RACLIVAGO · Rapid Climate variability in response to changing glacial and orbital boundary conditions during the Mid-Pleistocene transition

Final Activity Report Summary - RACLIVAGO (Rapid climate variability in response to changing glacial and orbital boundary conditions during the mid-pleistocene transition)

The main objective of RACLIVAGO was to investigate how and why millennial and submillennial climate variability has evolved as orbital and glacial boundary conditions have changed during the Pleistocene and to consider whether these long-term processes set up the boundary conditions in which short-term processes operate. We recognised from the outset that the problem is a difficult one to be addressed, but it is of fundamental importance from the societal standpoint. We did not expect to fully answered the question: "What is the origin of abrupt climate events?" but we expected to obtain a better understanding of the factors that affect the probability of rapid climate change in the North Atlantic region. We have used sediment cores obtained by ocean drilling to address several questions pertaining to the history of millennial-scale variability in surface and deep-water hydrography of the North Atlantic during the Mid-Pleistocene Transition. This was perhaps the most important climate transition in the Quaternary period, and it was the time when quasi-periodic (c. 100 kyr), high-amplitude glacial variability developed in the absence of any significant change in the character of orbital forcing. The rationale behind the choice of this time interval is that appealing evidence suggests that millennial-scale climate variability is amplified during times of intense forcing changes, but this rapid variability has not been thoroughly explored yet at the time when the major changes in climate periodicity occurred. One of the main results of RACLIVAGO was obtained by investigating the climatic evolution of a sector of the North Atlantic Ocean from 910,000 to 790,000 years BP; this time interval corresponds to a very important event of the Mid-Pleistocene climate shift, represented by the build up of the first distinctly larger Northern Hemisphere ice sheets. We have isolated the millennial-scale component of our climate records, and used time series analyses to estimate the variance distribution as a function of frequency, as well as the coherence and phase relationship between records. Our results indicate significant variability is centred on periods of 10.7 and 6 kyr: this timing is particularly interesting because it is close to peaks expected from the second and the fourth harmonics of the precessional component of the insolation forcing. We have suggested that the timing of abrupt climate changes, as well as the amplitude of millennial scale oscillations, may be strongly influenced by the orbital insolation through frequency and amplitude modulation. These results are quite novel because numerous mechanisms have been proposed to explain millennial-scale oscillations, but few if any of them include the orbital or astronomical input as a plausible forcing because of the marked difference between the typical period of the millennial oscillations (c. 10.7 and 6 ky) and the shortest period of the insolation (c. 19 ky). We have also suggested that the source of this part of the climate signal at our Site is low latitude insolation, with the equatorial response being advected to the high latitudes through oceanic and atmospheric circulation, after being possibly amplified by moisture feedback. The fact that some of the millennial scale variability observed at our North Atlantic site was influenced by the equatorial and tropical regions confirms the important role played by these regions in the response of the climate system to the astronomical forcing. While the regional extent of these events needs to be verified by future work, the observation that insolation determines, in the end, the timing of abrupt climate change is quite intriguing in the context of future climate change; since the insolation is certainly predictable, these results would suggests the challenging possibility that, when climate models become reliable enough, abrupt climate change may also become predictable.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз