Instrumental records, climate modeling, and paleoclimate evidence all show that a portion of the secular trend of rising 20^th century mean annual temperatures may be anthropogenic in origin, reflecting the influence of greenhouse trace gases. Several studies have concluded that 20^th century mean annual Northern Hemisphere temperatures exceed those of the last 1000 years, including the period known as the Medieval Warm Period (MWP) ~1000-1300 AD. Moreover, these patterns cannot be explained solely by solar and volcanic forcing. But it is still not yet clear to what degree 20^th century temperature variability is anomalous in the context of natural centennial and multi-decadal climate variability related to changes in the North Atlantic’s thermohaline circulation (THC). For example, changes in the hydrological balance in high latitudes have been linked to centennial scale changes in THC during the Medieval Warm Period and Little Ice Age. Processes such as the North Atlantic Oscillation (NAO), a measure of the normalized sea level pressure difference (SLP) between the Azores and Iceland, is also a major source multi-decadal wintertime Northern Hemispheric climate variability that has been associated with changes in North Atlantic thermohaline circulation (THC). Annually-resolved tree ring and ice core records have been successful in reconstructing NAO-associated processes during the Little Ice Age. However these records deal mainly with changes in SLP and do not cover the important period prior to 1000 AD.

The Chesapeake Bay sedimentary record has yielded paleoclimate evidence indicating rapid (<100 yr) shifts of ~2-4º C occurred in Chesapeake Bay temperature ~2100, 1600, 950, 650, 400 and 150 years before present based on reconstructed temperatures from magnesium/calcium (Mg/Ca) paleothermometry. The data indicate large temperature excursions during the Little Ice Age (~1400-1900 AD) and the Medieval Warm Period (~800-1300 AD) possibly related to changes in the strength of North Atlantic thermohaline circulation (THC). Evidence also exists for a long period of sustained regional and North Atlantic-wide warmth with low-amplitude temperature variability between ~450 and 1000 AD. In addition to centennial-scale temperature shifts, the existence of numerous temperature maxima between 2200 and 250 yr BP (average ~70 years) suggests that multi-decadal processes typical of the North Atlantic Oscillation (NAO) are an inherent feature of late Holocene climate. However, late 19^th and 20^th century temperature extremes in Chesapeake Bay associated with NAO climate variability exceeded those of the prior 2000 years by 2-3° C, suggesting anomalous recent behavior of the climate system.