Another New Hockeystick!

Two temperature reconstructions: 2000 years backward (left), 500 years backward (right) using proxy and also instrumental data (1880-1960) for calibration. Source: Christiansen and Ljunqvist, Climate of the Past, 2012.

Christiansen (Danish Meteorological Institute, Copenhagen) and Ljunqvist (Stockholm University) have published a two millennia (o-2000 AD) and a five centurys (1500-2000 AD) temperature reconstruction for extra-tropical Northern Hemisphere (The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability, Climate of the Past, 8-2012)! The two scientists used diverse proxys (tree ring widht and density, ice-cores, (varved) lake sediments and speleothems, widespread on Northern Hemisphere) and also instrumental data for calibration. As reconstruction based on a variety of proxys it is eminently reliable and thus comparable to hockeystick, the pioneering Millenia temperature reconstruction of Michael Mann and colleagues (The Hockeystick – A Milestone in Climate Science )!

Like the previous ones the new reconstruction shows a long term cooling trend and a strong natural variability in temperature. Long term cooling trend is caused by decreasing angle of Earth´s axial tilt leading to a cooler world in the long run. Cooler summers, less melting of snow. Balmier winters, increased evaporation of water, more snow. Extending Polar ice sheets are reflecting more sunlight thus cooling down planet Earth.

Natural variability of climate due to variations in solar irridiance and changing ocean currents seems to be stronger than usually estimated.
A big surprise: In Medieval Warm Period (MWP) a Global Warming came up like nowadays! However, current warming started from a subjacent level compared to MWP due to long term cooling trend.
But in the 1930th temperatures on Northern Hemisphere reached almost the same level as today, too. That´s even a result of instrumental data alone!
Jens Christian Heuer

A New Hockeystick

Temperature reconstruction of last 2000 years with tree ring proxys and instrumental data.  Source Esper et al. (Nature Climate Change, March 2012)

In March 2012 Jan Esper (Department of Geography, Johannes Gutenberg-University, Mainz, Germany) and his collegues published a paper about a Two Millennia Reconstruction of Temperatures on Northern Hemisphere (Esper J, Frank DC, Timonen M, Zorita E, Wilson RJS, Luterbacher J, Holzkämper S, Fischer N, Wagner S, Nievergelt D, Büntgen U (2012) Orbital forcing of tree-ring data. Nature Climate Change).

Esper et al. used proxys (tree ring data from North Scandinavian Pines) and also instrumental data for calibration. Measuring the maximal latewood density with x-rays (MXD), they used a more reliable method than often used tree ring width.

The results were a little surprise: There is a long term cooling trend caused by orbital forcing like in other temperature reconstructions, too. But there is also a strong natural variability in temperature, in all likelihood a consequence of changes in Solar irradiation and fluctuating ocean currents. Warming during Medieval Warm Period (MWP) and Roman Climate Optimum (RCO) were almost the same as present-day!

Long term cooling by orbital forcing refers to decreasing angle of Earth´s axial tilt leading to a cooler world in the long run. Cooler summers, less melting of snow. Balmier winters, increased evaporation of water, more snow. Extending Polar ice sheets are reflecting more sunlight thus cooling down planet Earth.

Jens Christian Heuer


56 Mio years ago, Global Warming fueled by greenhouse gases in abundance occurred once before! This event is called the Paleocene Eocene Thermal Maximum (PETM).

Due to an initial warming, maybe caused by exceeding volcanic eruptions, clathrate hydrates in oceans became unstable, releasing Methane, a potent greenhouse gas. A positive feedback started!

The global mean temperature on Earth rised from averaged 18°C up to 23°C, causing major changes in marine and terrestrial ecosystems. 

PETM is indicated by three proxies:

Indicators of PETM in the oceans (IPCC, 2007).

Global rise in temperature decreased O18/O16 ratio in oceans through enhanced evaporation of heavier isotope. There is a temperature-sensitive fractioning of oxygene isotopes via differential evaporation from oceans to atmosphere. 

Whereas the release of methane decreased C13/C12 ratio. Carbon isotope C13 is sparse in Methane, as this originates from microbes and life (on Earth) discriminates enzymatically against it. Hence a release of methane means a diminished C13/C12 isotope ratio.

Still in the oceans a part of released Methane was oxidated to CO2 or carbonic acid, dissolving carbonates (CO3) on the seefloor. 

Jens Christian Heuer

The Hockeystick – A Milestone in Climate Science

Prof. Michael E. Mann holding a tree grate with tree rings Source: Homepage Michael E. Mann

In 1999 climate scientist Michael E. Mann and his colleagues published a reconstruction of average temperatures last millenium-round on northern hemisphere  come to be known as “hockeystick”-curve (M.E.Mann,R.S.Bradley, M.K.Hughes: “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations”, Geophysical Research Letters, 1999). This scientific paper was really a pioneering feat! It was the first reliable reconstruction of temperatures in global scale over such a long time.

The team of scientists used proxy data from tree rings, corals and ice cores on the one hand and instrumental temperature data on the other hand such as it was. The “hockeystick” demonstrated at first how extraordinary (human-made) global warming during 20th century really was! For that reason all climate skeptics dislike this curve up to the present day.

The Hockeystick-Curve. The graph resembles a hockeystick in shape: with a long part of curve declining slightly as the shaft and with a short part of curve upturning suddenly as the blade.   Source: Mann, Bradley und Hughes 1999

Assembling and evaluation of all data for “hockeystick” happened in a very sophisticated way. The proxy-data, mainly tree rings extending to 1980 were validated by instrumental data back to 1854, occasional to 18th century yet. Long proxy record and shorter instrumental record overlapped a good way. Using only instrumental data from time intervall 1902 to 1980 for calibration of proxy data, Mann and his collegues were able to use leftover intervall from 1854 to 1901 (and the rare earlier instrumental data from 18th century) to verify this calibration. Calibration and validation of proxy data  worked accurately, meaning that „hockeystick“ is correct in this regard.

CO2 is a greenhouse gas but it also acts as a fertilizer, especially for trees in high elevations. More CO2 let tree ring width and tree ring density (of summerly late wood) mimic an additional rise in temperature that does not really exist. Thus the tree ring data were readjusted.

Above: Records of two tree ring data series (ITRDB Millenial, North American Treeline), which diverged from 19th century on due to influence of CO2. ITRDB Millenial includes trees in high elevations much sensitive to dunging effect of CO2. Below: Variations in atmospheric CO2 and residual between the two data series, used for readjustment. Source: Mann, Bradley und Hughes, 1999

Two examples how carefully Mann and his team had gone about it! All that and then some you can read in original paper, downloadable on Mann´s homepage (see sources below). It´s worth it! Michael E. Mann was also a lead author of 3rd. IPCC-Report about global warming in 2001.

In the course of General Assembly of European Geosciences Union (EGU) in April 2012 Michael E. Mann received the Oeschger-Medal for his research! Hans Oeschger (1927) was a Swiss scientist, famous because of his ice core research. Together with Willi Dansgaard he discovered a series of abrupt climate changes (Dansgaard-Oeschger Events) during the last glacial period by analysis of Greenland ice cores. Also he measured first the glacial-interglacial change of atmospheric CO2 encased in ice core´s tiny air bubbles.

But also some climate sceptics tried again and again to discredit Mann scientifically and personally. Fortunately they failed yet.  I hope it stays that way!

Jens Christian Heuer

Sources: European Geosciences Union (EGU) , Homepage Michael E. Mann


Land temperatures from 1750-2000.  The shaded regions show the statistical uncertainties. Prior land results from the other research groups are also plotted.

An IPCC-independent study, the Berkely Earth Surface Temperature (BEST) has confirmed global warming! The research team analyzed land-surface temperature data from 36000 meteorological stations going back to 250 years where it was possible, about 100 years further than previous studies. Temperatures have rised in 2/3 of the stations, 1/3 indicated a cooling. The Urban heat island effect, a frequently cited argument of the so called climate skeptics is negligible and gives no explanation about.

Jens Christian Heuer