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.