: Extract of IPCC Technical Summary pages 28-29

The shown in Figure 2-1 left-hand page, show the global average of tropospheric temperatures derived using weather satellites (upper graph) and the lower stratospheric temperatures from balloons and satellites. This is enlarged below in Figure 2-2. As can be seen by comparing the atmospheric data to the hockey stick, graph a different trend is observed. However, the graph of satellite-measured tropospheric data was omitted altogether from the IPCC TAR Summary for Policymakers. As soon as the IPCC Report came out, the hockey stick version of climate history became canonical – it became the “consensus” view (Part 7 of the Global Warming Summary discusses how consensual the consensus is.)

Figure 2-2: Larger View of Satellite and Balloon Data From IPCC Technical Summary page 28

Scientists try to estimate local climate histories over past centuries using various techniques, including temperature proxies of various types. (Part 1 of the Global Warming Summary discusses the various long-term temperature trend graphs). Proxies include a wide range of measures that are, potentially, sensitive to local temperature trends, such as tree ring widths, ice cores, and ground boreholes. Boreholes drilled into the ground have a vertical temperature profile that can be inverted to yield an estimate of the historical temperature sequence at the surface. Michael Mann’s data is composed of various proxy data sets, mainly based on tree rings. In 1998 Nature published the first Mann hockey stick paper, commonly called “MBH98.” Mann et al. followed up in 1999 with a paper in Geophysical Research Letters (“MBH99”) extending their results from AD1400 back to AD1000. In early 2000 the IPCC released the first draft of the TAR. The hockey stick was the only paleoclimate reconstruction shown in the Summary, even though most other scientists disagree with the elimination of the Medieval Warm Period and Little Ice Age.

Ross McKitrick of the University of Guelph did an in-depth study of Michael Mann’s data used to produce the hockey stick graph. What he found was that Mann used a data analysis method that essentially forced the data sets (mostly tree-ring proxies, but also other proxies) to take the shape of strongest shapes found in any data set. Most of the data sets exhibited no trend. But those that did were weighted in an unrealistic manner. Detailed information about his analysis can be found at http://www.uoguelph.ca/~rmckitri/research/trc.html.

Figure 2-3 shows two of Mann’s tree-ring proxy data sets. Both series are the same length, but due to the increasing trend in the 20^th century in the top panel, Mann’s algorithm gives that series 390 times the weight of the bottom series.

Figure 2-3: Two tree ring chronologies from the MBH98 data set. Top: Sheep Mountain, CA, USA. Bottom: Mayberry Slough

McKitrick found many errors in Mann’s data analysis methods, which have been published [McIntyre, Stephen and Ross McKitrick (2005a) “The M&M Critique of the MBH98 Northern Hemisphere Climate Index: Update and Implications.” Energy and Environment 16(1) pp. 69-100; (2005b) “Hockey Sticks, Principal Components and Spurious Significance” Geophysical Research Letters Vol. 32, No. 3, L03710 10.1029/2004GL021750 12 February 2005. See www.climateaudit.org.]

Figure 2-4 shows one of the results of McKitrick’s analyses where invalid data were removed. The hockey stick (dashed line) ends up showing the occurrence of the Medieval Warm Period (something that Mann denies occurred).

In 2006 a National Academy of Sciences (NAS) panel investigated the hockey stick graph used by the (IPCC) to promote the idea that humans are causing rapid and unprecedented global warming. McKitrick had argued that the hockey stick relied for its shape on the inclusion of a small set of invalid proxy data (called bristlecone, or “strip-bark” records). If they are removed, the conclusion that the 20th century is unusually warm compared to the pre-1450 interval is reversed. Hence the conclusion of unique late 20th century warmth does not hold up under minor variations in data or methods. The NAS panel agreed, saying Mann’s results are “strongly dependent” on the strip-bark data (pp. 106-107), and they went further, warning that strip-bark data should not be used in this type of research (p. 50).

McKitrick’s analysis was further backed up by a study by Wegman etc al [Edward J. Wegman, George Mason University, David W. Scott, Rice University, and Yasmin H. Said, The Johns Hopkins University.], which verified that Mann’s hockey-stick graph was invalid.

Figure 2-4: Dashed line: MBH98 proxy-based Northern Hemisphere temperature index reconstruction. Solid line: Series resulting from using corrected PCs (retaining 5 PCs in the North America network), removing Gaspé extrapolation and applying CO₂ fertilization adjustment to full length of bristlecone pine series

The Medieval Warm Period (MWP) was previously accepted as a warm period that occurred in the time frame of about 1000-1400 AD. That is, until the advent of IPCC and Mann’s hockey stick – now many claim the MWP never happened, or that it was a minimal regional event. But many others do not accept the recent IPCC-inspired rejection of the MWP, which was based on Mann’s flawed study.

Figure 2-5: Red Line – 1980’s Accepted Temperature Trend, Blue Line – IPCC 2001 Hockey Stick, Black Line – Moberg et. al (see Fig 1-3 and 1-4)

The IPCC 1990 report showed the temperature graph of the red line in Figure 2-5, which indicates the MWP and the Little Ice Age (LIA). But in the IPCC 1995 report the only pre-instrumental temperature trend uses a northern hemisphere temperature reconstruction, starting in 1400 -thus leaving out the MWP. The IPCC 2001 report (TAR) shows temperature reconstructions starting from 1000 AD from Mann, Jones and Briffa and prominently displays the hockey-stick graph as shown in Figure 2-1. The TAR states: “the posited Medieval Warm Period appears to have been less distinct, more moderate in amplitude… The Northern Hemisphere mean temperature estimates of Jones et al. (1998), Mann et al. (1999), and Crowley and Lowery (2000) show temperatures from the 11th to 14th centuries to be about 0.2°C warmer than those from the 15th to 19th centuries, but rather below mid-20th century temperatures. Thus current evidence does not support globally synchronous periods of anomalous cold or warmth over this timeframe, and the conventional terms of "Little Ice Age" and "Medieval Warm Period" appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries ”

This is at odds with the findings of other scientists. For example, research at the Harvard-Smithsonian Center for Astrophysics report on a recent paper using proxies, which verifies the occurrence of the MWP: [http://cfa-www.harvard.edu/press/pr0310.html] “A review of more than 200 climate studies led by researchers at the Harvard-Smithsonian Center for Astrophysics has determined that the 20th century is neither the warmest century nor the century with the most extreme weather of the past 1000 years. The review also confirmed that the Medieval Warm Period of 800 to 1300 A.D. and the Little Ice Age of 1300 to 1900 A.D. were worldwide phenomena not limited to the European and North American continents. While 20th century temperatures are much higher than in the Little Ice Age period, many parts of the world show the medieval warmth to be greater than that of the 20th century.”

The University of Southern California course material on climate change in the last millennium [http://earth.usc.edu/geol150/evolution/lastmillenia.html] also provides supporting evidence of the MWP and LIA (although after citing evidence from all continents they say it may be just regional existing only in the northern hemisphere).

Many studies can be found exhibiting the MWP. One example is shown in Figure 2-6.

[http://www.wsl.ch/staff/jan.esper/publications/QRS_Cook_2004.pdf] One of the authors of the study said "The Medieval Warm Period is in some sense comparable up to 1990 in the 20th century," said Cook. "But that does not say that the 20th century hasn't been perturbed by greenhouse gases. The real challenge is to factor out the natural variability from" manmade causes of global warming.”

[http://www.usatoday.com/news/science/climate/2002-02-03-tree-rings.htm]

Figure 2-6: Temperature Trend Exhibiting MWP Around Year 1000

One of the most recent multi-proxy reconstructions was published in 2005 by Moberg et al (Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data
Nature, Vol. 433, No. 7026, pp. 613 - 617, 10 February 2005 -- Anders Moberg, Dmitry M. Sonechkin, Karin Holmgren, Nina M. Datsenko & Wibjörn Karlén) (This is the red line shown in Figure 1-4 in Part 1. of this document) [http://www.ncdc.noaa.gov/paleo/pubs/moberg2005/moberg2005.html] and [http://stephenschneider.stanford.edu/Publications/PDF_Papers/MobergEtAl2005.pdf] Their paper states “These reconstructions have mainly used tree-ring data and other data sets of annual to decadal resolution. Lake and ocean sediments have a lower time resolution, but provide climate information at multicentennial timescales that may not be captured by tree-ring data. Here we reconstruct Northern Hemisphere temperatures for the past 2,000 years by combining low-resolution proxies with tree-ring data, using a wavelet transform technique to achieve timescale-dependent processing of the data. Our reconstruction shows larger multicentennial variability than most previous multi-proxy reconstructions, but agrees well with temperatures reconstructed from borehole measurements and with temperatures obtained with a general circulation model. According to our reconstruction, high temperatures - similar to those observed in the twentieth century before 1990- occurred around AD 1000 to 1100, and minimum temperatures that are about 0.7K below the average of 1961-90 occurred around AD 1600. This large natural variability in the past suggests an important role of natural multicentennial variability that is likely to continue… The main implication of our study, however, is that natural multicentennial climate variability may be larger than commonly thought, and that much of this variability could result from a response to natural changes in radiative forcings.”

Figure 2-7 is from their paper. The MJ line in 2-7 a) is the Mann – Jones multi-proxy study while the MBH line in 2-7 d) is the Mann et al proxy study (the hockey stick), further evidence of how Mann’s hockey stick does not match reality. One of the proxies included in Moberg et al is the July temperatures at Lake Tsuolbmajavri (Finland) shown in Figure 2-8 based on diatoms in lake sediments.

Although the IPCC denies the existence of the MWP and LIA as global events, many studies exist from around the world verifying the events. (The fact that the current warming varies significantly around the world seems irrelevant to them – they want to deny it to justify their hockey stick). A couple of examples:

“The Little Ice Age and Medieval Warm Period in the Sargasso Sea” (Science 29 November 1996:
Vol. 274. no. 5292, pp. 1503 – 1508) by Lloyd D. Keigwin (Woods Hole Oceanographic Institution)

[http://www.sciencemag.org/cgi/content/abstract/274/5292/1503?rbfvrToken=b3527f8140d1ddfd7f0fdac765ac49b01f52eacb] which states: “Results from a radiocarbon-dated box core show that SST was 1°C cooler than today 400 years ago (the Little Ice Age) and 1700 years ago, and 1°C warmer than today 1000 years ago (the Medieval Warm Period). Thus, at least some of the warming since the Little Ice Age appears to be part of a natural oscillation.”

“Was the Medieval Warm Period Global?” (Science 23 February 2001: Vol. 291. no. 5508, pp. 1497 - 1499) by Wallace S. Broecker (Lamont-Doherty Earth Observatory, Columbia University) [http://www.sciencemag.org/cgi/content/summary/291/5508/1497] summary states: “During the Medieval Warm Period (800 to 1200 A.D.), the Vikings colonized Greenland. In his Perspective, Broeker discusses whether this warm period was global or regional in extent. He argues that it is the last in a long series of climate fluctuations in the North Atlantic, that it was likely global, and that the present warming should be attributed in part to such an oscillation, upon which the warming due to greenhouse gases is superimposed.”

Figure 2-7: Moberg 2005 Multi-Proxy Temperature Reconstruction (2000 Years)

Figure 2-8: July temperatures at Lake Tsuolbmajavri (years before present)

“Little Ice Age and Medieval Warm Periods in Eastern China as Read from the Speleothem Records” (American Geophysical Union, 2002) by H. Li and T. Ku (University of Southern California, Department of Earth Sciences) [http://adsabs.harvard.edu/abs/2002AGUFMPP71C..09L] which states “The long-term climatic pace has often been interrupted by short-term abrupt changes. The Little Ice Age and Medieval Warm Period represent the two most important such changes over the last two millennia. Largely due to a dearth of high-resolution climatic records, our knowledge on the spatial extent, duration, and moisture characteristics of these two events is incomplete, and this has hampered our understanding of the driving force causing them as well as the recent global warming trend. … The records show that in eastern China, the Medieval Warm Period started around 1000 AD and lasted until 1500 AD. A brief cooling during this warm interval occurred around 1150 AD. The Little Ice Age in China started at around 1500 AD and ended in the mid-1800s.”

“Tree-ring and glacial evidence for the medieval warm epoch and the little ice age in southern South America” (Climate Change Journal Volume 26, Numbers 2-3 / March, 1994) by Ricardo Villalba (Laboratorio de Dendrocronologia, Argentina) [http://www.springerlink.com/content/x0214563n1n44731/ ] which states: “A tree-ring reconstruction of summer temperatures from northern Patagonia shows distinct episodes of higher and lower temperature during the last 1000 yr. The first cold interval was from A.D. 900 to 1070, which was followed by a warm period A.D. 1080 to 1250 (approximately coincident with the Medieval Warm Epoch).”

Although the IPCC no longer uses the hockey stick grpah, they have replaced it with a graph of multiple proxy studies, as shown in Figure 2-9. Recently IPCC AR4 reviewer Steve McIntyre described his review of the usage of bristlecone and foxtail pines in the IPCC temperature proxy figure (shown below) [http://www.climateaudit.org/?p=2245].

Figure 2-9: Box 6.4 Figure 1 from the IPCC 4AR (Chapter 6)

In his review, McIntyre critcized the IPCC for including proxies that were based on bristlecone and foxtail pines (since tree rings from these trees have been shown in various studies to exhibit no correlation with temperature). McIntyre calculated correlations for the included studies and found that some had zero correlation to temperatures at the study locations. The IPCC lead author of the section rejected the reviewer’s comment stating: “the purpose of this Figure is to illustrate in a simple fashion, the variability of numerous records that have been used in published reconstructions of large-scale temperature changes”. The figure shown above also includes the discredited MBH1999 hockey stick.

Dr. David Deming (University of Oklahoma, College of Earth and Energy) said in his testimony to congress [http://epw.senate.gov/hearing_statements.cfm?id=266543] “I had another interesting experience around the time my paper in Science was published. I received an astonishing email from a major researcher in the area of climate change. He said, "We have to get rid of the Medieval Warm Period.” The existence of the MWP had been recognized in the scientific literature for decades. But now it was a major embarrassment to those maintaining that the 20th century warming was truly anomalous.”