Final Report Summary - MILLEVARIABILI (Origin and character of MILLEnnial-scale climate VARIABILIty in the North Atlantic during different climate boundary conditions of the Pleistocene)
Reliable predictions of future global climate rely upon understanding its complex history in the geological past. It has long been recognised that the long-term climatic variations deduced from the geological records are driven by changes in the distribution of the Sun’s energy over the Earth, which in turn arise due to changes in the geometry of the Earth-Sun system. In particular, the obliquity of the earth's axis (with a period of about 41,000 years) and the precession of the equinoxes (periods of about 19,000 and 23,000 years) are the underlying, controlling variables that influence climate through their impact on planetary insolation; in addition, the eccentricity (with a period of about 100,000 years) is the only parameter that can change the total energy received by the earth each year. Since the late 1970’s, a plethora of studies has demonstrated a correlation between orbital variations and climatic change but, despite more than four decades of research, information on how these changes in orbital boundary conditions affected the frequency and amplitude of rapid climate variability is still fragmentary. Indeed, climate variability is expressed on different timescales and rapid climate variability, which operates at centennial/millennial timescales, is of societal importance because it has the potential to produce dramatic changes over human timescales. A possible feature, which has hindered the inclusion of the orbital or astronomical input as a plausible forcing for rapid climate variability, has been the difference between the primary orbital periods (i.e. the shortest ~19,000 years (19 kyr)) and the timing of abrupt climate changes (a few kyr). However, the climate system may behave as a highly nonlinear system and it is conceivable that forcing with frequencies much lower (e.g. a few kyr) than those of its own free oscillations (e.g. ~100, 41, 19-23 kyr) can influence its response.
Based on models of climate change and previous palaeoclimate research, it has been hypothesised that the circulation of the Atlantic Ocean in particular may play a key role in determining the magnitude and character of major climate anomalies right across the globe. These observations have been largely restricted to the last glacial cycle (i.e. between 110,800 and 11,700 years ago) because of the reduced number of available high-resolution palaeoclimate records from older intervals. Taking advantage of the long, continuous, high-resolution Pleistocene sedimentary sequences recovered from the North Atlantic by Integrated Ocean Drilling Program (IODP) Expedition 306, we have undertaken a multiproxy study of centennial-scale changes in sea-surface and deep-water conditions, the dynamics of thermohaline deep-water circulation and ice-sheet ocean interactions. IODP Site U1313 was raised from a water depth of 3426 m at 41°00'N 32°58'W, on the upper middle western flank of the Mid-Atlantic Ridge. At present, surface waters in this region are derived from the North Atlantic Current; moreover, the site is in the path of the deep North Atlantic Deep Water western boundary current as it exits the northernmost Atlantic. Site U1313 constitutes a reoccupation of Deep Sea Drilling Project Site 607, which has provided many important advances in palaeoceanography during the last 20 years, including yielding valuable insights into the long-term surface and deep-water circulation patterns in the mid-latitude North Atlantic. In a complementary way, in our work, special emphasis is placed on assessing the presence and the characteristics of the rapid variability in North Atlantic sea-surface and deep-water hydrography. We examine the record of climatic conditions from c. 925,000 to 610,000 years ago (during Marine Isotope Stage 24-16) in order to evaluate the climate system's response in the millennial band to known orbitally induced insolation changes and investigate whether the orbital insolation influenced the amplitude and frequency of rapid climate variability. Special emphasis is placed on assessing the stability and duration of interglacial periods, because past warm periods provide a basis for comparison with the present interglacial (Holocene) and its future evolution. In this sense, Marine Isotope Stage (MIS) 19 - an interglacial period centred at around 785 ka, during which the insolation appears comparable to the current orbital geometry - provides an opportunity to pursue this question. It is characterised by a subdued amplitude of precession as a consequence of the modulating effect of the 400 kyr eccentricity cycle; the phasing between precession and obliquity is also very similar during the Holocene and MIS 19, making this marine isotopic stage a potential astronomical analogue for the Holocene and its future evolution, if this remains governed by natural forcing
Benthic and planktonic foraminiferal oxygen and carbon isotope values indicate relatively stable conditions during the peak warmth of MIS 19, but sea-surface and deep-water reconstructions start diverging during the transition towards the glacial MIS 18, when large, cold excursions disrupt the surface waters whereas low amplitude millennial scale fluctuations persist in the deep waters as recorded by the oxygen isotope signal. The glacial inception during MIS 19 occurred at ~779,000 years ago, in agreement with an increased abundance of tetra-unsaturated alkenones, reflecting the influence of icebergs and associated meltwater pulses and high-latitude waters at the study site.
Using a variety of time series analysis techniques, we evaluate the evolution of millennial climate variability in response to changing orbital boundary conditions during the early-middle Pleistocene. Suborbital variability in both surface and deep-water records is mainly concentrated at a period of ~11 kyr and, additionally, at ~5.8 and ~3.9 kyr in the deep ocean; these periods are equal to harmonics of precession band oscillations. The fact that the response at the 11 kyr period increased over the same interval during which the amplitude of the response to the precessional cycle increased supports the notion that most of the variance in the 11 kyr band in the sedimentary record is nonlinearly transferred from precession band oscillations. Considering that these periodicities are important features in the equatorial and intertropical insolation, these observations are in line with the view that the low-latitude regions play an important role in the response of the climate system to the astronomical forcing. The precise mechanism by which the energy is transported into high latitudes away from equatorial regions is not easy to reconstruct from available proxy records, but experiments with Earth System models are in progress in order to provide useful insights into the way the climatic signal is transferred into the North Atlantic. On the other hand, although insolation seems to be the most plausible trigger for millennial scale variability, its extent depends also on the overall state of the climate system, as it occurred during the transition towards MIS 18 when large, cold excursions disrupted the surface waters due to instability of the Northern Hemisphere ice sheets.
In summary, our results suggest that the variables measured by proxies are replicated in cycles apparently paced by orbital changes, suggesting the climate system is to a significant extent understandable and deterministic, being contingent upon both forcing and previous history. If other climate records from different time intervals and locations confirm our conclusions - that is, the insolation determines, in the end, the timing and amplitude of rapid climate change - these results will have important implications in the context of future climate change. Cross-correlation coefficients between forcing and our proxy records show that during MIS 19 the orbital forcing at millennial-scale periodicities, apparently echoed by the climatic response, is very close to that which we might expect at present and in the near future. This in turn implies the intriguing possibility that, as recently stated by Rial and Yang (AGU Monograph Series, 173, 2007), “since the insolation is certainly predictable, when climate models become reliable enough, abrupt climate change may also become predictable” to the extent that the behaviour of the climate system operates within the framework of its behaviour earlier in the recent geological record.
The results of this project will be of benefit to:
- Palaeoceanographers and palaeoclimatologists because of an improved insight into the response of the ocean environment to rapid climate change;
- Climate and ocean modellers. The project has provided palaeodata on the temporal response of the North Atlantic system, which can be compared with model output;
- Global Change scientists and policy makers, who need to understand the behaviour of the North Atlantic system for a better assessment of the oceanic and atmospheric forcing on European climate.
Based on models of climate change and previous palaeoclimate research, it has been hypothesised that the circulation of the Atlantic Ocean in particular may play a key role in determining the magnitude and character of major climate anomalies right across the globe. These observations have been largely restricted to the last glacial cycle (i.e. between 110,800 and 11,700 years ago) because of the reduced number of available high-resolution palaeoclimate records from older intervals. Taking advantage of the long, continuous, high-resolution Pleistocene sedimentary sequences recovered from the North Atlantic by Integrated Ocean Drilling Program (IODP) Expedition 306, we have undertaken a multiproxy study of centennial-scale changes in sea-surface and deep-water conditions, the dynamics of thermohaline deep-water circulation and ice-sheet ocean interactions. IODP Site U1313 was raised from a water depth of 3426 m at 41°00'N 32°58'W, on the upper middle western flank of the Mid-Atlantic Ridge. At present, surface waters in this region are derived from the North Atlantic Current; moreover, the site is in the path of the deep North Atlantic Deep Water western boundary current as it exits the northernmost Atlantic. Site U1313 constitutes a reoccupation of Deep Sea Drilling Project Site 607, which has provided many important advances in palaeoceanography during the last 20 years, including yielding valuable insights into the long-term surface and deep-water circulation patterns in the mid-latitude North Atlantic. In a complementary way, in our work, special emphasis is placed on assessing the presence and the characteristics of the rapid variability in North Atlantic sea-surface and deep-water hydrography. We examine the record of climatic conditions from c. 925,000 to 610,000 years ago (during Marine Isotope Stage 24-16) in order to evaluate the climate system's response in the millennial band to known orbitally induced insolation changes and investigate whether the orbital insolation influenced the amplitude and frequency of rapid climate variability. Special emphasis is placed on assessing the stability and duration of interglacial periods, because past warm periods provide a basis for comparison with the present interglacial (Holocene) and its future evolution. In this sense, Marine Isotope Stage (MIS) 19 - an interglacial period centred at around 785 ka, during which the insolation appears comparable to the current orbital geometry - provides an opportunity to pursue this question. It is characterised by a subdued amplitude of precession as a consequence of the modulating effect of the 400 kyr eccentricity cycle; the phasing between precession and obliquity is also very similar during the Holocene and MIS 19, making this marine isotopic stage a potential astronomical analogue for the Holocene and its future evolution, if this remains governed by natural forcing
Benthic and planktonic foraminiferal oxygen and carbon isotope values indicate relatively stable conditions during the peak warmth of MIS 19, but sea-surface and deep-water reconstructions start diverging during the transition towards the glacial MIS 18, when large, cold excursions disrupt the surface waters whereas low amplitude millennial scale fluctuations persist in the deep waters as recorded by the oxygen isotope signal. The glacial inception during MIS 19 occurred at ~779,000 years ago, in agreement with an increased abundance of tetra-unsaturated alkenones, reflecting the influence of icebergs and associated meltwater pulses and high-latitude waters at the study site.
Using a variety of time series analysis techniques, we evaluate the evolution of millennial climate variability in response to changing orbital boundary conditions during the early-middle Pleistocene. Suborbital variability in both surface and deep-water records is mainly concentrated at a period of ~11 kyr and, additionally, at ~5.8 and ~3.9 kyr in the deep ocean; these periods are equal to harmonics of precession band oscillations. The fact that the response at the 11 kyr period increased over the same interval during which the amplitude of the response to the precessional cycle increased supports the notion that most of the variance in the 11 kyr band in the sedimentary record is nonlinearly transferred from precession band oscillations. Considering that these periodicities are important features in the equatorial and intertropical insolation, these observations are in line with the view that the low-latitude regions play an important role in the response of the climate system to the astronomical forcing. The precise mechanism by which the energy is transported into high latitudes away from equatorial regions is not easy to reconstruct from available proxy records, but experiments with Earth System models are in progress in order to provide useful insights into the way the climatic signal is transferred into the North Atlantic. On the other hand, although insolation seems to be the most plausible trigger for millennial scale variability, its extent depends also on the overall state of the climate system, as it occurred during the transition towards MIS 18 when large, cold excursions disrupted the surface waters due to instability of the Northern Hemisphere ice sheets.
In summary, our results suggest that the variables measured by proxies are replicated in cycles apparently paced by orbital changes, suggesting the climate system is to a significant extent understandable and deterministic, being contingent upon both forcing and previous history. If other climate records from different time intervals and locations confirm our conclusions - that is, the insolation determines, in the end, the timing and amplitude of rapid climate change - these results will have important implications in the context of future climate change. Cross-correlation coefficients between forcing and our proxy records show that during MIS 19 the orbital forcing at millennial-scale periodicities, apparently echoed by the climatic response, is very close to that which we might expect at present and in the near future. This in turn implies the intriguing possibility that, as recently stated by Rial and Yang (AGU Monograph Series, 173, 2007), “since the insolation is certainly predictable, when climate models become reliable enough, abrupt climate change may also become predictable” to the extent that the behaviour of the climate system operates within the framework of its behaviour earlier in the recent geological record.
The results of this project will be of benefit to:
- Palaeoceanographers and palaeoclimatologists because of an improved insight into the response of the ocean environment to rapid climate change;
- Climate and ocean modellers. The project has provided palaeodata on the temporal response of the North Atlantic system, which can be compared with model output;
- Global Change scientists and policy makers, who need to understand the behaviour of the North Atlantic system for a better assessment of the oceanic and atmospheric forcing on European climate.