Ice age rethink puts warming into context

A 10 cm diameter ice core from Antarctica, drilled down to a depth of 2,250 m, which is more than 150,000 years old. The core is cleaned, measured, and catalogued after drilling. Image: Hans Oerter, Alfred Wegener Institute

A 10 cm diameter ice core from Antarctica, drilled down to a depth of 2,250 m, which is more than 150,000 years old. The core is cleaned, measured, and catalogued after drilling. Image: Hans Oerter, Alfred Wegener Institute

Both the northern and southern hemispheres’ climates – particularly coming into and out of ice ages – may be driven by energy falling directly on them from the sun. That might not sound surprising, but scientists have made just such a claim this week, challenging a fundamental theory of climate held since the 1970s. While it shows climate researchers are willing to question their assumptions, that claim does not undermine the idea that humans are causing climate change, explains Thomas Laepple.

Laepple, a researcher at the Alfred Wegener Institute (AWI) for Polar and Marine Research, Bremerhaven, Germany, makes his claim in relation to when ice ages happen. Our understanding of why they have occurred in the patterns in which they have, known as the glacial-interglacial cycle, is hampered by the mists of time. “There is a significant difference in looking at the glacial-interglacial cycle and the modern climate,” Laepple told Simple Climate. “In the glacial-interglacial cycle we are really digging into the dark, things are built on much less evidence.”

The fundamental understanding that Laepple and his AWI colleagues have challenged is related to the Milankovitch theory, devised by Serbian geophysicist Milutin Milanković in the early 20th century. “He was stating that changes in the orbit of the Earth around the sun are influencing the incoming radiation and this is the pacemaker for glacial-interglacials,” Laepple explained. “There is no doubt about this. The question is just: ‘How does it work?’”

North versus south

The inclination of the axis of the Earth (obliquity) changes in a rhythm of about 41,000 years between 22 and 24.5 degrees. Reeling of the axis of the Earth (precession) changes its direction towards the fixed stars with a rate between 19,000 and 23,000 years. The deviation of Earth's orbit ellipse from a circle (eccentricity) varies with a rhythm of about 100,000 to 400,000 years. These three cycles change the distribution of solar radiation in different seasons. Image: Thomas Laepple, Alfred Wegener Institute

The inclination of the axis of the Earth (obliquity) changes in a rhythm of about 41,000 years between 22 and 24.5 degrees. Reeling of the axis of the Earth (precession) changes its direction towards the fixed stars with a rate between 19,000 and 23,000 years. The deviation of Earth's orbit ellipse from a circle (eccentricity) varies with a rhythm of about 100,000 to 400,000 years. These three cycles change the distribution of solar radiation in different seasons. Image: Thomas Laepple, Alfred Wegener Institute

Earth’s motion in space can be seen in the 24-hour rotation on its axis that gives us our days, and the 365¼ day circuit around the Sun that gives the seasons. However, the orbit of the Earth around the Sun is not stable, but changes in cycles of 20,000-400,000 years.

One of the most frequent variations in Earth’s orbit is precession around the planet’s axis, which is completed once every 21,000 years. Precession means a motion like a spinning top that is beginning to slow down. It changes the Earth’s orientation to the Sun, varying the contrast between the seasons in the different hemispheres. It took until the 1970s for strong evidence to be found to support Milanković’s idea that motions like this influenced climate.

“Hays, Shackleton and Imbrie analyzed sediment cores which contain remains of plankton falling down into mud from the ocean bottoms,” Laepple explained. “In the variations of the fossil plankton they found the rhythms which were predicted by Milankovitch. So this was then a very big breakthrough in climate science.” Imbrie and his colleagues then went on to analyze a range of sediment cores in both hemispheres and map the influence of the Milankovitch cycles.

From this pattern, they concluded that changes in the summer radiation reaching the Arctic in the northern hemisphere are triggering global climate variations. “They were the ones who then said the radiation had to come through the northern hemisphere and influence the climate,” Laepple said. “Our findings now show that Antarctic ice cores cannot be used to confirm this link between north and south.”

Laepple’s challenge is based on a new interpretation of Antarctic temperatures dating back 350,000 years published in top scientific journal Nature on Wednesday. Those temperature measurements come from cylindrical “cores” drilled from the ice that contain compacted snow from far back in history. To do this, they take advantage of the fact that there are different types of each chemical element in nature, known as isotopes.

“What is measured in the ice cores is the ratio between isotopes, heavier water against lighter water,” Laepple explained. “This depends on the temperature during snowfall. Various evidence suggests that where the ice cores are, there is more snow falling in winter and some snow is evaporating in summer.” Consequently, the winter temperatures have a stronger influence on the temperature record than the summer temperature. While this doesn’t strongly affect the overall record, it changes the rhythm related to orbital precession that previously linked Antarctic temperature to the northern hemisphere’s influence.

We know from snow

Detail of an ice core from 2,668 metres depth. Image: Sepp Kipfstuhl, Alfred Wegener Institute

Detail of an ice core from 2,668 metres depth. Image: Sepp Kipfstuhl, Alfred Wegener Institute

Laepple and his colleagues recalculated the effect of changes in radiation from the sun in the Antarctic on the rhythm in the ice core, taking into account the variations in snowfall across the year. “The surprising result is that, accounting for the snowfall variations, the signal coming from local radiation changes matches the ice-core just as well as the northern hemispheric summer insolation,” Laepple said. “The Sun-Earth geometry is creating lots of variations which look the same, so it’s very hard to distinguish where it comes from by just looking at the signal.”

“Because it doesn’t prove that it has to come from the north, it also shows that other theories based on southern hemispheric insolation could be important for glaciation,” Laepple continued. The idea that the southern hemisphere’s climate is influenced by the sun falling there also works better in terms of broader climate science, the German scientist added. “If you apply a modern atmosphere-ocean climate model to a problem, then it works better with the local hypothesis,” he said.

Paleoclimatic changes occur over much longer timescales than the temperature increases the world is currently going through. Nevertheless, Laepple emphasises that how the heat flowing into and out of the planet has changed in these historical periods does provide information relevant to our current situation. “The paleoclimate is telling us that the Earth is very sensitive to small perturbations,” he said. “In the past climate it’s this slow changing in the solar radiation coming via the sun. In the modern climate it’s the human-caused CO2 increase. We can see that they’re having a very strong impact.”

And when Milankovitch theory is taken into account, we should be heading for an ice age, Laepple adds, albeit slowly. “It looks like we are even stopping the change,” he said. “At least if you try and model the orbital forcing against the impact we are having it looks like we could stop the next glacial. For the last million years, we had this regular rhythm. It could be that we just skip the next one. That’s a very uncertain prediction, but it’s what most scientists are thinking.”

One Response to “Ice age rethink puts warming into context”

  1. Climate News and Blog Recap: 2011 03 05 « The Whiteboard Says:

    […] Space & Earth) Ice age rethink puts warming into context Extance @ Simple Climate The Sun has been in the news a lot lately because it’s beginning to […]


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