RealClimate logo


Temperature Variations in Past Centuries and the so-called "Hockey Stick"

Instrumental data describing large-scale surface temperature changes are only available for roughly the past 150 years. Estimates of surface temperature changes further back in time must therefore make use of the few long available instrumental records or historical documents and natural archives or ‘climate proxy’ indicators, such as tree rings, corals, ice cores and lake sediments, and historical documents to reconstruct patterns of past surface temperature change. Due to the paucity of data in the Southern Hemisphere, recent studies have emphasized the reconstruction of Northern Hemisphere (NH) mean, rather than global mean temperatures over roughly the past 1000 years.

The term “Hockey Stick” was coined by the former head of NOAA’s Geophysical Fluid Dynamics Laboratory, Jerry Mahlman, to describe the pattern common to numerous proxy and model-based estimates of Northern Hemisphere mean temperature changes over the past millennium. This pattern includes a long-term cooling trend from the so-called “Medieval Warm Period” (broadly speaking, the 10th-mid 14th centuries) through the “Little Ice Age” (broadly speaking, the mid 15th-19th centuries), followed by a rapid warming during the 20th century that culminates in anomalous late 20th century warmth (Figure 1). Numerous myths regarding the “hockey stick” can be found on various non-peer reviewed websites and other non-scientific venues.

Estimates of Northern Hemisphere average temperature changes from climate model simulations employing estimates of long-term natural (e.g. volcanic and solar) and modern anthropogenic (greenhouse gas and sulphate aerosol) radiative forcings of climate agree well, in large part, with the empirical, proxy-based reconstructions. One notable exception is a study by Gonzalez-Rouco et al (2003) that makes use of a dramatically larger estimate of past natural (solar and volcanic) radiative forcing than is accepted in most studies, and exhibits greater variability than other models (Figure 2). Yet, as in all of the other simulations, even in this case unprecedented warmth is indicated for the late 20th century.

The simulations all show that it is not possible to explain the anomalous late 20th century warmth without including the contribution from anthropogenic forcing factors, and, in particular, modern greenhouse gas concentration increases. A healthy, vigorous debate can be found in the legitimate peer-reviewed climate research literature with regard to the precise details of empirically and model-based estimates of climate changes in past centuries, and it remains a challenge to reduce the substantial uncertainties that currently exist. Despite current uncertainties, it nonetheless remains a widespread view among paleoclimate researchers that late 20th century hemispheric-scale warmth is anomalous in a long-term (at least millennial) context, and that anthropogenic factors likely play an important role in explaining the anomalous recent warmth.

Reviews of past scientific research in this area can be found in the following peer-reviewed journal articles:

FIGURE 1 [reprinted from Mann et al, 2003, Eos, (C) American Geophysical Union]. Comparison of proxy-based Northern Hemisphere (NH) temperature reconstructions (Jones et al., 1998; Mann et al., 1999; Crowley and Lowery, 2000) with model simulations of NH mean temperature changes over the past millennium based on estimated radiative forcing histories (Crowley, 2000; Gerber et al., 2002–results shown for both a 1.5oC/2xCO2 and 2.5oC/2xCO2 sensitivity; Bauer et al., 2003). Also shown are two independent reconstructions of warm-season extratropical continental NH temperatures (Briffa et al., 2001; Esper et al., 2002) and an extension back through the past two thousand years based on eight long proxy temperature series chosen for their ability to retain long-term trends (Mann and Jones, 2003). All reconstructions have been scaled to the annual, full Northern Hemisphere mean, over an overlapping period (1856-1980), using the NH instrumental record (Jones et al., 1999) for comparison, and have been smoothed on time scales of >40 years to highlight the long-term variations. The smoothed instrumental record (1856-2003) is also shown. The gray/pink shading indicates estimated two-standard error uncertainties in the Mann et al. (1999) and Mann and Jones (2003) reconstructions. Also shown are reconstructions of ground surface temperatures (GST) based on appropriately areally-averaged (Briffa and Osborn, 2002; Mann et al., 2003) continental borehole data (Huang et al., 2000), and hemispheric surface air temperature trends, determined by optimal regression (Mann et al., 2003) from the GST estimates. All series are shown with respect to the 1961-90 base period. [added 1/12/05: It should be noted (thanks to S. Huang for pointing this out), that the two borehole curves shown in this plot suffer from a small error in the areal weighting normalization. Further discussion, and corrected versions of the borehole temperature estimates can be found in Rutherford and Mann (2004) (and in the Jones and Mann, 2004 article discussed above)]

FIGURE 2 [reprinted from Jones and Mann, 2004, Reviews of Geophysics, (C) American Geophysical Union]. Model-based estimates of northern hemisphere temperature variations over the past two millennia. Shown are 40 year smoothed series. The simulations are based on varying radiative forcing histories employing a hierarchy of models including one-dimensional energy based models (Crowley, 2000), two-dimensional reduced complexity models (Bauer et al, 2003; Bertrand et al, 2002; Gerber et al, 2003), and full three-dimensional coupled atmosphere-ocean general circulation (’GKSS’-Gonzalez-Rouco et al, 2003; ‘CSM’–Ammann et al., submitted). Shown for comparison is the instrumental northern hemisphere record 1856-2003 (Jones et al, 1999), and the proxy-based estimate of Mann and Jones (2003) extended through 1995, with its 95% confidence interval. Models have been aligned vertically to have the same mean over the common 1856-1980 period as the instrumental series (which is assigned zero mean during the 1961-1990 reference period).

References:

Bauer, E., M., Claussen, and V. Brovkin, Assessing climate forcings of the earth system for the past millennium, Geophys. Res. Lett., 30 (6), doi: 10.1029/2002GL016639, 2003.

Bertrand C., M.F. Loutre, M. Crucifix, and A. Berger, Climate of the Last millennium: a sensitivity study, Tellus, 54(A), 221-244, 2002.

Briffa, K.R., and T J. Osborn, Blowing Hot and Cold, Science, 295 2227-2228, 2002.

Briffa, K.R., T.J. Osborn, F.H. Schweingruber, I.C. Harris, P.D. Jones, S.G. Shiyatov and E.A. Vaganov, Low-frequency temperature variations from a northern tree-ring density network. J. Geophys. Res., 106, 2929 2941, 2001.

Cook, E.R., J. Esper, and R.D. D’Arrigo, Extra-tropical Northern Hemisphere land temperature variability over the past 1000 years, Quat. Sci. Rev., 23, 2063-2074, 2004.

Crowley, T.J., Causes of Climate Change over the Past 1000 Years, Science, 289, 270-277, 2000.

Crowley, T.J., and T. Lowery, How Warm Was the Medieval Warm Period?, Ambio, 29, 51-54, 2000.

Esper, J., E.R. Cook and F.H. Schweingruber, Low-frequency signals in long tree-line chronologies for reconstructing past temperature
variability, Science, 295, 2250-2253, 2002.

Gerber, S., F. Joos, P. Brügger, T. F. Stocker, M. E. Mann, S. Sitch, and M. Scholze, Constraining temperature variations over the last millennium by comparing simulated and observed atmospheric CO2, Climate Dynamics, 20, 281-299, 2003.

Gonzalez-Rouco, F., H. von Storch, and E. Zorita, Deep soil temperature as proxy for surface air-temperature in a coupled model simulation of the last thousand years, Geophys. Res. Lett., 30, 2116, doi:10.1029/2003GL018264, 2003.

Huang, S., H. N.Pollack and P.-Y. Shen, Temperature Trends Over the Past Five Centuries Reconstructed from Borehole Temperature, Nature 403, 756-758, 2000.

Jones, P.D., K.R. Briffa, T.P. Barnett and S.F.B. Tett, High-resolution palaeoclimatic records for the last millennium: Integration, interpretation and comparison with General Circulation Model control run temperatures, Holocene, 8, 455-471, 1998.

Jones, P.D., M. New, D.E. Parker, S. Martin, and I.G. Rigor, Surface air temperature and its changes over the past 150 years, Reviews of Geophysics, 37, 173-199, 1999.

Jones, P.D., T.J. Osborn, and K.R. Briffa, The Evolution of Climate Over the Last Millennium, Science, 292, 662-667, 2001.

Mann, M.E., R.S. Bradley, and M.K. Hughes, Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations, Geophysical Research Letters, 26, 759-762,
1999.

Mann, M.E., Jones, P.D., Global surface temperature over the past two millennia, Geophysical Research Letters, 30 (15), 1820, doi: 10.1029/2003GL017814, 2003.

Mann, M.E., Rutherford, S., Bradley, R.S., Hughes, M.K., Keimig, F.T., Optimal Surface Temperature Reconstructions Using Terrestrial Borehole Data, Journal of Geophysical Research, 108 (D7), doi: 10.1029/2002JD002532, 2003.


6 Responses to “Temperature Variations in Past Centuries and the so-called "Hockey Stick"”

  1. 1
    donald baker says:

    Dear RC: I am grateful to find this site. For the most part, it succeeds in presenting a scientific, non-political tone, with lots of useful information. As long as you maintain this non-political stance, the site can do great good. Science is becoming more and more politicized and is in danger, on certain subjects such as this, of losing all credibility in the way that social and political science have done, generally. As a result, we get The Day After Tomorrow and State of Fear, both highly political (the former hysterical as well)and obviously partisan representations of issues. If you can avoid such phrases as “right-wing” or “left-wing” you can contribute much to this important discussion. Yours, Don Baker

  2. 2
    Steve Funk says:

    Does anyone know what caused the medieval warming? I would assume it is not anthropogenic.

    [Response: In the modeling studies shown in Figure 2 above, the relatively warm temperatures between AD 800-1200 or so are due to a combination of factors including (1) a relative lack of explosive tropical volcanic eruptions (which can a substantial global cooling influence) in comparison with later centuries. and (2) relatively high estimated values of solar irradiance (though solar reconstructions exhibiting very large century-scale variability, such as that used in the "GKSS" simulation shown in Figure 2, have recently been called into question--see this article in Science by Foukal et al, and references therein). -mike]

    How do we know that these processes are not responsible for current climatic variation?

    [Response: The same simulations referred to above indicate that natural factors such as volcanoes or solar variations are not sufficient to produce the observed 20th century forcing. The observed 20th century warming ins these simulations can only be produced through the addition of anthropogenic forcing to the simulations. -mike]

    How sensitive are estimates of historic temparature variation to changes lasting less than 30 years?

    [Response: In general quite sensitive (many historical and "proxy" climate indicators resolve year-to-year temperature variations quite well). -mike]

    Why is the 6th century cooling, a disaster which caused widespread starvation in the British Isles, shown as a relatively small blip in the graphs?

    [Response:This, again gets at the important distinction, discussed here (see "myth #2") between regional (e.g. British) and truly hemispheric or global-scale temperature changes. -mike]

  3. 3
    Steve Funk says:

    “This, again gets at the important distinction, discussed here (see “myth #2″) between regional (e.g. British) and truly hemispheric or global-scale temperature changes.”

    http://news.independent.co.uk/world/science_medical/story.jsp?story=487550
    (There are probably much better sources) but the sixth century cooling is variously attributed to a comet or Krakatoa, neither of which happened in the western hemisphere. Why would one assume it is just a regional phenomenon?

    [Response: It is very difficult to to diagnose the potential climatic response to a forcing we don't even know (the climatic response to a comet impact is quite different from that to a volcano!). More directly relevant to your question, however, recent modeling studies suggest that an explosive tropical eruption associated with a relatively moderate radiative forcing (imposing perhaps only a few tenths of a degree C cooling in global mean temperature), may nonetheless be associated with much larger regional temperature responses in areas far remote from the actual eruption. These large regional responses result from changes in atmospheric circulation (basically, the pattern of undulation of the jet stream), that are influenced by the vertical and latitudinal gradient in the associated radiative forcing, and which redistribute warmth over the surface of the earth, but have little impact on the average surface temperature. See for example this article: Shindell, D.T., G.A. Schmidt, M.E. Mann, and G. Faluvegi 2004. Dynamic winter climate response to large tropical volcanic eruptions since 1600. J. Geophys. Res. 109, D05104, doi:10.1029/2003JD004151." and references therein. -mike]

  4. 4

    When Climatologists Attack!!
    The climatologists are angry, and are on the warpath against the industry-funded Tech Central Station: RealClimate » Temperature Variations in Past Centuries and the so-called "Hockey Stick": …coined by the former head of NOAA’s Geo…

  5. 5
    Steve Hemphill says:

    There seems to be a lot of dancing around Mann’s Hockey Stick. I have a simple question and am looking for just a yes or no answer:

    If random numbers are put into Mann’s procedure, does a hockey stick shape result?

    Thank you
    Steve Hemphill

    [Response: Thanks for your question, and the opportunity it provides for clearing up yet further "Hockey Stick" disinformation. First, I'll give you a two word short answer: Absolutely Not! For a detailed explanation, see the new post "On Yet Another False Claim by McIntyre and McKitrick".

    Let me also politely take issue with your use of the phrase "Mann's Hockey Stick". Use of this term perpetuates the myth that "The Hockey Stick" Reconstruction is based solely on two publications by climate scientist Michael Mann and colleagues (Mann et al, 1998;1999). This is far from being the case. Secondly, the use of "Mann" alone dismisses the contributions of the other members of the team of researchers (Bradley and Hughes) involved in the Mann et al publications in question. -mike]

  6. 6

    Some Glaciers are Melting – And Some Are Growing
    A quick response to a question from Stu’s blog. Dave Turner asks: Please explain the growing glaciers in 90% of the Antarctic, the increasing glaciers in the Southern Alps (New Zealand) and even on Mt. St. Helens. Worldwide most glaciers…

    [Response: This seems to be lacking some context... who says 90% of antarctic glaciers are growing? - William]


Switch to our mobile site