Evidence from North Atlantic deep sea cores reveals that abrupt shifts punctuated what is conventionally thought to have been a relatively stable Holocene climate. During each of these episodes, cool, ice-bearing waters from north of Iceland were advected as far south as the latitude of Britain. At about the same times, the atmospheric circulation above Greenland changed abruptly. Pacings of the Holocene events and of abrupt climate shifts during the last glaciation are statistically the same; together, they make up a series of climate shifts with a cyclicity close to 1470 ± 500 years. The Holocene events, therefore, appear to be the most recent manifestation of a pervasive millennial-scale climate cycle operating independently of the glacial-interglacial climate state. Amplification of the cycle during the last glaciation may have been linked to the North Atlantic's thermohaline circulation.

[1] Using a climate model of intermediate complexity, we simulate the response of the El Niño-Southern Oscillation (ENSO) system to solar and orbital forcing over the Holocene. Solar forcing is reconstructed from radiocarbon production rate data, using various scaling factors to account for the conflicting estimates of solar irradiance variability. As estimates of the difference since the Maunder Minimum range from 0.05% to 0.5% of the solar ''constant,'' we consider these two extreme scenarios, along with the intermediate case of 0.2%. We show that for large or moderate forcings, the low-pass-filtered east-west sea surface temperature gradient along the equator responds almost linearly to irradiance forcing, with a short phase lag (about a decade). Wavelet analysis shows a statistically significant enhancement of the century-to-millennial-scale ENSO variability for even a moderate irradiance forcing. In contrast, the 0.05% case displays no such enhancement. Orbitally driven insolation forcing is found to produce a long-term increase of ENSO variability from the early Holocene onward, in accordance with previous findings. When both forcings are combined, the superposition is approximately linear in the strong scaling case. Overall, the sea surface temperature response is of the magnitude required, and is persistent enough, to induce important climatic perturbations worldwide. The results suggest that ENSO may plausibly have acted as a mediator between the Sun and the Earth's climate. A comparison to key Holocene climate records, from the Northern Hemisphere subtropics and midlatitudes, shows support for this hypothesis.