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While the IPCC and global warming alarmists continue to claim climate change is controlled by atmospheric CO2 levels, most knowledgeable scientists will tell you that climate change is caused by variation in Earth's orbit and orientation. These periodic changes in movement and attitude are called the Milankovitch Cycles. A new paper, to be published in Science, confirms that glacial terminations are caused by Earth's orbital cycles, not carbon dioxide.
There are three components that comprise the Milankovitch Cycles: Orbital Eccentricity, Axial Obliquity, and Precession of the Equinoxes. The cycles are named for Milutin Milankovitch, a Serbian engineer, who mathematically theorized that variations in these three orbital parameters determine climatic patterns on Earth. Milankovitch's work, which was done in the mid-20th century, was hindered by the need to calculate orbital data by hand. Because of some inaccuracies in his results, the theory was not accepted by the scientific main stream until later revisions by Wally Broecker and J. van Donk. In 1970, they published a paper that detailed temperature changes going back 400,000 years and resolved a number of the apparent discrepancies in Milankovitch's theory. From their work it became widely accepted that orbital cycles cause the glacial-interglacial cycles that have governed ice age conditions over the last few million years.
Earth's orbit goes from measurably elliptical to nearly circular in a cycle that takes around 100,000 years. Presently, Earth is in a period of low eccentricity, about 3%. This causes a seasonal change in solar energy of 7%. The difference between summer and winter is a 7% difference in the energy a hemisphere receives from the Sun. When Earth's orbital eccentricity is at its peak (~9%), seasonal variation reaches 20-30%. Additionally, a more eccentric orbit will change the length of seasons in each hemisphere by changing the length of time between the vernal and autumnal equinoxes.
Variation in Axial Obliquity, Orbital Eccentricity, and Polar Precession. NOAA.
The variation in eccentricity doesn't change regularly over time, like a sine wave. This is because Earth's orbit is affected by the gravitational attraction of the other planets in the solar system. There are two major cycles; one every 100,000 years and a weaker one every 413,000 years.
The second Milankovitch cycle involves changes in obliquity, or tilt, of Earth's axis. Presently Earth's tilt is 23.5°, but the 41,000 year cycle varies from 22.1° to 24.5°. This tilt is depicted in the upper-left panel of the illustration above. The smaller the tilt, the less seasonal variation there is between summer and winter at middle and high latitudes. For small tilt angles, the winters tend to be milder and the summers cooler. Cool summer temperatures are thought more important than cold winters for the growth of continental ice sheets. This implies that smaller tilt angles lead to more glaciation.
The third cycle is due to precession of the spin axis. As a result of a wobble in Earth's spin, the orientation of Earth in relation to its orbital position changes. This occurs because Earth, as it spins, bulges slightly at its equator. The equator is not in the same plane as the orbits of Earth and other objects in the solar system, as shown in the illustration below.
Precession of Earth's axis of rotation.
The gravitational pull of the Sun and the Moon on the equatorial bulge tries to bring Earth's spin axis into perpendicular alignment with the orbital plane. Earth's rotation is counter clockwise; gravitational forces make Earth's rotational axis move clockwise in a circle around its orbital axis. This is called precession of the equinoxes because, over time, the retrograde axial rotation causes the seasons to shift.
Until recently, variations in the intensity of high-latitude Northern Hemisphere summer insolation, driven largely by precession, were widely thought to control the timing of glacial terminations. However, it has been suggested that changes in Earth’s obliquity may be a more important mechanism. According to the paper by R. N. Drysdale et al., “Our record reveals that Terminations I and II are separated by three obliquity cycles and that they started at near-identical obliquity phases.”
During the Late Pleistocene, the period of glacial-to-interglacial transitions (or ‘terminations’) has increased when compared to the Early Pleistocene. The length of the cold glacial periods shifted from ~40,000 years to ~100,000 years in length some 700,000 years ago. Why this change took place is still a matter of debate, though we know that CO2 levels did not cause the change (see “Change In Ice Ages Not Caused By CO2”). Although many different explanations have been proposed for the shift, the most widely accepted one invokes changes in the intensity of high-latitude Northern Hemisphere summer insolation (NHSI). These changes were thought to be primarily driven by precession, which produces relatively large seasonal and hemispheric insolation variation.
The new work by Drysdale et al. claims that obliquity, not precession, is the proximate cause of glacial terminations. Moreover, based on a detailed study of the last two terminations (T-I and T-II), it was found that glacials can span multiple obliquity cycles. The researchers make the case for obliquity as the forcing mechanism:
Based on our results, both T-I and T-II commence at the same phase of obliquity and the period between them is exactly equivalent to three obliquity cycles (~123 ky). Obliquity is clearly very important during the Early Pleistocene, and recently a compelling argument has been advanced that Late Pleistocene terminations are also forced by obliquity, but that they bridge multiple obliquity cycles. Under this model, predominantly obliquity-driven total summer energy is considered more important in forcing terminations than the classical precession-based peak summer insolation model, primarily because the length of summer decreases as the Earth moves closer to the sun.
Timing of the Termination II was established by matching a uranium–thorium (U–Th) chronology derived from a high-resolution speleothem δ18O time series to the T-II marine sediment record from the Iberian margin in the northeast Atlantic Ocean. A speleothem is a secondary mineral deposit formed in caves. The figure below, taken from the paper's preprint online, compares timing, insolation and obliquity data for the two terminations.
Figure 3 from Drysdale et al., Science express 13 Aug 2009.
Shown above is a comparison of the benthic δ18O record through T-I (orange crosses; plotted on the upper timescale) and T-II (black crosses; plotted on the lower timescale), showing similarities in the duration of both terminations. Southern Hemisphere summer insolation at 65°S (blue) and obliquity curves (red) for T-I (dashed lines) and T-II (solid lines), and obliquity. The gray vertical bar marks the commencement points for both terminations, revealing an age difference of ~123,000 years, which is equivalent to three obliquity cycles of ~41,000 years each. As the author's put it: “Our record reveals that Terminations I and II are separated by three obliquity cycles and that they started at near-identical obliquity phases.”
What this means is that another nail has been put into the coffin of carbon dioxide as the primary driver of climate change. While the earlier paper by Hönisch et al. showed that CO2 could not be the driver of glacial-interglacial transitions (again see “Change In Ice Ages Not Caused By CO2”), this paper shows that change in obliquity is the probable trigger for the onset of global warming. Indeed, obliquity also nicely matches the previous ~40,000 year glacial-interglacial cycle that had been dominant prior to ~700,000 years ago.
As we discussed in The Resilient Earth, the major drivers of long-term, medium-term, and short-term cycles all seem to have extraterrestrial origins. Henrik Svensmark's recent paper confirming the linkage between galactic cosmic rays and low level cloud formation provides a mechanism for both long-term climate cycles, those acting over tens of millions of years, and short-term decadal variations. By tracking the water content of low clouds following solar coronal mass ejections a causal link is established. According to Svensmark et al, “a link between the sun, cosmic rays, aerosols, and liquid-water clouds appears to exist on a global scale.”
The Sun's activity moderates cosmic rays, influencing Earth's climate.
That linkage allows cosmoclimatology, as framed by Svensmark and expanded on by Nir Shaviv and Jan Veizer, to explain long term trends as the passage of the solar system through the arms of the Milky Way galaxy. The same mechanism links solar activity on the decadal scale to events like the Medieval Warm Period and the Little Ice Age. Now the cause of middle-term changes, like the glacial-interglacial cycles, can be confidently attributed to changes in Earth's orbital cycles, perhaps with an assist from solar variation. Throw in some random events, like meteor strikes, near by supernovae and volcanic eruptions, and there remains no role for CO2 as a driver of terrestrial climate.
Yet the global warming doomsayers and climate change catastrophists continue to spout their nonsense, threatening the world with plague, famine and destruction. They claim it is time for action and I agree—we need to disband the IPCC and redirect its funding toward solving the world's real problems. Earth's climate is driven primarily by extraterrestrial events: supernovae, cosmic rays and orbital dynamics. It seems the Bard got it wrong—when it comes to global warming the fault is not in ourselves, but in our stars.
Be safe, enjoy the interglacial and stay skeptical.