Doubt Cast on the Origin of Ice Ages?
A new paper, published in Geology, has cast doubt on one of the main causes of Ice Ages. The climate changes which trigger massive glaciations were thought to be caused by cyclic changes in the Earth’s orbit around the sun but now a study of southern hemisphere glaciations suggests this might not be the case.
Today this effect, known properly as ‘Milankovitch cycles‘ are considered to trigger Ice Ages in the Northern Hemisphere. These cycles, named after Milutin Milankovitch, the Serbian astronomer who discovered them, describe the slight wobbles and tilts that affect the Earth as it orbits the sun. These eccentricities affect the intensity of solar radiation which reaches the Earth and the periodicity of the cycles seemed a good match for the climate fluctuations observed in the geological record.
This new study, by an international team of geologists, examined glaciers in New Zealand and measured when they had grown and contracted by dating glacial moraine deposits. The dating method they used measured the amount of the nuclide, beryllium-10, which is formed over time when a rock is struck by cosmic rays. The results showed that the New Zealand glaciers grew large at the same time as those in Canada and Scandinavia. This was during the last Ice Age, 20,000 years ago. Such a result would make sense if the whole world was cold simultaneously, causing the growth of ice sheets across the planet, however, climate affected by Milankovich cycles should show opposite effects on glaciers in the northern and southern hemispheres. This strongly suggests that something else was at work during the last Ice Age.
So if not Milankovitch cycles then what? The study highlighted instead the impact of sea surface temperatures and atmospheric CO2 levels in generating a feedback in the climate. This means that as the temperature falls glaciers grow, which then reflect more sunlight. lowering the temperature further. Feedback like this seems to be common in climate systems and serves to amplify the effect of small initial changes. Findings like this improve our understanding of the climate as a global system and allows scientists to construct more robust models to potentially predict future changes.
Reference: Doughty, A.M., et al. 2015. Mismatch of glacier extent and summer insolation in Southern Hemisphere mid-latitudes. Geology. doi: 10.1130/G36477.1
Featured Image: Portage Glacier, Alaska. Source: Wikimedia Commons