The “Hockey Stick” Controversy

One of the most significant climate change controversies started when researchers noted that a sharp increase in the carbon dioxide (CO2) concentration in the atmosphere beginning in the 1970’s was accompanied by a significant rise in global mean temperature (Figure 1). [1] The shape of the resulting trend of surface temperature over the past 1000 years became known as the “hockey stick” diagram (see right panel in Figure 2 below).  An inference was made that the rise in temperature was directly caused by a simultaneous increase in CO2 concentration. There are several problems with this conclusion: (1) an equally sharp rise in temperature in the early 1900’s was only accompanied by a modest increase in CO2 (Figure 1), (2) global temperatures from land-based observing stations ceased rising in the late 1990’s (more on this in a later blog), (3) a Medieval warm period observed in historical temperature records from about 1100 to 1400 AD disappeared from the Intergovernmental Panel on Climate Change (IPCC) reports between 1990 and 2001 (Figure 2), and (4) some question whether CO2 increases have much if any effect on the Earth’s temperature.

A recent unpublished report [2] questioned the reliability of the Global Average Surface Temperature (GAST) data set used for decision-making worldwide. The GAST data set extends from 1880 to the present and is based on temperature observations over land (mostly from airport sites) and from ocean-going vessels and buoys. The report shows that cyclical patterns in the raw temperature data have been removed by adjustments that reduced the magnitude of warm periods in the early to mid 20th century (such as the 1930’s and early 1950’s) while increasing mean temperatures in more recent decades. The purpose of the adjustments was to correct for changes in reporting locations and instruments over land, and methods of collecting temperatures from ships over oceans. The result has been a sharpening of the upward trend in the records to one that resembles the “hockey stick.” The claims are supported by graphs originally from official sources such as NOAA, EPA, the U. K. Met Office, the Hadley Centre at East Anglia University, and NASA.

While the 2001 IPCC report included additional types of data in the longer period of record (~1000 years), such as tree ring analyses, it was thought that the conclusion of a modern spike in global temperatures was exaggerated relative to historical temperature records.  Further doubt was cast when researchers found that the statistical method used to create the updated trend line (Principle Component Analysis) was flawed, emphasizing those data series (referred to as “proxies”) that showed a temperature trend favoring the “hockey stick” shape with time. [3,4] The result is that the medieval warm period and a cool period known as the Little Ice Age in the early 1800’s were nearly eliminated (Figure 2). Other scientists have been unable to replicate the “hockey stick” trend in their independent analyses.


Figure 1. Trend of CO2 concentration (ppm) since 1880 (solid black line) compared with annual mean global temperature (°F) for the same period. Horizontal reference line is mean temperature for the period (57.6°F). (Source: NOAA/National Climate Data Center)


Figure 2. Trends of global temperature (°C) for the past 1000 years as shown by the Intergovernmental Panel on Climate Change (IPCC) reports for 1990 (left) and 2001 (right).(From McKitrick, 2005)

In regards to the longer, 1000 year trend of global temperatures, an independent study based on data from over 6,000 sub-surface borehole records (formed by layers of petrified mud in lake bottoms) that go back 20,000 years clearly showed that the Medieval Warm Period and Little Ice Age existed. [5] In fact, the medieval warm period was significantly warmer than what we have seen in the late 20th century. Nevertheless, the results of this study were ignored by the IPCC. It is interesting to note that the “Hockey Stick” figure was removed entirely in both the 2007 and 2013 IPCC reports. The only temperature trends shown in those reports were in the 1850-2005 year range. So while we should still be concerned about future climate change, recent global trends in surface temperature do not appear to be extreme based on trends over the past millennium.


Figure 3. Trend in global mean temperatures (°C) over the past 1,000 years inferred from over 6,000 soil cores (adapted from figure in Huang et al. 1997). The large peak centered between 1300 and 1400 is the Medieval Warm Period, and the dip centered near 1800 is the Little Ice Age. Top and lower lines show the range of uncertainty.

The role of CO2 in warming the Earth’s climate has been questioned by some. One researcher, the late Dr. William Gray, argued that the expected increase in CO2, even a doubling of CO2, will not bring the anticipated increase in global temperatures. Instead, he believed that warming of only half a degree C or less will occur, not the 2-4°C or more expected by the IPCC.[6]  This claim has historical basis. Around 1900, Swedish physicist Knut Angstrom determined that CO2 concentration beyond about 50 ppm has little effect on the Earth’s temperature, although the results of that experiment were disputed. Dr. Gray believed that changes in the strength of the inter-ocean circulation (such as the Atlantic Thermohaline Current) which controls the salinity of the oceans has led to the 0.8°C increase we have seen in the past century, not man-made greenhouse gases.

Another interesting theory that support’s a diminished role of CO2 in climate change is that the recent increases in the Earth’s temperature were not due to increased CO2 concentrations, but to the depletion of ozone in the upper atmosphere [7]. Our ozone layer, with peak concentrations at around 20 km altitude, absorbs most of the dangerous ultraviolet-B (UV-B) radiation from the sun before it reaches the Earth. According to the Planck-Einstein relationship, energy is proportional to the frequency of electromagnetic radiation (EMR). Thus, EMR from the sun (UV-B), provides almost 50 times more energy than infrared EMR emitted from Earth which has a much lower frequency (i.e., longer wavelength), and thus is more effective in producing temperature change at the surface.

Depletion of atmospheric ozone due to release of man-made chlorofluorocarbons (CFCs) into the upper atmosphere, which began in the 1960’s, continued into the early part of the 21st century.  A plot of Northern Hemisphere temperature anomalies versus total column ozone (Dobson Units – DU) from 1927 to 2013 is plotted in Figure 4. Although there is considerable scatter, the decrease in ozone of 30 DU explains a temperature increase of 0.9°C, which is close to that observed. The scatter in the diagram is believed to be due to (1) emission of volcanic aerosols and (2) sudden, large ozone emissions due to tests of nuclear devices and volcanic lava flows. The Montreal Protocol, signed in 1989, began to alleviate this problem by reducing emission of CFCs.

Figure 4. A decrease in yearly total column ozone at mid-latitudes of the northern hemisphere by 30 Dobson Units (DU) from 1927 to 2013 corresponds  to a 0.9°C increase in temperature anomaly. Numbers are the years of observations. (from Ward, 2016)

In short, a rise in global mean temperature of about 1°C has occurred since the beginning of the Industrial Revolution over the past 150 years but that is not an alarming amount based on historical temperature records for the past millennium, which includes a medieval warm period believed to be significantly warmer than today. Carbon dioxide levels, meanwhile, have risen significantly to about 400 ppm since the latter part of the 20th century, but the resulting temperature increase (0.8 °C) has not been consistently in step. For example, there has been little or no rise in global mean temperature from 1998 to 2013.


1, Mann, M. E., Bradley R. S., and Hughes, M. K., 1998: Global-scale temperature patterns and climate forcing over the past six centuries. Nature, 392, pages 779-787

2. Wallace, J. P., J. S. D’Aleo, and C. D. Idso, 2017: On the validity of NOAA, NASA and Hadley CRU Global Average Surface Temperature data and the validity of EPD’s CO2 endangerment finding. Abridged Research Report. June 2017, 30 pages. (available online at: 

3. McKitrick, R., 2005: What is the hockey stick debate about? Asia-Pacific Economic Cooperation (APEC) Study Group, Canberra, Australia, April 4, 2005, invited paper

4. Muller, R., 2004: Global Warming Bombshell. MIT Technology Review,

5. Huang, S., Pollack, H. N., and P. Y. Shen, 1997: Late Quaternary Temperature Changes Seen in Worldwide Continental Heat Flow Measurements. Geophys. Res. Letters 24, pages 1947-1950.

6. Gray, William M., 2012: Physical Flaws of the Global Warming Theory and Deep Ocean Circulation Changes as the Primary Climate Driver. Unpublished manuscript prepared for the Heartland Institute’s 7th Int’l Conference on Climate Change, Chicago, May 21-23, 2012.

7. Ward, Peter L., 2016: Ozone Depletion Explains Global Warming. Current Physical Chemistry, Volume 6, 21 pages.

Next: Recent Satellite-observed Trends and the “Pause” in Global Warming


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