Systematic Training Cycle Essays On Global Warming

1. Introduction

Global temperature has oscillated at least since the beginning of the Phanerozoic Eon 542 million years (My) ago, manifest as long climate cycles that repeat with a period of 135–150 My [1,2,3]. Faster and smaller temperature oscillations are superimposed on the cooling phase of the most recent long cycle beginning at least ~55 million years before present (Mybp) [4], forming the earliest Marine Isotope Stages (MISs; [5]), or global glacial cycles (“ice ages”). MISs recurred initially every ~41 thousand years (Ky) [5], the same repetition period as the integrated Obliquity/Precessional Orbital Cycle (OPOC; [6,7]), and grew larger by more than an order of magnitude over millions of years as the Earth cooled an additional ~1.5 °C [5]. MISs shifted to a quasi-regular period of ~80–120 Ky and doubled in amplitude at the Mid-Pleistocene Climate Transition 1.25–0.75 My ago [5,8,9,10,11,12,13,14]. This temperature-oscillation pattern continues to dominate multi-millennial climate variability up to the present, inducing quasi-regular global glaciations separated by shorter warm interstadials. The contemporary climate constitutes a prolonged warm interstadial comparable to those that have alternated in the past with cold stadials corresponding to global glacial maxima.

Superimposed upon these multi-millennial climate cycles are numerous shorter global and regional climate cycles ranging in period from several millennia down to a few weeks. Included among these faster oscillations are millennial-scale cycles, particularly the Bond cycle [15,16,17,18,19,20,21,22,23], and centennial-scale cycles, notably the Antarctic Oscillation (AAO) known also as the Southern Annular Mode (SAM) and tracked quantitatively by means of the Southern Oscillation Index (SOI) [24,25,26,27,28,29,30,31]. These interdependent Southern Hemisphere (SH) temperature-proxy oscillations exhibit both centennial [24,25,26,27] and decadal [28,29,30,31] frequency components. Similar periodicity appears in independent reconstructions of more contemporary temperature proxies from James Ross Island [27] and snow accumulation in stacked records from snowpits at Vostok [32].

These and other centennial-scale temperature oscillations recurred with regularity in the past and are, therefore, likely to continue to oscillate at similar periods to affect future climate [33]. Understanding these cycles can therefore improve the empirical basis for climate projection across time scales most immediately pertinent to human affairs, decades to centuries [34,35]. Analysis of oscillatory climate regimens and their interactions on diverse time scales also promises to improve our understanding of regional and global climate dynamics, including especially the balance between human and natural forcing of present and future climate change, which are among today's most compelling environmental and policy issues.

Toward these ends we report here a previously-unexplored centennial-scale temperature-proxy oscillation, the Antarctic Centennial Oscillation (ACO), recorded in stable isotopes frozen in ice cores at Vostok, Antarctica [36,37] and three additional Antarctic drill sites distributed widely on the East Antarctic Plateau (EAP), namely, EPICA Dronning Maud Land, EPICA Dome C and Talos Dome. Paleoclimate records from these four drill sites have been synchronized on the most precise ice-core chronology available, the Antarctic Ice Core Chronology of 2012, or AICC2012 [38,39], enabling the most accurate possible estimates of the relative timing of ACO occurrence at different drill sites.

Centennial-scale temperature oscillations in the Vostok paleoclimate record were recognized qualitatively nearly two decades ago [40], but their nature and spatial extent have not been characterized previously, “precluding a definitive conclusion about their meaning” [40] (p. 111). Here we characterize the ACO quantitatively, document its occurrence at the aforementioned four ice-core drill sites on the EAP, explore the ACO contribution to Dansgaard-Oeschger (D-O) oscillations in the Northern Hemisphere (NH), draw parallels with contemporary climate cycles including particularly the Antarctic Oscillation (AAO), and evaluate possible implications for the current and future climate.

In particular, the contemporary global warming increase of ~0.8 °C recorded since 1850 has been attributed widely to anthropogenic emissions of carbon dioxide (CO2) into the atmosphere. Recent research has shown, however, that the concentration of CO2 in the atmosphere has been decoupled from global temperature for the last 425 million years [3] owing to well-established diminishing returns in marginal radiative forcing (ΔRF) as atmospheric CO2 concentration increases. Marginal forcing of temperature from increasing CO2 emissions declined by half from 1850 to 1980, and by nearly two-thirds from 1850 to 1999 [3]. Changes in atmospheric CO2 therefore affect global temperature weakly at most. The anthropogenic global warming (AGW) hypothesis has been embraced partly because “...there is no convincing alternative explanation…” [41] (p. 12). The ACO provides a possible alternative explanation in the form of a natural climate cycle that arises in Antarctica, propagates northward to influence global temperature, and peaks on a predictable centennial timetable.

2. Methods

Open-access databases containing stable isotope records [36,37,38] and computed planetary orbital (Milankovitch) parameters and consequent surface insolation [6,7] were analyzed using conventional methods detailed in the Supplementary Materials (SM). The cycle nomenclature proposed by Wunsch [42] was modified for the centennial-scale Antarctic cycles identified here. The start time for this analysis, 226,408 years before 1950 (yb1950), is the oldest for which the sampling resolution of the Vostok temperature-proxy record is sufficient to detect centennial-scale events in compliance with the Nyquist-Shannon sampling-frequency criterion of two samples per cycle (SM). Oxygen and hydrogen isotope ratios are used widely and interchangeably as paleoclimate temperature proxies [43,44,45,46,47], although the former contains a proxy component of ice volume and other environmental signals not present in the latter [43,48]. We use both proxies in this study as is conventional.

Three definitions were used to describe temperature-proxy oscillations comprising ACOs: oscillations larger than 0.25 °C in amplitude (~90% of 546 cycles identified over 226 millennia), oscillations from 0.045 to 0.25 °C in amplitude (~10% of 546 cycles), and small oscillations characterized by a positive inflection of the temperature-proxy record followed by a negative inflection on a rising or falling background temperature-proxy curve (an additional 104 cycles creating a maximum sample size n = 650). These smallest ACOs are accompanied by larger homologs in climate records from other drill sites, showing that they are genuine temperature-proxy oscillations and not noise. The third definition was used only to compute cycle period; excluding it increased mean computed period by ~15%. All other properties of ACO cycles were computed using only the first and second ACO cycle definitions. ACO properties evaluated include period (trough-to-trough and peak-to-peak), amplitude (half the excursion of the warming phase plus half the excursion of the cooling phase), and warming and cooling duration and rate, as detailed in the SM. Reduction of temperature-proxy and atmospheric carbon dioxide (CO2) concentration data to analytic format and color-coding of centennial-scale climate cycles depicting the above three definitions appear in SM Table S1, enabling replication of this study and supporting further independent analysis.

This study is limited to East Antarctic temperature-proxy records from ice cores extracted from four drill stations on the EAP synchronized on the AICC2012 chronology: Vostok, EPICA Dome C (EDC), Talos Dome (TD) and EPICA Dronning Maud Land (EDML) [38]. Meaningful comparison of ACO cycles across these diverse climate records required equalizing corresponding sampling resolutions as closely as possible to Vostok by averaging (filtering). The averaging protocols for non-Vostok records were established using a simple arithmetic protocol (SM) and are required to replicate the results of our study. Averaging protocols are therefore reported in figure legends as the bin width over which averages were computed from the original raw temperature-proxy data followed in parentheses by the start time of the averaging. Quantitative analysis of the timing of homologous cycles in different climate records was restricted to the most recognizable cycles associated with well-established climate events. These unambiguous “signpost” ACO cycles can be matched objectively based on their position relative to known climate events, following the methodology established by numerous previous climate investigators [38,40,49].

We used standard quantitative methodologies in this study, including Fourier (spectral) analysis, auto- and cross-correlation, and conventional t-tests. Our results show, however, that climate data including those evaluated here do not necessarily comprise a stationary time series, implying that modern advanced methods that assume neither linearity nor stationarity may be more appropriate. Such methodologies include, to various degrees, Wavelet Analysis (which is nonetheless based on the Fourier transform), Empirical Mode Decomposition, Hilbert-Huang transform, etc. We consider these more advanced analytic approaches unnecessary in this initial analysis of the ACO, but our findings nonetheless imply that their absence here constitutes a methodological limitation. As developed in the Discussion, we consider this limitation inconsequential because we mainly evaluated short time segments of the climate record that comprise relatively stationary time series as confirmed by auto- and cross-correlation results.

We evaluated the major potential sources of noise in the reconstructed climate records, including frequency aliasing, dating uncertainty, variance in cycle amplitude, cycle periodicity, sample resolution and stratigraphic noise from non-climate signals in the original ice-core records, and concluded that they did not affect the present interpretations significantly (SM). The SAS JMP software used here for spectral power analysis employs Fisher’s Kappa Test Statistic (κ) as a white-noise test (SM). The κ statistic returns the probability that the distribution analyzed is generated by Gaussian (random) white noise against the alternative that the distribution is nonrandom (SM) and is reported in the figure legends. To compute confidence intervals of spectral density peaks, Fourier coefficients were estimated using forward stepwise regression (SM). Such stepwise selection can exaggerate the precision of the alpha estimates by as much as an order of magnitude [50], however, and we therefore conservatively set the confidence limit threshold for spectral peaks at an order of magnitude higher (0.005, 99.5%) than usual (0.05, 95%).

3. Results

3.1. Cycle Nomenclature

We used the cycle nomenclature developed by Wunsch [42] as modified here (SM). By this nomenclature, the ACO documented in the present study is designated as the TOC350V cycle (Temperature-proxy OscillationCentennial−scale350year mean periodVariable) based on its mean repetition period of 352 years over the time period evaluated, 226.4 to 0.149 Kyb1950, and based on the observed systematic variation of ACO period and amplitude in the time series as documented below. The acronyms “ACO” and “TOC350V” are used interchangeably throughout this paper. A second, millennial-scale oscillation, termed the TOK1500V cycle using the modified Wunsch nomenclature, appears homologous with the previously-identified Bond cycle in the NH [17,18,19,20,21] and is also evaluated here using temperature-proxy data from the SH.

3.2. Cycle Form

The TOC350V cycles analyzed here are evident in raw temperature-proxy records as separate, individually-identifiable oscillations, each composed of several sampling data points (Figure 1). These centennial temperature-proxy cycles are superimposed on ascending temperature phases of MISs such as the Last Glacial Termination (LGT; Figure 1a) and recovery from the Antarctic Cold Reversal (ACR, Figure 1b), on descending phases of the temperature-proxy record such as the cooling phase of the ACR (Figure 1b), and on relatively flat regions of the Vostok temperature-proxy record including the Last Glacial Maximum (LGM) and the Holocene (Figure 1a,d, respectively). All 650 TOC350V cycles identified in this study over the 226.4 Ky evaluated using the three definitions described in the Methods are labeled starting with the most recent cycle at 149 yb1950 on the Vostok GT4 glaciological chronology using alphanumeric symbols, following the convention adopted for MISs [5] (Figure 1). These time series show qualitatively an apparent progressive decline in cycle period (an increase in frequency) combined with a proportional increase in cycle amplitude, both of which are confirmed quantitatively below.

3.3. Cycle Periodicity

Spectral analysis of Antarctic paleoclimate records was done previously for the Holocene for the time period from 11,500 ybp to the present [51]. We extend this analysis here using the more accurate data synchronized on the AICC2012 chronology and including climate records from Vostok, EDC, EDML and TD; [38,39]. We evaluate these climate records over broader time spans than previously done, including the Holocene, and over older and longer time periods, to enable detection of a broader complement of cycle periodicities. The resulting spectral power density periodograms show peaks at decadal, centennial, millennial and multi-millennial periodicities (Figure 2). Spectral peaks occur at similar frequencies in ice-core records from different drill sites (cf. Figure 2a,b), consistent with the hypothesis that the same temperature cycles manifest at different Antarctic drill sites located on the EAP.

Broadband periodograms covering several hundred millennia exhibit the primary MIS frequency peak at a period of 106–108 Ky, as well as secondary and tertiary peaks of Milankovitch cycles at the orbital obliquity (41 Ky) and precession (21 Ky) cycle periods (Figure 2a,b), replicating previous results [36] (cf. our Figure 2a with Figure 4a in [36] (p. 432)). Spectral analysis of shorter records shows possible representations of the obliquity orbital cycles in the Talos Dome climate record at 38.3 Ky (Figure 2c), but corresponding peaks at EDML differ in period by up to 25% and hence their etiology is correspondingly less certain. The spectral periodograms constructed over shorter time periods (Figure 2c,d) also lack the longer MIS cycle (Figure 2c,d) as expected, since the time period evaluated is too short to detect it.

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