New Ideas about Late Holocene Climate Variability in the Central Pacific

Abstract

Pacific archaeologists, geographers, and other social scientists have long used a model of Late Holocene climate change based largely on other regions of the world. In high-latitude regions, two major climate periods have been recognized: the Medieval Warm Period, dated to ca. AD 900-1200, and the Little Ice Age, dated to ca. AD 1550-1900. However, new evidence from long-lived Pacific corals, along with more general climate modelling, suggests that while the rest of the world was experiencing the Medieval Warm Period, conditions in the tropical Pacific were cool and possibly dry. Similarly, during the Little Ice Age the central Pacific was comparatively warm and wet and stormy conditions more common. A significant body of new evidence points to substantial climate variability in the central Pacific over the past millennium. Changing background climate, El Niño-Southern Oscillation variability, and the potential for regional variation are here considered with an eye to understanding the potential influence of climate on prehistoric human populations in the central Pacific region. For the past 50-odd years, Pacific archaeologists, geographers, and other social scientists have been using a model of Late Holocene climate change based largely on climate variability known from other regions of the world (e.g., Fagan 2000; Grove 1988; Lamb 1965). Until recently, there was little reason to suspect that conditions in the central Pacific diverged from those of the Northern Hemisphere, where two major climate periods are recognized: the Medieval Warm Period (MWP, also known as the Little Climatic Optimum), dated to ca. AD 900-1200, and the Little Ice Age (LIA), when temperatures were up to 1.2ЊC cooler (relative to the long-term average), dated to ca. AD 1550-1900 (following Jones, Osborn, and Briffa 2001, 665). Increasingly, however, there is evidence to suggest global variability in the timing, duration, and character of these two periods (Jones et al. 1998; Jones, Osborn, and Briffa 2001; Jones and Mann 2004). In the central Pacific, both observational studies and climate modelling now suggest that the MWP was relatively cool and the LIA relatively warm, the inverse of Northern Hemisphere conditions. Further, other parameters of climate may have been variable within the Central Pacific region. While not promoting an environmental determinist perspective, I argue that it is necessary to ᭧ 2006 by The Wenner-Gren Foundation for Anthropological Research.

Simulated climate variability in the region of Rapa Nui during the last millennium

“…Therefore, the model results do not corroborate the assumption of Nunn (2000) that a temperature decrease associated with a large-scale cooling from the Medieval period to the Little Ice Age and an associated precipitation change could have influenced the climate on Rapa Nui around 1300 AD. Likewise, a cool and dry MWP and comparatively warm and relatively wet LIA as seen in paleoclimate reconstructions for the central Pacific (Allen, 2006) cannot be found for the region around Rapa Nui in the model results.…”

“…Another assumption is that the socalled "AD 1300 event" -taken as the transition between the Medieval Warm Period (MWP) and the Little Ice Age (LIA) in this region -caused the vegetation change on Rapa Nui due to uncommonly heavy precipitation associated with rapid cooling (Nunn, 2000). Reconstructions for the central Pacific, however, suggest that the MWP was cool and dry and the Little Ice Age was comparatively warm and relatively wet (Allen, 2006). Numerical experiments with the coupled Pacific ocean-atmosphere Zebiak-Cane model support paleoclimate evidence of an El Niño-like state in the tropical Pacific during the LIA and a La Niña-like state during the MWP (Mann et al, 2005).…”

Altered Ecologies: Fire, climate and human influence on terrestrial landscapes: Terra Australis 32

“…But on every point, from cooling to sea-level change to resource depletion, Nunn's assertions can be challenged. Allen (2006) answers most of Nunn's assertions decisively, indeed devastatingly. Using recent palaeoclimatic data from the region itself, she shows that the previous Medieval Warm Period (MWP) was a time of colder weather in the equatorial Pacific, while the LIA was warmer -the opposite of the Northern Hemisphere-derived model Nunn is using to underpin his views.…”

A Marquesan coral reef (French Polynesia) in historical context: an integrated socio‐ecological approach

“…New evidence from the region at large suggests that the conditions outlined above may have been exacerbated by climate change over the past millennium. In particular, new studies suggest that the 'Little Ice Age' best known from the Northern Hemisphere, may have been a time of warm and wet conditions in the central Pacific (see summary in Allen, 2006). Evidence from fossil corals on Palmyra in the equatorial Line Islands points to the 17th century as a period of particularly intense ENSO activity, with some El Nin˜o events exceeding those of recent times (Cobb et al, 2003).…”

“…Similarly, the late 19th century occupation layer on the Teavau'ua coastal flat has a higher clay content, which also suggests an increase in the transport of sediments from more interior areas. Two potential causes for these sedimentary changes, both discussed in more detail below, are climate variability around the 17th century (Cobb et al, 2003;Allen, 2006) and the introduction of exotic herbivores, particularly goats, in the late 18th century.…”