Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations

Building on recent studies, we attempt hemispheric temperature reconstructions with proxy data net works for the past millennium. We focus not just on the reconstructions, but the uncertainties therein, and important caveats. Though expanded uncertainties prevent decisive conclusions for the period prior to AD 1400, our results suggest that the latter 20th century is anomalous in the contest of at least the past millennium. The 1990s was the warmest decade, and 1998 the warmest year, at moderately high levels of confidence. The 20th century warning counters a millennial-scale cooling trend which is consistent with long-term astronomical forcing.