Wednesday, October 3, 2012
An 1800 Year Oceanic Tidal Cycle Driving Climate Change
I am not copying the whole paper here but it underlines what I have come to conclude myself. That the driving force for changes in the heat flux into the Arctic is a tidal pulse in the Antarctic Circumpolar Current. This explanation is both simple, persistent and profoundly elegant.
It is also reasonable that this tidal effect will have a comparable effect on the opposite side of the globe and thus we likely have specific local effects at the nine hundred year mark also. In any case, we have a choke point in the South Atlantic that is affected at least every 1800 years and reasonably every 900 years which pretty closely reflects the apparent rough 1100 year cycle suggested by the data at hand. What the data really reflects is that the data does have a cycle that spans centuries. Having a natural tidal cycle allows us to understand that the data presented is a shift in the distribution curve.
At present we are getting more heat and this allows ice loss to run along in a variable fashion. When this heat is shut off, the reverse will occur. The data itself at any point will be confusing and misleading.
At present the tide is in and conditions are warming. This should last a while. My best estimate is around one additional century.
The 1,800-year oceanic tidal cycle: A possible cause of rapid climate change
Charles D. Keeling* and Timothy P. Whorf
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0244
Contributed by Charles D. Keeling, February 2, 2000
Variations in solar irradiance are widely believed to explain climatic change on 20,000- to 100,000-year time-scales in accordance with the Milankovitch theory of the ice ages, but there is no conclusive
evidence that variable irradiance can be the cause of abrupt ﬂuctuations in climate on time-scales as short as 1,000 years. We propose that such abrupt millennial changes, seen in ice and sedimentary core records, were produced in part by well characterized, almost periodic variations in the strength of the global oceanic tide-raising forces caused by resonances in the periodic motions of the earth and moon. A well deﬁned 1,800-year tidal cycle is associated with gradually shifting lunar declination from
one episode of maximum tidal forcing on the centennial time-scale to the next. An amplitude modulation of this cycle occurs with an average period of about 5,000 years, associated with gradually
shifting separation-intervals between perihelion and syzygy at maxima of the 1,800-year cycle. We propose that strong tidal forcing causes cooling at the sea surface by increasing vertical mixing in the oceans. On the millennial time-scale, this tidal hypothesis is supported by ﬁndings, from sedimentary records of ice-rafting debris, that ocean waters cooled close to the times predicted for strong tidal forcing.
High resolution ice-core and deep-sea sediment-core records over the past million years show evidence of abrupt changes in climate superimposed on slow alternations of ice-ages and interglacial warm periods. In general these abrupt changes are spaced irregularly, but a distinct subset of recurring cold periods, on the millennial time-scale, appears to be almost periodic. Such events, however, are not clearly apparent in ice-core data after the termination of the most recent glaciation, about eleven thousand years (11 kyr) BP (kyr before A.D. 2000). This absence of recent events has led to the hypothesis that their underlying cause is related to internal ice-sheet dynamics (ref. 1, p. 35).
Interpretations of sediment-cores by Bond et al. (1, 2) indicate, however, that a 1- to 2-kyr periodicity persisted almost to the present, characterized by distinct cooling events, including the Little Ice Age that climaxed near A.D. 1600. Although evidence that cooling was more intense during glacial times may be explained by some aspect of ice-dynamics, a continuation of cooling events throughout the postglacial Holocene era suggests an alternative underlying mechanism.
The 1- to 2-kyr Ice-Rafted Debris (IRD) Cycle. In a comprehensive comparison of ice-core and sediment-core data, Bond et al. (1) found persistent episodic cooling events recorded as increases in the amounts of IRD in deep sea sediments of the North Atlantic Ocean basin over the past 80 kyr. Consistent periodicity was demonstrated by averaging the times between inferred cool events over 12-kyr time intervals. This ‘‘pacing’’ of events was found to be restricted to a narrow range between 1,328 and 1,795 years, with a grand average of 1,476 6 585 yr, that they termed a ‘‘1–2 kyr climate cycle.’’
Similar to these oceanic cooling events, but less frequent and regular, are Dansgaardy Oeschger events seen in 18Oy16O data of glacial ice. Although sudden warming events are more conspicuous, these data also show cooling events. Bond et al. (1) found that Dansgaardy Oeschger cooling events coincide with nearly every IRD event during Stage 3 of the last glaciation when DansgaardyOeschger events were best developed (ref. 1, p. 51).
They also found that Heinrich events, massive discharges of icebergs at intervals of 6–9 kyr in the North Atlantic Ocean, were in most cases preceded by IRD events by 0.5 and 1 kyr (ref. 1,p. 48), suggesting a close link between the two phenomena.
Bond et al. (1) proposed ‘‘that the millennial scale climate variability documented in Greenland ice cores and North Atlantic sediments through the last glaciation was not forced by ice sheet instabilities, but instead arose through modulation of a pervasive 1–2 kyr cycle. The persistence of the cycle through
virtually the entire 80 kyr record points to a single forcing mechanism that operated independently of the glacia linterglacial climate states’’ (ref. 1, p. 55). They did not, however, propose a specific mechanism.
The IRD events identified by Bond et al. (1, 2) show high spectral power density in a broad band centered at about 1,800 years (0.55 6 0.15 cyclesykyr). The authors do not explain why this period is so much larger than the 1,476-year average pacing of cool events, but the time-distribution of pacing (ref. 1, Fig. 6C; G. Bond, private communication) suggests that a majority of the events were about 2,000 years apart, with occasional additional events occurring about half-way between, evidently too infrequent to cancel out a dominant spectral peak near 1,800 years.
Bond et al. (2) in addition found a spectral peak near 5,000 years whose possible cause was also not explained. We now propose an oceanic tidal mechanism that may explain the basis for both of these spectral peaks, consistent with the actual times of IRD events.
A Proposed Tidal Mechanism for Periodic Oceanic Cooling. In a previous study (3) we proposed a tidal mechanism to explain approximately 6- and 9-year oscillations in global surface temperature, discernable in meteorological and oceanographic observations. We first briefly restate this mechanism.
The reader is referred to our earlier presentation for more details. We then invoke this mechanism in an attempt to explain millennial variations in temperature.
We propose that variations in the strength of oceanic tides cause periodic cooling of surface ocean water by modulating the intensity of vertical mixing that brings to the surface colder water from below. The tides provide more than half of the total power for vertical mixing, 3.5 terawatts (4), compared with about 2.0 terawatts from wind drag (3), making this hypothesis plausible.
Moreover, the tidal mixing process is strongly nonlinear, so that vertical mixing caused by tidal forcing must vary in intensity interannually even though the annual rate of power generation is constant (3). As a consequence, periodicities in strong forcing, that we will now characterize by identifying the peak forcing