Geophysical Research Letters - Vol. 26, No. 6
Year 1998 is the warmest in a millennium
Tree rings, varved sediments, ice cores, corals, and historical records can be used to reconstruct climate for the period preceding the instrumental record. Mann et al.  reconstruct the Northern Hemisphere mean annual surface temperature and report that the 1990s is likely the warmest decade, with 1998 being the wamest year in at least a millenium. The authors find that the late 11th, late 12th, and late 14th centuries are comparable to mean 20th-century temperature levels. Cooling following the 14th century, in their reconstructed series, marks the initial onset of the Little Ice Age.