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
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