Paleoclimate Cycles are Key Analogs for Present Day (Holocene) Warm Period

Guest Post By Renee Hannon

Abstract

Detailed pattern correlation of Earth’s temperature changes during the past 450 kyrs reveals observations about several cyclic climate patterns. The past four glacial cycles are increasing in duration from 89 kyrs to 119 kyrs. Within these glacial cycles, two warm periods occur about 200 kyrs apart and have strikingly similar temperature characteristics. These two warm patterns suggest processes modifying Earth’s temperature could be repeatable and predictable. In contrast, two other warm periods have different and distinct characteristics. These two warm periods occur during a predominantly elliptical orbit and a predominantly circular orbit, respectively, and on approximately 400 kyr cycles.

Preliminary simplified models of astronomical and oceanic controls on temperature variations for the past four warm periods have been developed. Although process interactions are very complex, separating out predominate causes and effects on global temperature should help improve future climate mathematical simulation models. Climate models need to include astronomical as well as oceanic and atmospheric forcing to reliably predict the duration and temperature changes of the future Holocene interglacial Warm period.

Introduction

The Holocene Warm Period was compared to four interglacial warm periods and their glacial cycles during the past 450,000 years using EPICA Dome C isotope ratios and temperature estimates to identify pattern similarities and trends.  Interestingly, a hierarchy of correlative events and common patterns occur amongst the glacial cycles and warm periods.

Warm periods are anomalous events referred to as interglacial periods within a glacial cycle. Glacial cycles last for approximately 100 kyrs and warm periods range from 10 to 30 kyrs. For simplicity, these glacial cycles and warm periods are referred to as I through V, with I being present day and V being the oldest as defined in Figure 1. Glacial/interglacial transitions known as Terminations (I-V) and common usage names from literature, marine isotope stages(“MIS”), and approximate age are also noted in Figure 1.

Figure 1: EPICA Dome C isotope temperature estimates over the past 450 kyrs show four warm periods prior to the present-day Holocene warm. Warm periods last approximately 10-30 kyrs.

Calculated temperatures from the Antarctica Dome C data are multiplied by 0.5 to correct to approximate global temperatures rather than polar temperatures. Uncorrected, the magnitude of the delta degree C would be double than what is shown in the figure.

Correlation of Glacial Cycles

Figure 2 is a traverse which displays the four past glacial cycles. The temperature curves for each cycle are turned sideways with time plotted on the vertical axis. This technique is similar to creating a geologic cross section and enables correlation of events between the past glacial cycles. The key repeating event for each cycle is the rapid onset of warming following abrupt terminations of glacial periods. This significant event was used as a datum for each glacial cycle. Datuming is a useful tool that allows recognition of relative changes between cycles. Very cold is highlighted in blue and warm periods in red. The blue, yellow and green lines are an interpretation of internal correlations within each cycle bounded by bold red lines.

Figure 2: A traverse of the past five interglacial-glacial cycles. Cycles are datumed on the Terminations/Onsets. EPICA Dome C isotope temperature estimates are plotted as curves in 1 degree C increments on the horizontal scale (cold to left and warm to right). The vertical scale is time in 20 ka increments. Actual age is plotted on each cycle. Interglacial warm periods are highlighted in red and the coldest portion of the glacial period in blue. Dark red is calculated ratios of warm:cycle.

These glacial cycle patterns are what geologists call bottom-loaded sequences. They are well-behaved cycles with the warmest interglacial period, shaded in red, always following the termination of the previous glacial period. The green line within the warm period highlights a brief cooling event that approximately correlates to the 8.2 kyr event in the present-day Holocene. The blue correlation line is the base of the coldest full glacial period shaded in blue which occurs at the end of each cycle. The full glacial period ranges in duration from 35 to 60 kyrs in Cycle V to Cycle II, respectively. In the middle of each cycle there is a mild glacial period consisting of smaller cold stadials and warm interstadials (Dansgaard-Oeschger cycles). The yellow lines attempt to correlate these minor stadial events.

While there is uncertainty of +/- 3 kyrs in picking the exact timing of events this is not enough to change the main trends and observations. Although these curves are not stretched, stretching would likely improve correlation of the higher frequency events such as the stadials and interstadials. Several observations are evident from the glacial cycle traverse:

• Duration of glacial cycles are progressively increasing from 89 kyrs in Cycle V to 119 kyrs in Cycle II. Cycle II is 34% longer in duration than Cycle V. The cycles are not spaced equally at 100 kyrs apart. Javier previously challenged the 100 ky cycle (see his Table 1 and Figure 5) over the past 800 kyrs using interglacial peak to peak duration.
• The full glacial period at the top of each cycle is also increasing in duration from past to present but not necessarily getting much colder between cycles (+/- 1 degree C).
• Cycle V has the longest warm period and Cycle III the shortest initial warm period. Maximum average warm temperatures are not that different (+/- 1 degree C).
• Cycle III stands out as having an abbreviated initial interglacial warm followed by a second warm period (MIS 7c). This has been recognized in the literature and is frequently debated as to whether the second warm period is a true interglacial or an interstadial. A similar suppressed interstadial can be correlated to Cycle II (yellow lines). Regardless, MIS 7c is considered to be part of the larger Cycle II and its onset is not as significant as the Termination event.
• There appears to be an internal event within the interglacial warm periods that correlates to the 8.2 kyr Holocene event. Stadials, or cooling events, also appear correlative within the glacial periods (yellow lines) that have similar patterns suggesting a similar natural process was repeated.

Interglacial Warm Periods Comparison

Patterns in historical temperature changes were also evaluated for the past four interglacial periods and compared to the present-day Holocene. On the largest scale as seen in Figures 1 and 2; Warm II, III, and IV have asymmetric patterns with rapid initial warming and slower cooling. Warm V has more of a symmetrical pattern, with the climate optimum occurring towards the end of its warm period. Onset of present day warm appears similar to the beginning of a symmetrical pattern and perhaps analogous to Warm V.

In the following sections, warm patterns are compared in more detail for the duration of the warm interglacial period, the warming onset and cooling period.

Warm Period Duration Patterns

The warm interglacial periods over the past 450 kyrs range from 9 kyrs to 32 kyrs in duration using a delta ^oC temperature cut-off of minus 1 (Figure 2, Table 1). All past warm periods have been approximately 2 degrees C warmer than the Holocene. Most interglacial durations have bimodal patterns and tend to be asymmetrical. Warm V is an exception with a more symmetrical pattern (Figure 2).

Table 1: Warm Period Durations over the past 450 kyrs (dark red on Figure 2 which corresponds to a minus 1^oC delta cut-off).

Warm periods II and IV are discussed together due to their similar patterns and Warm periods III and V are discussed separately due to their different patterns. Javier (his Figures 13 and 14) and others have also recognized that Warm periods III (MIS 7E) and V (MIS 11) have different interglacial characteristics.

Warm Periods II and IV: The Holocene warm pattern is generally bimodal and similar to Warm II (MIS 5) and IV (MIS 9) with the exception of the onset warming and a lower initial climate optimum (Figure 3). These three warm periods have an initial brief climate optimum of 2 to 3 kyrs like the early Holocene. This is followed by a brief cooling with a V-shape pattern like the Holocene 8.2 kyr event and then a longer more stable warm period lasting about 7 to 8 kyrs like the Middle Holocene warming.

Figure 3: Holocene temperatures in red overlain on Warm II and IV periods. Bottom horizontal axis corresponds to the Holocene time and top horizontal axis is the past warm time in thousands of years. Colored bar at top refers to the older warm phases. The red text corresponds to the Holocene events.

There is only one key stadial cooling event during Warm II and IV periods. This key stadial event drops temperatures by about 2 degrees C and lasts for a couple thousand years. It also appears similar in timing to the Holocene 8.2 kyr stadial (Figure 3). This intervening stadial may represent a recurring event that happens after the initial climate optimum and will be discussed later.

There are numerous oscillating events during the warm periods with minor temperature variations (<1.5 deg C) that are short duration (tens to hundreds of years). These events would be like the Holocene events that include the Roman Climate, and the Medieval warm followed by the Little Ice Age which are discussed extensively in the literature. They are difficult to see on the graphs in Figure 3 and appear more as background noise during the Holocene warm period due to the compressed scales used here.

These minor events are probably unique to each warm period and have been attributed to long term ocean cycles, Pacific and N. Atlantic multi-decadal and decadal climate cycles, orbital obliquity, solar variability, and greenhouse effects.

Additionally, the bimodal patterns for Warm Periods II and IV are strikingly similar as previously discussed by Hannon. The main difference is Warm IV is compressed by about 6 kyrs. This is demonstrated by stretching the Warm IV curve to match Warm II as shown in Figure 4. These warm periods are approximately 200 kyrs apart. The striking similarity of these two warm periods suggests that the sequence and interaction of natural causes (solar and oceanic) are comparable and repeatable.

Figure 4: Graphs with Warm II (blue) and IV (green) overlain. Horizontal axis for Warm II is in kyr and plotted at top of each graph. Warm IV horizontal axis is plotted on bottom of each graph. Vertical axis is in 1 deg C units and same for all datasets. In the second graph, Warm IV was stretched by ~6 ka.

Warm III and V Anomalous Periods:

Warm III (MIS 7e) and V (MIS 11) exhibit less well-behaved patterns during their warm period (Figure 5). Warm III has a very brief initial climate optimum lasting 2 kyrs and then begins to enter a significant cooling period with nearly coincidental timing as seen in the Holocene 8.2 kyr event (Figures 2 and 5). Then temperatures stabilize for about 4 kyrs before resuming a rapid cooling descent.

Warm V has a similar initial temperature as the Holocene. However, it quickly enters a brief cooling (Figure 5). It is not certain whether this brief stadial at 423 kyrs is equivalent to the Holocene Younger Dryas stadial or the 8.2 kyr cooling (Figure 2). Warm V is unique because it has an extended warming period that is even warmer than the initial climate optimum. Warm V’s second extended warm is almost 2 degrees C warmer than present-day (Figure 5). EPICA discusses Warm V compared to the present-day in more detail here.

Figure 5: Holocene temperatures in red overlain on Warm III and V periods. Bottom horizontal axis corresponds to the Holocene time and top horizontal axis is the past warm time in thousands of years. Colored bar at top refers to the past warm phases. The red text corresponds to the Holocene events.

Warming Onset Patterns

Warm II, IV, and V warm periods display a rapid linear warming to their climate optimum (Figure 6). Warm II and IV have a similar steep slope of m=0.89 and 0.85 respectively. Whereas Warm V has a flatter slope of m=0.67, giving it more of a symmetrical pattern (Figures 1 and 2).

The initial Holocene warming is characterized by two events; the Boiling/Alleröd (B/A) interstadial and the Younger Dryas stadial, referred to as a stair-step pattern (Figures 3 and 5). The only interglacial warm period during the past 450 kyrs that exhibits a similar behavior is Warm III. Warm III has a B/A interstadial equivalent at 248 kyr, but it is not as warm as the B/A. An onset slope calculated for Warm III immediately following this interstadial is quite steep (m=1.1). A slope was not calculated for the Holocene due to the stair-step pattern.

As previously mentioned, the climate optimums of both Warm II and IV are approximately 2 degrees C higher than the Holocene present-day. Perhaps the Younger Dryas cooling during the Holocene warming onset intervened and prevented the Holocene from reaching the initial climate optimums seen in Warm II and IV. This was also observed by Javier, his Figure 14.

Figure 6: Comparison of the onset patterns and slopes. Past warm periods have a linear increase with an excellent regression. The linear equation and R² are presented on each chart.

Cooling Patterns

Final cooling slopes for the warm periods are more gradational than the onset warming and are similar with slopes ranging from 0.38 to 0.31 (Figure 7; Table 2). Warm II, IV, and V have a gradual cooling over 5 -10 kyr and then continue into a series of stadial and interstadial periods during the next mild glacial phase. Warm III has an initial rapid cooling, stabilizes and then a final cooling (Figure 5). Different slopes were calculated for Warm III (Figure 7). The initial rapid cooling slope is very steep (m=0.8), however the overall cooling is like the other warm periods.

If the Holocene Warm I behaves similarly to Warm IV, the cooling onset should begin very soon and within hundreds of years (Figure 3). If the Holocene Warm I behaves like Warm II, the cooling onset should begin within a few thousand years (Figure 3). If the Holocene Warm behaves more like Warm V, cooling onset is probably 10 thousand years away (Figure 5).

Figure 7: Comparison of cooling patterns and slopes. Cooling of past interglacial periods have a linear decrease with an excellent regression. The linear equation and R² are presented on each chart and slopes range from 0.40 to 0.31.

Table 2: Comparison of Onset and Cooling slopes for Warm Periods

Hierarchy of Events and Accuracy Observations

Scientists have attributed interglacial warming and pacing to the Milankovitch cycles. The Milankovitch cycles consist of eccentricity (elliptical orbit), obliquity (axial tilt), and precession (wobble) of Earth’s orbit resulting in cyclical variation in summer insolation in the northern hemisphere. A strong case for obliquity dominance has been made by Javier and Tzedakis, and precession is favored by Ellis and Palmer. These papers provide excellent overviews of the Milankovitch processes.

It is interesting to note that scientists compare astronomical data with precise accuracy to geologic timescales with uncertainties on the order of +/- 5 kyr. Picking the exact onset age of an interglacial period, the precise peak of a climate optimum, and their durations can vary +/- 3 kyr depending on the criteria used. Precession cycles occur on an approximately a 20 kyr scale and obliquity on a 41 kyr scale. The geologic timescale and interpretation error is approximately 25% of a precession cycle and 12.5% of an obliquity cycle.

In the following analyses, Milankovitch processes (eccentricity, obliquity, and summer insolation) are correlated to different events based on degree of temperature variation and duration. These events are the longer term glacial cycle, rapid onset of interglacial warm periods, and interglacial warm duration and cooling. All references to insolation/summer insolation in this post are “Northern Hemisphere Summer Insolation at 60 degrees North” (Berger, 1992).

Glacial Cycle Control Observations

In general, the cycle of eccentricity from circular to more elliptical and back to circular takes approximately 100 kyrs. Additionally, each 100 kyr cycle can be predominately more circular or predominantly more elliptical (Figure 8).

There are five intervals of Glacial Cycles I-V over the past 450 kyrs that increased in duration from past to present (89 kyr long to 119 kyr long). They are not precisely 100 kyr events. Closer evaluation of the eccentricity cycles demonstrates that each cycle also varies in duration and are not precisely 100 kyr events.

Figure 8: Glacial cycles I – V plotted with Eccentricity (orange) and summer insolation (blue). Duration (kyr) between eccentricity cycles in orange, between summer insolation in blue, and between termination/onset cycles in gray. See Figure 2 for detailed glacial cycle correlations.

Eccentricity and precession/summer insolation appear to correlate with the duration of the glacial cycles (Figure 8). Each eccentricity cycle has become increasingly longer in duration over the past 450 kyrs. The eccentricity cycles generally correlate with the progressively longer glacial cycles. However, the duration of summer insolation cycles, which are strongly influenced by eccentricity, coincides nicely with the glacial cycle duration (Figure 8).

An interesting observation is Glacial Cycle V occurs when eccentricity cycles are predominately circular (Ecc 5). Glacial Cycle V is the shortest glacial cycle over the past 450 kyrs and lasted only 89 kyrs. Because Earth is currently in a circular orbit, the Holocene Glacial Cycle appears more likely to be analogous to Glacial Cycle V with a shorter glacial cycle.

Glacial Cycles II and IV occur when the eccentricity cycles are between circular and elliptical (Ecc 2 and Ecc 4). These cycles are 200 kyrs apart. Ecc 2 has an asymmetrical pattern because it is more elliptical initially and then continues to decrease to an almost circular orbit. Glacial Cycle II occurs during Ecc 2 and has the longest mild and full glacial period.

Glacial Cycle III occurs when the eccentricity cycle is the most elliptical (Ecc 3). Glacial Cycle III also is an exceptional cycle which has two warm periods; Warm period III (MIS 7e) and a second warm period (MIS 7c). During elliptical cycles, both obliquity and summer insolation are amplified.

Warm periods occur at the beginning of each eccentricity cycle as Earth is going from a circular orbit to elliptical. In the middle of each eccentricity cycle, as Earth’s orbit is going from elliptical back to circular, mild glacial to full glacial periods exist, except for Glacial Cycle III which has a second stunted warm period.

Warm Onset Controls

The most significant event of the interglacial/glacial cycle is the termination of the glacial period and rapid onset of the interglacial period. This has happened five times over the past 450 kyrs.

In geologic terms, the warm onset event would be described as an unconformity representing a significant geologic episode such as continental uplift or massive erosional periods. Therefore, the observed rapid onset of warm interglacial periods should be caused by powerful events such as an alignment of external astronomical forces (eccentricity, obliquity, and precession/insolation). Warm onsets only occur when three external forces are increasing: 1) eccentricity, 2) obliquity, and 3) summer insolation as shown in Figures 8 and 9. All major warm onsets have commonality of these three increasing astronomical forces escalating in combination. All three play a role although not always equally.

Figure 9: Obliquity (red) plotted with Dome C isotope/temperature data (gray). Each obliquity cycle is 41 kyrs apart. As discovered by Javier, ~a 6.5 kyrs shift demonstrates a good correlation with interglacial warm period duration.

Warm III (MIS 7e) is a good example. As Javier points out in his Figure 10, MIS 7e onset did not initiate when the obliquity cycle was increasing because insolation was decreasing. As soon as summer insolation began to increase combined with increased obliquity and increased eccentricity, then warming onset began.

Warm V (MIS 11) was also initiated by increasing summer insolation and eccentricity. However, the increase in obliquity was not far behind. Warm V has a lower onset slope possibly due to the predominance of insolation and a more circular orbit. This slower onset results in a more symmetrical pattern for the warm period. Warm periods tend have a slower onset such as Warm I and V during the time when the eccentricity cycle is predominantly circular (Table 2).

There are an additional five occurrences when obliquity increases without initiating a subsequent warm interglacial. These occur when eccentricity is changing from elliptical to circular and insolation is below 550 W/m2 (Javier, Table 5). Obliquity is only successful in initiating an interglacial warm when eccentricity transitions from circular to elliptical and summer insolation is increased. The only exception is a second stunted warm period (MIS 7c) that occurred during the most elliptical cycle. During elliptical cycles, both obliquity and summer insolation are amplified and enhanced obliquity could explain a stunted warm period.

There are approximately ten occurrences when precession/insolation increases without initiating a subsequent interglacial. During this time eccentricity was changing from elliptical to circular orbit and/or obliquity tilt was decreasing.

Steeper and stronger onsets occur when Earth’s orbit is more elliptical and more gradual onsets when the orbit is more circular (Table 2, Figure 8). Obliquity and precession/insolation are more amplified during an elliptical orbit and less pronounced during a circular orbit.

Warm Duration Controls

The past five warm periods last from approximately 10 kyr to 30 kyr (Table 1). The duration of the interglacial warm period correlates well to obliquity which appears to be a dominate control as proposed by Javier (Figure 9 and his Figures 9 and 12). Obliquity cycles are 41 kyr in duration and most of the warm cycles are less than the obliquity cycle. Obliquity increases precede the warm periods by about 6 kyrs due to Earth’s thermal inertia (Javier). Summer insolation cycles have a higher frequency of about 21 kyrs (11 kyrs for ½ cycle) and do not correlate as well as obliquity does with warm periods (compare Figures 8 and Figure 9).

Obliquity is the greatest control on duration and cooling of the warm periods. Most of the warm periods ended due to decreasing obliquity. Although it appears summer insolation frequently decreases during this time.

Warm Periods II and IV have very similar patterns as discussed in the previous section and Figure 5. These two interglacial warm periods occur during a semi-elliptical eccentricity cycle with a similar insolation and obliquity cycle (Figures 8 and 9).

Warm periods III and V are extremely different end members. Glacial Cycle III and Warm III occurs when eccentricity is the most elliptical. Glacial Cycle V and Warm V occurs when eccentricity is predominantly circular.

During the most elliptical orbit, obliquity has the greatest tilt ranging from 22.1 to 24.5 degrees and insolation is the greatest ranging from approximately 430 to 550 W/m2. When [insolation] is greater than 550 W/m2, it can result in an interglacial period (Javier, his Figure 12). Warm III has two warm periods during this elliptical orbit with amplified obliquity tilt and summer insolation cycles. This initial warm period (MIS 7e) was short lived because obliquity began to decrease and ended the Warm period prematurely (Javier). A second stunted interglacial occurs during this cycle, MIS 7c, and was initiated by amplified obliquity.

Interestingly, Warm V is the longest warm period in duration but has the shortest glacial cycle. The short Glacial Cycle V is consistent with the shortest eccentricity/insolation cycle as previously discussed in the eccentricity control. Javier has attributed the extended duration of Warm V (MIS 11) to an additional increasing insolation cycle in the middle of its period as shown in Figure 8.

2^nd Order Control Observations

Several smaller warm events (interstadials) and abridged coolings (stadials) which last only thousands of years occur within each warm period. They are easily recognized in Figures 3 and 5. Many of the Holocene intermediate events have been extensively studied and named including; Boiling/Allerod warming interstadial followed by the Younger Dryas Cooling Stadial, and the intervening 8.2 kyr Stadial or cooling during Holocene optimum. Key stadial and interstadial events occur less frequently during warm periods than the Dansgaard-Oeschger cycles which occur primarily during mild glacial periods.

A cooling stadial that is equivalent to the 8.2 kyr event is present in both Warm II and IV shortly after the climate optimum causing the bimodal warming pattern. It is interesting that this event occurs 2 to 3 kyrs after the warm onset. Perhaps a decrease in precession triggered the Holocene 8.2 kyr cooling event as well as in Warm II and IV. The result was a meltwater pulse where glacial lake melting modifies the AMOC (Atlantic Meridional Overturning Circulation) resulting in a cool period.

Cycles of precession/ summer insolation which occur more frequently than obliquity cycles may explain the occurrence of smaller stadials and interstadials within the warm periods. Insolation and obliquity have different frequency cycles. Obliquity full cycles occur every 41 kyrs and insolation varies from 18 to 23 kyrs. Insolation could be a second order modifier on obliquity and initiate an overprint creating a stadial or interstadial during or between obliquity cycles.

Conceptual Models for Warm Periods

A generalized model for Warm II and IV is proposed in Figure 10 to demonstrate the hierarchy and timing of astronomical processes and oceanic processes on these similar Interglacial Warm periods. It is simplified from the detailed science describing these processes and serves as a guide for whether astronomical or oceanic processes dominate. The model assimilates the processes discussed in the previous section for the Holocene and by Javier.

Figure 10: Summary of processes for Warm II (blue) and IV (green) warm periods. Horizontal axis is relative time for kyr. Onset occurs during increasing Eccentricity, Obliquity, and precession/insolation. Obliquity is dominant control over cooling and thus, warm duration. Higher frequency precession/insolation creates stadials and interstadials.

Warm II and Warm IV occur during a semi-elliptical eccentric cycle (Ecc 2 and 4) and are approximately 200 kyrs apart as show in Figure 8. The rapid warming onset require the Milankovitch cycles (eccentricity, obliquity, and precession/Insolation) to be increasing simultaneously or within close timing of each other. These combined cycles terminate the previous glacial period and initiate the onset of the warm interglacial period. Most warm duration culminations coincide with obliquity which subsequently controls the duration. Warm durations can be interrupted by higher frequency insolation creating either a stadial (8.2 kyr event) as in the case of Warm I, II and IV or an extended warm period as in the case of Warm V. The minor temperature fluctuations of +/- 1.5 degrees C during the flatter portion of the warm durations are dominated by oceanic processes (Javier).

A conceptual model is also proposed for the anomalous Warm III and V periods in Figure 11. Eccentricity is very small or circular for Warm V and very elliptical for Warm III. As discussed in the onset section, increasing insolation may have initiated both warming periods with obliquity increasing shortly thereafter.

Figure 11: Proposed processes for anomalous Warm III and V periods. Horizontal axis is time in Kyr. Onset occurs during increasing Eccentricity, Obliquity, and precession/insolation. Vertical axis is relative global temperatures. Curves are Dome C isotope/temperature data.

Warm III had the steepest onset and cooling slopes. The first initial warm period was short-lived. A second stunted warm period occurred approximately 40 kyrs later. The glacial cycle was also one of the longest at 113 kyrs. During the most elliptical cycle, both precession and obliquity are amplified and can initiate a stunted “interglacial” period like MIS 7c.

Interglacial warm periods and glacial cycles show the following characteristics during Elliptical orbits:

• Astronomical forces dominate during the elliptical orbits. Both Earth’s axial tilt is more extreme and Earth’s wobble is more dramatic.
• Glacial cycles tend to be longer (>100 kyrs).
• Temperature changes are dramatic and abrupt with rapid warm onsets and faster cooling slopes.
• Obliquity increases can result in a couple of interglacial periods.
• Predominantly elliptical orbits repeat approximately every 400 krys.

When Earth is in a circular orbit like Warm V and present day Warm I, Earth’s tilt and wobble are less. Subsequently, astronomical forces play a lesser role in the interglacial warm periods. Obliquity tilt and summer insolation still control the warm initiation and closure. However, onset and cooling are more gradual.

Warm V onset was primarily driven by an early increase in insolation and a second increase by obliquity. The warm period was extended by a second summer insolation pulse resulting in its optimum. The warm duration finally ended by decreasing obliquity. The quiet or more stable portions of the long warm period were probably dominated by oceanic processes.

Circular orbits show the following characteristics:

• Circular orbits have less obliquity tilt, lower precision wobble and less summer insolation.
• Glacial Cycles tend to be shorter (<90 kyrs).
• Warm onsets and cooling are more gradual resulting in more symmetrical patterns.
• Oceanic processes dominate during the warm duration creating minor temperature changes (+/- 1.5 degrees C).
• Predominately circular orbits repeat approximately every 400 kys.

These models were an attempt to compile astronomical and oceanic processes discussed by previous authors (Javier and Ellis and Palmer) and to explain the various repeatable patterns seen in the Dome C isotope/temperature data. Although process interactions are very complex, separating out predominate causes and effects on global temperature should help improve future climate simulation models. Climate models need to include astronomical as well as oceanic and atmospheric forcing to reliably predict future temperature changes and the duration of the Holocene interglacial warm period.

Astronomical processes appear to be the key control for significant temperature changes. Oceanic and atmospheric processes create minor temperature fluctuations during warm periods. However, the culmination and eventual cooling is astronomically controlled. It is impossible for humans to control Earth’s orbit, tilt, or wobble. Since astronomical processes affecting significant climate changes are out of our control, a focus on adaptation instead of climate manipulation is a much better use of resources.

Summary and Conclusions

A Glacial Cycle traverse illustrates that the duration of interglacial/glacial cycles progressively increases from past to present over the past 450 kyrs and are not a simple 100 kyr cycle. Eccentricity and its influence on summer insolation appears to play a dominate role in the duration of glacial cycles. Circular orbits tend to have shorter cycles (<100 kyrs) and more elliptical orbits tend to be longer (>100 kyrs).

Conceptual models are proposed using astronomical and oceanic processes described in the literature to explain repeatable patterns observed in past interglacial warm periods. Prioritizing dominate processes operating in different warm periods may provide general guidelines for future climate models.

Past Warm II and IV periods are well behaved and exhibit strikingly similar warming/cooling patterns suggesting a repeatable interplay of astronomical, oceanic, and atmospheric processes. These repeatable patterns occur every 200 kyrs during semi-elliptical eccentric cycles.

On the other hand, anomalous Warm III and V periods tend to have less predictable patterns and are unique. Warm III occurred during the most elliptical orbit and Warm V during the most circular orbit. Glacial cycles during elliptical orbits tend to have rapid onset and several warm/cool periods because obliquity is amplified and summer insolation dominates. Warm periods during circular orbits tend to have slower warm onsets and are more symmetrical. Oceanic processes may play a greater role during the warm periods but play a minor role in controlling the onsets or eventual cool periods.

During the last 450 kyrs, the five major warm onsets with rapidly increasing temperatures are triggered by increases in the eccentricity, obliquity, and precession of Earth’s orbit. The nearly concurrent increase in these three astronomical forces appears a necessary component for a major warm onset. Obliquity is the dominate control for ending these major warm periods and entering a cooling phase.  Higher frequency procession/summer insolation appears to play a secondary role in overprinting the duration pattern with a stadial event such as the Holocene 8.2 kyr or extending a warm period like in Warm V. Oceanic processes dominate during periods of minor temperature changes (+/- 1.5 degrees C).

Dome C isotope ratios and their associated temperature estimates in combination with astronomical data provide ample evidence that astronomical forces control warming and cooling cycles. Because the astronomical processes affecting significant climate changes are beyond human control our focus should be on adaptation rather than climate manipulation. It is not a question if cooling will occur but simply a question of when.

Acknowledgements:

Special thanks to Andy May and Donald Ince for reviewing and editing the article.