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