Sun & Cycles Heat Up Ice Age Interglacials

Since the Mid-Brunhes Event, around 430,000 years ago, interglacial periods have grown warmer and their CO2 levels higher. Research confirms that Croll and Milankovitch were right: Earth's orbital cycles seem to be the cause of these documented cases of true global warming, with CO2 playing a supporting role, not the lead. Many of the catastrophic events warned of by climate change alarmists turn out to be well within the range of natural variation. Moreover, new findings indicate that the effects of the cycle induced changes, through their impact on the environment in the Southern Hemisphere, are not correctly accounted for in the IPCC models.

One of the big questions in climate science comes from studying recent interglacial periods—those relatively warm periods between bouts of ice age glaciation. It has been known for some time, that average temperatures during recent interglacials were warmer than during older ones. Writing in the April, 2010, edition of Nature Geoscience, Q. Z. Yin and A. Berger propose an answer as to why the amplitude (i.e. warming) of the glacial interglacial cycles increased significantly after the Mid-Brunhes Event (MBE) with cooler interglacials before the MBE than after. In their paper, entitled “Insolation and CO2 contribution to the interglacial climate before and after the Mid-Brunhes Event,” they describe their work as follows:

In parallel to the reconstruction of palaeoclimate based on proxy records, climate models are used to better understand past climate behaviour. In particular, efforts have been made over the past decade on modelling the most recent interglacials, namely the Holocene, the Eemian and the past five interglacials. Here, we focus on the forcing and global response of the climate system at the interglacial peaks of the past 800 kyr, using snapshot simulations to try to understand the difference between the post-MBE and the pre-MBE interglacials. The model used is LOVECLIM, with the atmosphere, ocean, sea ice and vegetation components interactively coupled and the ice sheets kept as today.

The Mid-Brunhes Event, ~430,000 years ago, signaled significant long-term changes in global atmosphere and ocean circulation. As a consequence, there was a transition to more humid interglacial conditions in equatorial Africa, and in the Northern Hemisphere to more glacial oceanic conditions. In a paper in Science, “A Mid-Brunhes Climatic Event: Long-Term Changes in Global Atmosphere and Ocean Circulation,” J. H. F. Jansen, A. Kuijpers, and S. R. Troelestra document the event through marine and continental records from various latitudes. Their conclusion was that the change was probably due to a change in the eccentricity of Earth's orbit.

“We present evidence of a global climatic change 4.0 x 105 to 3.0 x 105 years ago on a time scale of 1 x 105 to 1 x 106 years which is superimposed on the glacial and interglacial cycles,” they report. “Unlike other Late Cenozoic climatic variations reported so far, the change shows opposite trends in the Northern and Southern hemispheres.” The trends referred to are warmer Northern Hemisphere winters and Southern Hemisphere summers, accompanied by the opposite for Northern Hemisphere summers and Southern Hemisphere winters, which are cooler. This shift also brought an increase in humidity to the south.

It was Jansen et al. who first proposed a mid-Brunhes transition to more humid, interglacial conditions in the southern hemisphere. Over the millions of years of the Pleistocene ice age, slow changes in ocean basin circulation due to tectonic activity (i.e. shifting continents) caused recognizable changes in climate, but the mid-Brunhes change represents something different. Both Jansen et al. and now Yin and Berger concluded that small changes in the pattern of solar radiation energy received at Earth's surface, insolation, were responsible. This was triggered by a change in one of the three Croll-Milankovitch cycles that affect Earth's orbit and attitude, primarily eccentricity.

I have talked about the Croll-Milankovitch cycles before (see “Confirmed! Orbital Cycles Control Ice Ages” or the more detailed explanation in our book, The Resilient Earth). The three variables involved are shown in the illustration below. Yin and Berger think that the interaction of precession and eccentricity are responsible for the change in insolation, and hence, climate. This is how it works.

First, the reason the seasons vary is because Earth's axis of rotation is tilted. This tilt (T) is also referred to as obliquity. Currently Earth's axial tilt is 23.44°, though this changes over time taking approximately 41,000 years to shift between a tilt of 22.1° and 24.5° and back again. If Earth's axis of rotation was perpendicular to its orbital plane (i.e. straight up and down in the diagram above) then every day would be the same length every where on the planet. It is because of the slight tilt of Earth's axis that days vary in length over the course of a year—shorter in winter and longer in summer. It is this variation in the amount of day light a hemisphere receives that causes the temperatures to change from winter to summer and back again. Short days mean little warmth from the Sun and colder weather. Long days mean lots of insolation and hotter conditions.

In a sense, tilt can be thought of as a seasonal variation control knob, with greater tilt causing larger variation between summers and winters. But the magnitude of changes in the summer and winter are not the same and vary with latitude. The annual mean insolation increases in high latitudes with increasing obliquity, while lower latitudes experience a reduction in insolation. It has been proposed that lower obliquity favors ice ages both because of the mean insolation reduction in high latitudes as well as the additional reduction in summer insolation. Cooler summers are suspected of encouraging the start of an ice age by melting less of the previous winter's ice and snow. Currently the Earth's tilt is about half way between its extreme values, though the tilt is decreasing. It will reach its minimum value around the year 10,000, which may be accompanied by a shift back to glacial ice age conditions. Somewhat counter-intuitively, no significant climate changes are associated with extreme axial tilts.

Second, because Earth's orbit around the Sun is elliptical, not perfectly round, our planet's distance from the Sun varies. The measurement of how far Earth's orbit is from being circular is called eccentricity (E). Naturally, radiation received from the Sun, called insolation, is higher when Earth is closest. Currently, Earth is closer to the Sun during the Northern Hemisphere's winter, which occurs when the Southern Hemisphere is in summer. This means that when the Southern Hemisphere is having its summer there is a slight increase in insolation, as compared with a Northern Hemisphere summer, because Earth is slightly closer to the Sun (91.402 vs 94.509 million miles). Again, there are a number of factors that cause the eccentricity of Earth's orbit to vary (eccentricity variation of ±0.012), with the major component of these variations occurring on a period of 413,000 years.

Third, the timing of summer/winter is controlled by the last parameter, precession. The direction that Earth's axis is tilted at any given point during its orbit around the Sun also varies—put another way, Earth wobbles on its axis. This slow wobble cause the seasons to slowly shift, so that eventually the occurrence of the Southern Hemisphere's summer will be when Earth is farthest from the Sun. This change was noted by astronomers in ancient times and called the “precession of the equinoxes,” nowadays just precession (P). Note, there are other changes in the alignment of Earth's axis, but their magnitude is much smaller than precession. For the axis to return to its same orientation takes ~25700 years.

The fourth and final thing to note is that the distribution of land and sea is different for the Northern and Southern Hemispheres. Specifically, the Southern Hemisphere has a much greater expanse of ocean water than the north. This changes the absorption of energy for the entire planet, depending on which hemisphere is pointed toward the Sun. With the cycle induced insolation increase during the Southern Hemisphere summer, sea surface temperatures (SST) would be expected to rise. Taking all of these variations into account, around 400,000 years ago conditions had changed enough to cause warmer interglacials. Data for the last ten interglacials is presented in the figure below, taken from Yin and Berger.

Marine δ18O, precession and obliquity around the past 10 interglacial peaks. The black bars localize δ18O minima, precession minima and obliquity maxima. The dates of δ18O minima and corresponding MISs are indicated.

The δ18O ratio is an indication of temperature using oxygen isotopes as a proxy measurement. The term MIS stands for “Marine Isotope Stage,” which refers to alternating warm and cool periods in the past, deduced from oxygen isotope data from deep sea core samples. The conclusions drawn by Yin and Berger from these data are as follows:

Analysis of the geographical features shows that the post-MBE interglacials are warmer in boreal winter and autumn (in a lesser degree in spring) with the largest warming over the high latitudes in both hemispheres, but that during boreal summer, most of the continents are cooler. Such analysis also shows that the changes in CO2, water vapour, sea ice and land vegetation amplify the astronomically induced boreal wintertime warming after MBE and counteract the astronomically induced boreal summertime cooling, finally leading to warmer interglacials after MBE on annual average.

Our simulations show that the post-MBE interglacials are warmer than the pre-MBE ones as expected from their higher GHG concentrations. However, insolation has a significant role at the seasonal and hemispheric scales. In the explanation of the warmer post-MBE interglacials, boreal winter—or equivalently austral summer—is a key season and the Southern Hemisphere has a more important role than the Northern Hemisphere, as it warms significantly during both seasons. We suggest that this might be considered when trying to understand the underlying causes of the higher CO2 concentration during the interglacials after the MBE.

Some interesting inferences can be drawn from the interglacial cycle data. The presence of similar orbital configurations and comparable atmospheric greenhouse gas concentrations have led a number of scientists to the suggestion that MIS 11 is the previous interracial that most closely matches the Holocene. MIS 11 spans the period from 420 to 360 thousand years ago, and represents the longest and warmest interglacial interval of the last half million years. In fact, it shows the most warming (i.e. highest temperatures) in the last 5 million years. MIS 11 is characterized by overall warm sea-surface temperatures in high latitudes, strong thermohaline circulation, unusual blooms of plankton in high latitudes, and higher sea level than the present. The most intriguing thing about MIS 11 is that it lasted twice as long as most other recent interglacial stages, raising the possibility that the Holocene will last much longer than previously expected.

This variation in interglacial duration should not be confused with the average interglacial spacing, the time taken by glacial periods. Up until about 1.2 million years ago, ice age glacial periods lasted about 40,000 years but since ~700,000 years ago ice-age cycles have lasted for around 100,000 years. As previously reported in “Change In Ice Ages Not Caused By CO2,” the Croll-Milankovitch cycles are thought to be responsible for the change in glacial duration as well. This change predated the MBE, and began before the Brunhes–Matuyama reversal, approximately 780,000 years ago. The Brunhes–Matuyama reversal, named after Bernard Brunhes and Motonori Matuyama, was the last time Earth's magnetic field underwent reversal. Because it affected the whole world more-or-less simultaneously, it is used to align ocean sediment core dates.

The other lesson to be learned is that all of the climatic “disasters” that global warming alarmists have predicted—warmer seas, melting glaciers, higher sea levels, etc—could be exactly what nature has in-store for our world. And if another ice age does not happen for another 30 or 40 thousand years, that too would not be “unprecedented.” In fact, most of the things that the alarmists point to, shrieking “see! Its global warming!” are not indications of anything out of the ordinary.

Looking to the South

It is widely accepted that insolation drives deglaciations, the transitions from glacial conditions to warmer interglacial ones. It is also known that a warming climate causes CO2 levels to rise, so the coincidence of GHG levels and temperature during interglacials is not unexpected. The upshot of Yin and Berger's work is that the Southern Hemisphere undergoes greater warming than the Northern Hemisphere during an interglacial and that has implications for other things happening in the Antarctic.

Sarah Gille, from UCSD's Scripps Institution of Oceanography, has a News and Views article in the same issue as the Yin and Berger paper. In it she emphasizes the importance of the Southern Ocean surface layer in understanding the sequestration of heat and carbon dioxide in the deep ocean. “Evaluations of future climate depend in part on understanding the processes by which the Southern Ocean mixed layer evolves over time,” she states. “[S]imulations from the ensemble of IPCC models generally agree in predicting an increasing index of the Southern Annular Mode in coming decades, but they disagree in their estimates of the resulting mixed-layer depths. If we are to develop meaningful projections for future ocean impacts on climate, then attention will need to be focused on accurately simulating mixed-layer physics.”

The Southern Annular Mode (SAM) is a hemispheric scale pattern of climate variability that describes variability in the “anomalous” atmospheric flow, that is, variability not associated with the seasonal cycle. By driving changes in the mixed-layer depth (MLD), the SAM significantly impacts both physical and biogeochemical processes. Changes in the upper ocean associated with the SAM influence both heat absorption and carbon uptake and storage in the Southern Ocean.

Gille's statements were based on yet another NatGeo paper by Sallée et al., linking the SAM with mixed-layer depth, the thickness of the top ocean layer created by active turbulence, and having nearly uniform temperature and salinity. The mixed layer plays an important role in climate but, as Sallée et al. state, “the response of the Southern Ocean mixed layer to changes in the atmosphere is not well known.” Their work, based on new data from Argo floaters, reinforces the importance of what happens between wind and water in the Southern Ocean. Previous studies were model based, mainly because there were no data available from deep within the waters of the Southern Ocean—the Argo data changes that. The old models predicted a much more limited effect than the empirical measurements indicate. Sallée et al. concluded:

Given the influence of the mixed layer on the carbon cycle and heat storage by the ocean, it is critical that climate models can represent the mean state and variability of the mixed layer. However, the current class of Intergovernmental Panel on Climate Change models vary widely in their ability to reproduce the observed mean depth of the mixed layer and its seasonal cycle. Models that fail to reproduce the mean state of the Southern Ocean mixed layer are unlikely to do a good job of simulating anomalies in MLD. Indeed, a recent coarse-resolution coupled climate model thought to be representative of the state of the art does not capture the observed zonally asymmetric response of the MLD to the SAM. This raises concerns that the current generation of models may not yet adequately capture the interaction between the ocean and atmosphere, as mediated through the surface mixed layer.

So let's connect the dots: The slowly changing Croll-Milankovitch cycles have caused greater warming in the Southern Hemisphere during interglacials over the past 400,000 years or so; An increasingly positive SAM in recent decades is one of the strongest climate trends observed in the Southern Hemisphere; and an active SAM affects the MLD in such a way that both heat absorption and carbon uptake increase. Climate driven by the Sun, influenced by orbital cycles, causing change in the circulation of wind and water in the Southern Ocean. Once again nature is doing things that science was unaware of and are still poorly understood. Model studies have once again led science astray, corrected only when empirical data became available. Now we know that another “critical” climate factor was unaccounted for in the IPCC suite of models, the models that the global warming scare is based on. Ask me again why I am a skeptic.

Be safe, enjoy the interglacial and stay skeptical.

[ PS: For more information related to interglacials and the Croll-Milankovitch cycles you may wish to read “Ice Age Terminations: Orbital Cycles, Ocean Circulation and Shifting Monsoons,” “Melting Antarctic Ice Part of Natural Cycle,” “The Long Road Ahead,” and “The Grand View: 4 Billion Years Of Climate Change.” ]

excellent article

I have been a global warming sceptic (prefer realist) for years, and it was knowledge of the Milankovitch cycles and their impact on insolation and glaciation that formed part of the reason for climate realism. It is especially good news if this interglacial lasts as long or longer than MIS 11.

Doug, please come over to and join in on the forum.

Climate Realist.

ice age

will another ice age will occure in the future ???

Next Ice Age

Yes, the odds are overwhelmingly in favor of a return to glacial conditions. It may take longer than the previous interglacial, due to Earth's current orbital configuration, but sometime between 5,000 and 25,000 years from now look for a significant cooling.