Skip to content

Figuring Out Ice Ages

Doug L Hoffman, The Resilient Earth

Science is far from figuring out ice ages and Earth’s climate in general.

A lot has been written about melting ice caps and new mini-ice ages recently. Seems that science can’t decide if we are going to drown in rising oceans or starve because summer will be a thing of the past. This leaves the layperson justifiably confused as to who to believe—the climate change alarmists who back rapid global warming or those who warn of a new glacial period.There is little certainty when it comes to science but one thing that can be counted on is our ignorance. Quite simply, scientists cannot predict with any certainty what Earth’s climate will do next. If someone tries to tell you different they are lying.

For over a million years, Earth’s climate has been cycling between interglacial periods, like the current temperate Holocene, and glacial periods, commonly referred to as ice ages. Scientists have devise a number of ingenious ways to figure out the historical timing of these alternating bouts of warm and cold: biogenic carbonate from the skeletal remains of marine organisms; ice-rafted-debris (IRD) in North Atlantic sediments; changes in loess grain-size in lake sediments; land-based pollen records and other proxies. In particular, benthic δ18O records are an indicator of the ice-volume signals from the both hemispheres.

The ratios in which the two stable isotopes of oxygen (16O and 18O) are precipitated in carbonates and phosphates depends upon the oxygen isotopic composition of the fluid from which the mineral precipitated and also on the temperature at which this took place. Marine δ18O records from benthic foraminifera show that the growth and reduction of global ice volume has exhibited quasi-periods of 100,000 since the Mid-Pleistocene transitions of 1200−800 kyr ago. One of the primary ways of identifying past changes in global climate is using δ18O to demarcate what are called Marine Isotope Stages (MIS). We are currently in MIS 1, know also as the Holocene, and the last glaciation is represented by MIS 2.

Not all interglacials are the same, they can vary in length and in temperature. Nicholas Shackleton, a pioneer in the field, was the first to correctly identify MIS 5e within the Eemian interglacial using land-based pollen records. At that time, approximately 125,000 years ago, the isotopic composition of the ocean indicated there was even less ice on the continents than there is today. Indications are that sea-levels were ~6m higher than today, largely because the Greenland ice sheet was greatly diminished. As can be seen from this information, there is nothing “unprecedented” about today’s temperatures or sea-levels and the effects of a warmer climate was far from “irreversible.” It goes without saying that there were few coal plants and SUVs 125,000 years ago.

The last glacial period peaked roughly 22 kya and by most estimates we have been in an interglacial for 14,000 years. Normal interglacials generally span two or three axial precession cycles with a maximum durations of ~60,000 years. Indeed, the temperature record of the last 800 kyr from the EPICA ice core shows a strikingly consistent sequence. There have been some major variations in this celestially driven parade of ice ages and warm spells, one of which is investigated in a new paper published in Nature. In “Extra-long interglacial in Northern Hemisphere during MISs 15-13 arising from limited extent of Arctic ice sheets in glacial MIS 14,” Qingzhen Hao et al, document an unusually long stretch of warm climatic conditions that occurred half a million years ago. Here is the abstract:

Knowledge of the behavior of Northern Hemisphere (NH) ice sheets over the past million years is crucial for understanding the role of orbitally driven insolation changes on glacial/interglacial cycles. Here, based on the demonstrable link between changes in Chinese loess grain-size and NH ice-sheet extent, we use loess grain-size records to confirm that northern ice-sheets were restricted during marine oxygen isotope stage (MIS) 14. Thus, an unusually long NH interglacial climate of over 100 kyr persisted during MISs 15−13, much longer than expected from marine oxygen isotope records. Taking a global view of the paleoclimate records, MIS 14 inception seems to be a response to changes in Antarctic ice-sheets rather than to NH cooling. Orbital configuration in the two Polar regions shows that the onset of MIS 14 was forced by austral insolation changes, rather than by boreal summer insolation, as Milankovitch theory proposes. Our analysis of MIS 14 raises the possibility that southern insolation forcing may have played an important role in the inception of several other glacials. We suggest that the extra-long NH interglacial climate during MISs 15−13 provided favorable conditions for the second major dispersal episode of African hominins into Eurasia.

In many ways, MIS 14 was an anomalous glacial period. “In the MIS sequence, MIS 14 stands out as a short and mild glacial epoch in many records,” the authors report. “However, records from the Northern Hemisphere suggest that MIS 14 was a much warmer glacial period than other glacial epochs in the last 800 kyr.”

The researchers correlated a number of different proxy measurements to try and isolate the cause(s) of this atypical period in the climate record. These data are shown in the figure below:

“The overall mild NH climate in MIS 14, with strong hemispheric asymmetry, is linked with both external forcing and internal processes within the Earth climate system,” they state. The external forcing comes from changes in insolation—energy received from the Sun—and the internal processes are those feedbacks that are part of Earth’s climate system—albedo change due to snow cover, thermohaline circulation, and such. Their conclusions strengthen orbital forcing, the so called Milankovitch Cycles, as the primary trigger for glacial/interglacial climate change. Further more, they also conclude that things are not quite as simple as they seem when it comes to the intensity and duration of such climate cycles.

Climate is sensitive to both the total amount of solar radiation falling onto Earth’s surface, and the latitudinal and seasonal distribution of that insolation. As the authors point out, the impact of the precession and eccentricity cycles on hemispheric insolation need to be viewed together. During MIS 14, Earth’s orbital cycles conspired to create a strong imbalance between the northern and southern Hemispheres, with the Southern Hemisphere strongly biased toward colder conditions.

Today, Earth’s orbit is slightly elliptical, with perihelion around the boreal (northern) winter solstice. This implies that it is at aphelion around the boreal summer solstice. When the orbit approaches a circle, these distance differences would have negligible effects. However, since some eccentricity applies, the solar radiation on illuminated places of the globe will be somewhat more intense in boreal winter (austral summer) than in boreal summer (austral winter). Effectively, this weakens the northern hemisphere’s seasonal contrast, whereas that on the southern hemisphere is strengthened. Similar to the conditions during MIS 13-15 but much less extreme. Such conditions are not conclusive but are a strong indication of things to come.

It has long been accepted that the glacial-interglacial cycles were synchronous in the Northern and Southern Hemispheres, triggered by summer insolation at high northern latitudes as proposed by Milankovitch. However, the key physical mechanisms are far from well understood. To understand the relationship of insolation and the glacial cycles, numerous investigations have focused on the orbital configuration or the insolation around glacial terminations because the magnitude and abruptness of changes at the terminations facilitate accurate identification of climate transitions. In contrast, the relationship between glacial inception and regional insolation forcing is less clear due to the gradual changes in the marine δ18O records at the interglacial-to-glacial transitions, as, for example, during marine oxygen isotope stage (MIS) 11/10 and MIS 9/8, although the timing of the two recent transitions at MIS 7/6 and 5/4 appears to be consistent with Northern Hemisphere forcing. Deciphering MIS 14 with strong hemispheric asymmetry decreases the uncertainty in the correlation of orbital forcing and gradual climate changes during glacial inception, and provides a convincing pointer to the pattern of insolation responsible for glacial inception.

What are we to take from this latest scientific work? First is that science is an iterative process, where yesterday’s answers are constantly refined and modified as new information comes to light. Second, what has been long known as the proximate cause for glacial/interglacial changes—namely orbital variation—has not been eclipsed by things so tenuous as trace levels of atmospheric greenhouse gases. To claim CO2 causes climate change is like blaming a whiff of smoke in the air for burning down your house.

Full post