What really does set the global climate state? The Million Year Ice Core Project (MYIC) (Follow on BlueSky), has been underway for several years, preparing to drill the oldest continuous ice core record from Antarctica. The project is a major element in the Australian Antarctic Program, led by the Australian Antarctic Division and other national and international collaborators to answer that very question.
Now, in 2025, the Australian Antarctic Division is leading one of the most ambitious and challenging scientific projects undertaken in Antarctica - the quest to drill an ice core containing over a million years of Earth's climate and atmospheric history. This continuous climate record will help solve a long-standing mystery about the timing and duration of past "ice ages" detected in marine sediment cores (Fig. 11). These sediment cores with their fossilized foraminifera reveal a proxy temperature record that indicates about one million years ago the cycle of "ice ages" shifted from a regular 41,000 year glacial-interglacial period, to a cycle every 120,000 years.
A prevailing theory is that declining atmospheric CO2 levels were the cause of the longer, colder ice expansions. The new million-year ice core record might provide the essential atmospheric gas record to test this theory.
We can see in the 1998 Vostok ice core data from Antarctica (below) that temperature leads the way with CO2 following by hundreds to thousands of years. At its simplest, this indicates that temperature not CO2 drives the climate change. What then drives the temperature change? Some might say "it's too complicated". I beg to differ.
Antarctic EPICA Dome C ice core temperature proxy data below is shown with Milankovitch orbital cycle forcing. While much correlation is discernable, it makes sense that higher obliquity increases insolation especially to the polar regions driving warmer temperatures and interglacials while lower obliquity provides less insolation, cooler temperatures and glacial expansion. Now, superimpose the 20k-year precession cycle where the effective obliquity angle could be modified by as much as 1 degree and the result is a confluence of high obliquity, high precession, and high eccentricity providing the insolation changes sufficient to break the cold feedback loops (water vapor and ice-albedo) of the previous glacial expansion period. After a 20k year interglacial, high eccentricity, low obliquity and low precession drive temperatures lower where the cold ice-albedo feedback loop takes hold and catches us in its hundred millennia icy grip.
Do the ice cores extracted from Greenland show the same climate variability and timing as Antarctica? The proxy temperature record below shows us the fine detail of a glacial-interglacial period from Greenland. The previous interglacial, the Eemian, around 120 thousand years ago, progressed to the follow-on glacial expanse period. As eccentricity and obliquity just begin to fall, we see temperature "jumps" (D-O events) in closely packed time regions. These hot blips are probably solar activity driven warm-ups. We can see the short duration of the events and a long cold recovery. As we approach the midway point of glacial expanse, we see some gaps where no temperature spikes occur. These correspond to low obliquity periods (there are 3 of these during the ice expanse). The periodic solar cycles that gave us the hot events during high obliquity cannot break through the colder low obliquity feedback wall. When high obliquity returns so do the undampened solar spikes. Continuing on to the confluence of high obliquity, high eccentricity where precession produces a series of significantly hotter northern hemisphere summers when the Earth is near its closest distance to the sun (apogee). This results in breaking the ice-albedo feedback and subsequent deglaciation and a 20k year interglacial period - this is our interglacial called the Holocene. We should not expect the fundamental planetary orbital processes that drive the Ice Age climate oscillation to all of a sudden be rendered ineffectual by a few parts per million of some trace atmospheric gases. To foretell the future of climate change then - in 10k years or so, with eccentricity still high and obliquity going low, precession produces a period where northern hemisphere summers occur near apogee with high eccentricity and low obliquity resulting in those summers getting a little warmer while southern hemisphere summers become significantly colder near perigee resulting in rapid sea ice expansion and the next 100k year glacial expanse period.
With this sampling of both polar regions, and the exceptional resolution from Greenland, we see the same basic climate shifts. During the last 800 thousand years or so, a seeming pattern of climate variability of 20k year warm interglacials followed by 100k year cold glacial periods has persisted. During the glacial expanse we see solar activity driven warming peeking through the high obliquity window with low obliquity shading and damping any solar influence. Great! Seems to explain a lot. Seems pretty clear. Milankovitch cycles and cold feedback loops. Does not explain the Great Shift though. Why did the periodicity of the glacial-interglacial cycle change from 41k years to 120k years around a million years ago (Fig. 11)? The 41k year obliquity cycle seems to predominate in the early Pleistocene glacial-interglacial cycle - for 2 million years. What changed? Further, what had set the earth's climate to oscillate in the first place?
The Great Shift
From The Story of Climate Change, I lay out the paradigm shifting ideas we can call the cosmic ray/cloud hypothesis (Svensmark). This cosmic radiation/ionization/cloud formation concept might now be expanded. Accepting the centrality of cosmic ray ionization in cloud modulation, and with observational and mechanistic support, I propose that climate is sensitive to the amount of lower tropospheric ionization caused by high energy Galactic Cosmic Rays (CR). More high energy radiation in the lower atmosphere means more ionization, more aerosols, more and whiter low-level clouds, thus cooler climates. Alternately, less radiation in the lower atmosphere means fewer clouds driving warmer climates. We can call this integrated process the cloud-albedo effect.
The long geologic timescale modulation of Earth's tropospheric cloud cover and global temperature are driven in a top-down process beginning with high energy galactic radiation variations originating in Earth's galactic orbit location within the Milky Way spiral structure. This produces low frequency climate responses: 50-million-year Hothouse periods with lower CR flux as we transit an inter-arm space and 100 to 200 million year Icehouse periods with higher CR flux as we cross a Milky Way spiral arm. Our solar system also experiences galactic orbital oscillatory out-of-plane perturbations. The plane polarized galactic cosmic radiation intensity is then further modulated by our in-and-out-of-plane orbital motion. This acts as a secondary driver of higher frequency climate responses: periodic 30-million-year warming-then-cooling-then-warming cycles. This signal is superimposed on the background spiral arm radiation signature.
The galactic radiation intensity sets the baseline temperature from which all other drivers vary. For the last 50 million years of our galactic orbit, we have been transiting the Sagittarius spiral arm. The increasing baseline spiral arm radiation leads us further into the colder chambers of the Icehouse. At the same time, the radiation intensity and temperature have been oscillating up and down in unison with our in-and-out-of-plane motion. We are now fully in-plane with cosmic rays at maximum. This promotes increasing tropospheric cloud-cover which can cool the Earth further. A glacial Ice Age can occur if the spiral arm radiation-driven baseline temperature of the planet falls to a critical minimum upon which our vertical galactic orbital oscillation places us nearly fully in-plane. Now we are vulnerable. Other shorter-term forces such as planetary orbital dynamics, ice-albedo feedback, plate tectonics, volcanism, solar activity, or proximate celestial events could push the Earth into a full-blown glacial Ice Age or "Snowball" scenario.
Here we are, in the middle of the Pleistocene Ice Age. So far, over the last 3 million years our climate has oscillated between cold and icy glacial climates and warm interglacial climates. The 41k year glacial-interglacial period driven by planetary obliquity dominated the Pleistocene for two million years. One million years ago the glacial-interglacial period switched to the current 120-thousand-year period. What has changed over the last 3 million years? The answer should be obvious, the temperature has changed. A million years ago the global temperature was colder than before and is ultimately colder today. One million years ago we were approaching the galactic plane and the bottom of this Pleistocene cold climate trough. The closer to the bottom the colder we got. So cold in fact, that high obliquity alone could not break the ice-albedo feedback loop. All three planetary orbital parameters - obliquity, eccentricity, precession - must act together in a 120-thousand-year cycle to melt our icy tomb to finally bask in a high obliquity sun once again - into the next warm interglacial.
Where are we now?
One million years after The Great Switch, here we are now. Our current climate state is warm interglacial. We have passed the interglacial optimum temperature (high obliquity) and are experiencing a continuing cooling as obliquity moves lower toward the next glacial inception point about 10 thousand years from now, where high eccentricity and low obliquity will drive rapid sea ice expansion and the next 100k year glacial period. In a wider view, assuming we are currently at the midpoint of the Pleistocene Ice Age and at the bottom of this current cold climate trough, symmetry suggests that we will experience slowly warming temperatures as we proceed out-of-plane with glacial-interglacial cycles becoming more interglacial dominant until finally dissipating in another 3 million years or so.
Finally
I am not sure that a new extensive Antarctic ice core record would change my thinking when it come to the ultimate drivers of global climate but it might be enlightening to visualize The Great Switch climate variability from a southern hemisphere perspective through Oxygen-18 isotope temperature proxies from water ice.