Millennial-scale climate variability during glacial, interglacial, and transitional climate states: Its causes and implications for biotic evolution and future climate change

In the early 1990s, climate records obtained from polar ice cores demonstrated that climate switched rapidly from warm to cold states on millennial time scales during the last glacial episode. Other climate records subsequently documented this climate instability over broad areas of the Earth indicating that it was a pervasive aspect of climate behavior during this cold climate state. Understanding how this natural climate variability evolved through the transitions to, and during, warm climate states (interglacial episodes) is essential for accurately predicting future climate. An archive of this past natural climate variability is contained in the rapidly accumulating sediment deposited in Santa Barbara Basin, California.; Climate records from Santa Barbara Basin demonstrate that the millennial-scale pacing of climate during the last glacial episode occurred continuously throughout the last two glacial-interglacial cycles (last 145,000 years). The specific periods of the millennial- to centennial-scale cyclicity are the same as those found in records of solar luminosity during the present interglacial; suggesting that variations in solar activity paced climate during glacial, interglacial, and transitional climate states. The magnitude of millennial-to-centennial climate variability may be modulated by longer-term variations in solar activity and/or insolation. Under elevated global temperatures, like which occurred during the penultimate interglacial and are predicted for the future, some components of the climate system may not respond linearly to feedbacks. The end of the penultimate interglacial began extremely abruptly suggesting that pronounced climate instability may arise in the future with little warning.; Benthic foraminiferal assemblages from the California margin indicate that the biota responded to the rapid climate oscillations not as communities, but individually by migration without speciation or extinction. Species associations were repeatedly reorganized every few thousand years, preventing long-term isolation and genetic differentiation. This is consistent with evolutionary models in which stasis prevails during highly unstable environmental conditions, and speciation and extinction occur when the mode of environmental variability changes. The absence of extinction and sensitivity of species to millennial-scale climate change suggests that broad segments of the global biosphere are presently well adapted to natural rapid climate variations.