The Story of Climate Change (Part 3)
"The sky is falling …" - Chicken Little
Our planet is currently traversing a region of the Milky Way galaxy that can trigger periods of intense cloudiness and cold. If our 675-million-year climate model reasonably reflects Earth's past climate trends, and if the relationship between galactic radiation flux intensity, the galactic orbital position of our solar system, and Earth's climate are indeed real, then we are currently passing through the Sagittarius arm, the Milky Way's largest spiral arm which can generate very high levels of galactic cosmic radiation and promote a shift towards a cloudy and colder climate characterized by polar ice caps and glacial periods. We are now in the middle of an Ice Age. This Ice Age is defined by general global cooling with periodic glacial expansions and retreats. The warmer, high frequency, out-of-plane galactic orbital oscillation driven climate response will eventually break the cooling feedback loops and end the Ice Age. But then, every 30 million years or so as we descend into the next in-plane cold climate trough, another Ice Age will likely occur until the entire arm region is crossed – about 180 million years for the Sagittarius arm. This galactic region was last transited by the Earth about 675 million years ago during the Cryogenian Period.
Cutting the Gordian Knot
We cycle between Hothouse and Icehouse global climate periods because galactic orbital exposure to a high energy cosmic radiation modulated by spiral arm structure is the predominant driver of the Earth’s global climate via ionization in the lower atmosphere leading to more or fewer clouds leading to warmer or cooler climates. Global baseline temperature is then modulated through the cloud-albedo effect. 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 Icehouse. At the same time, the radiation intensity here on Earth has been oscillating in and out of plane. Now, the in-plane radiation is nearing maximum. This promotes increasing tropospheric cloud-cover which cools 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 might push the Earth into a full-blown glacial period or “Snowball” scenario.
Ice Age - Glacial Inception
In 1941, Serbian mathematician and geophysicist Milutin Milankovitch proposed that variations in the earth’s orbit can cause climate variability through a local response to changes in insolation. The annual insolation of planet earth does not change during Milankovitch cycles. Instead, it is the distribution of solar energy with latitude and with seasons that influences the earth’s climate. As we continue into the Sagittarius arm cold climate zone the planet gets colder. Now the Milankovitch orbital cycles begin to really show their influence in the effectiveness and distribution of the sunlight received by the Earth. These orbital changes cycle every 100,000 years for eccentricity (elliptical orbit shape), 41,000 years for obliquity (tilt angle) and 21,000 years for precession (axis wobble). It should be noted that the precise way these three Milankovitch orbital variations work together to regulate the timing of glacial-interglacial cycles is not that well understood but let’s give it a try anyway, shall we?
How then might an Ice Age occur? Is it just the falling global temperatures? In 2017, Jung-Eun Lee and associates published model results which indicated that as the planet cools to a critical minimum temperature, and as obliquity moves lower (smaller tilt angle), and as planetary axial precession and obliquity effects drive both polar regions colder, a glaciation cascade via sea ice expansion can occur with subsequent ice-albedo and water vapor feedbacks maintaining the glacial state.
Glacial Progress
Let’s propose a possible explanation for the progression of glaciation. The early Pleistocene glacial-interglacial period (Fig. 11) where the ice expanded for 20k years then receded for 20k years appears to be driven by the Milankovitch obliquity cycle producing a 41k year period. This climate cycle is the result of the modulation of Earth’s tilt axis. The tilt angle changes from a minimum of about 23 degrees to a maximum of around 25 degrees in a continuous cycle. After glacial inception and as obliquity starts to move higher, the higher latitudes and the poles begin to get more sunlight which warms both regions. High obliquity then begins to melt the glaciers and sea ice which reduces the Earth’s albedo and initiates a 20k year warmer interglacial period. As obliquity again starts to move lower, cooling both polar regions, ice-albedo again fuels the cooling feedback where low obliquity then drives the next 20k year glaciation. This glacial-interglacial cycle had repeated for nearly 2 million years.
Starting about one million years ago, the oxygen isotope temperature proxies in the geologic record (Fig. 11) indicate that the late Pleistocene glacial - interglacial period makes a significant shift to 100k years. Even though the 100k year eccentricity cycle alone is the weakest of the three Milankovitch cycles to the degree that it affects solar radiation, it can modulate and enhance precession and obliquity drivers.
Oh instigator, thy name is Precession
As we continue to descend into this Sagittarius arm cold climate zone driven colder by the increasing in-plane galactic radiation, the planet continues to cool. Low obliquity cooling and cooling feedbacks can still initiate and maintain glaciation. High obliquity alone however, will not be sufficient to break the feedback loops of this colder world. During the late Pleistocene, as the global temperatures continue to fall toward the bottom of this cold in-plane trough, the 100k year glacial-interglacial period remains frigid and ice-bound for 80k years (two obliquity cycles). As the Earth’s orbit once again becomes more elliptical and with the added insolation from the Milankovitch eccentricity mode coupled with high obliquity, 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 with increased insolation, subsequent deglaciation and a 20k year interglacial period. The interglacial is then terminated when obliquity moves lower with eccentricity remaining high. Precession then 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 80k year glacial period (Table 5).
Snowball Earth Revisited
We have seen possible glacial-interglacial progression scenarios in our current Pleistocene cold climate trough, but what about the next cold in-plane trough 30 million years from now? This location in time and space produced the climate conditions that drove the Marinoan glaciation, 640 million years ago, and the Snowball Earth hypothesis. Our climate model (Fig. 6) indicates that the Marinoan trough is possibly the deepest and coldest in the entire Milky Way spiral arm structure with the associated highest in-plane galactic radiation flux driving cloudier and colder global climates. What would happen as the ancient Earth descended into that trough?
It is thought that in the deep past, changes in the principal astronomical frequencies due to shortening of the Earth-moon distance and the length of day, induced a shortening of the fundamental periods for obliquity and precession. The timing and pace of the Marinoan glacial-interglacial cycles may have been slightly different than ours today but the result would be the same.
In the Marinoan cold trough, as the cloud-albedo driven global temperatures fall to the critical minimum and after glacial inception has occurred, the Milankovitch obliquity cycle sets the pace of the early Marinoan glacial-interglacial period. As higher in-plane cosmic radiation drives temperatures lower, the coupled Milankovitch eccentricity mode produces the longer late Marinoan glacial-interglacial cycle. However, the Marinoan trough is colder than the Pleistocene. With higher radiation flux intensity, the coupled eccentricity insolation will eventually be insufficient to thaw the cold white planet thus maintaining an extended ice-albedo driven glacial state lasting perhaps several million years. This Snowball scenario played out on a predominantly equatorial continental configuration during the Cryogenian Period, leaving both hemisphere polar regions unencumbered to grow expansive sea ice. Further on however, as the out-of-plane high frequency climate driver produces warmer global temperatures, the Marinoan Ice Age will progress as the Pleistocene will.
Ice Age Termination
During our planet’s descent into the Pleistocene cold climate trough, after the critical minimum global temperature is reached and glacial inception has occurred, glacial-interglacial cycles can continue at a steady pace for a couple of million years driven by the 41k year Milankovitch obliquity cycle. As we descend further into this cold trough with in-plane radiation increasing, the global temperature continues to fall. The 100k year coupled eccentricity cycle now begins to predominate by modulating and enhancing precession and obliquity effects producing the 100k year glacial-interglacial period. In another million years or so, as the out-of-plane high frequency climate driver forces the global temperatures warmer, the 100k year eccentricity coupled glacial-interglacial cycle will eventually revert to the 41k year obliquity dominant cycle until continued warming temperatures over the following two million years finally terminate this Ice Age (Fig. 12).
The Climate in Our Future
So, where are we headed in the nearer term? In Figure 11, we can see that our current climate state (0 kyr) is warm interglacial. We have passed the interglacial optimum temperature (high obliquity) and are experiencing a continuing cooling toward the next glacial inception point about 10k years from now, where high eccentricity and low obliquity will drive rapid sea ice expansion and the next 80k 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 (Fig. 6), symmetry suggests that we will experience slowly warming temperatures with glacial-interglacial cycles becoming more interglacial dominant (Fig. 12) until finally dissipating in another 3 million years or so.
Hothouse transit
We will emerge from this Icehouse and enter the next Hothouse by exiting this cold Sagittarius spiral arm space and transiting the next hot inter-arm region. This Hothouse was last transited by our system 500 million years ago during the Cambrian Period which produced the “Cambrian Explosion”, the most intense burst of evolution ever known. As we cross this inter-arm space, the galactic radiation intensity is reduced with accompanying rising temperatures. Now, the out-of-plane oscillations reduce the radiation even further, driving global cloud cover to a minimum, initiating an even hotter period about 30 million years long. Each of the Milky Way’s inter-arm spaces is of unique size but can produce two to three of these very hot out-of-plane periods during the 50-to-100-million-year transit (Fig. 6). Further on in our galactic orbit, the radiation again increases as we approach the next high radiation spiral arm, the Perseus arm, home of the Ordovician glacial period.
The Gravity Wave
On the dark horizon
we see a sea of stars
rise on the swell
to ride the gravity wave again
accelerating to trough
through turbulent particle sprays
then slowing to crest
only to stand –
still on the beach
as the last of the captured foam recedes
– aground.
KS
end Part 3
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