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Lee1962

Ice Ages

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I'm not sure if this is a weather question, but it's something that's been on my mind for a long time. When ice ages occur do they happen in both hemispheres?

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Depends on the event. On the grand scale, the variations in the orbital mechanics of the Earth as described by Milankovitch cycles works symmetrically in both hemispheres. It makes sense if you think about the annual orbit of the Earth around the Sun. The total insolation at 60°S (for example) will be the same as  it is at 60°N over the course of a year.

 

However, abrupt climate change events with other causes such as the Younger Dryas are different. For the YD, the Southern hemisphere continued to warm while the Northern hemisphere plunged back into glacial conditions.

Edited by Yarmy
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Ice Ages tend to occur when the tilt of the Earth reduces a couple of degrees which leads to summers being markedly cooler as the total insolation near the summer solstice (for each respective hemisphere) then drops by 10% or more, other things being equal. This leads to a situation whereby snow that has fallen in the winter persists over a wide area (because of less incoming insolation) and this snow-cover continues to reflect much of the Sun's heat so that large areas of the sub-arctic and Antarctic remain much cooler. Snowfall the next winter falls on already snow-covered ground and new ice-sheets can build up over land at astonishing speed with the ice-sheets reflecting most of the slightly weaker summer sunshine.  Over the years as the ice-sheets thicken the top of it is exposed to cooler air higher in the atmosphere and this eliminates summer melting further so the ice sheets thicken even more. As the sea-level drops a little (due to so much water locked up in the ice) and cold airmasses flow off the ice into mid-latitudes winter and summer, this cold and the effective higher elevation of mid-latitude land areas means more snowfall in these areas in winter which could persist through the summer. Hence the ice sheet spreads into lower latitudes and even more of the Sun's heat is reflected back into space encouraging further global cooling. This leads rapidly to much more ice and the Earth plunges into a severe Ice Age.

One of the Milankovitch Cycles works to bring about opposite changes in solar input in each hemisphere: The Earth's orbit around the Sun is not totally circular but forms a bit of an ellipse. It means the Earth is closer to the Sun at a certain time of the year but further away six months later. In the past the ellipticity of the Earth's orbit has been such to cause the Solar Constant to vary by up to 25% over a year. Ice Ages happen when the Earth is at the farthest point in the Northern Summer which makes summer sunshine over the entire Northern hemisphere wide area 10% or more weaker and this allows snow that falls in winter (over the Arctic and mountainous areas of North America, Europe and Asia) to persist throughout the summer reflecting the Sun's heat and encouraging cooling and the spread of ice. In winter in the Northern Hemisphere the Earth would be closer to the Sun in this situation but with the Northern Hemisphere tilted away from the Sun in winter the Sun is low in the sky and so the warming effect of being closer to the Sun is mitigated by the fact that the Sun is low and so a 10% increase wont add a lot of heat. In any case a little more warmth in winter in higher latitudes means more moisture in the air which leads to heavier snowfalls where the air is still cold; thus this development encourages the rapid onset of new ice-sheets in high northern latitudes. But in summer, when the Sun is high in the sky and up for many hours each day a 10% drop in the Solar Constant has a massive impact and so the net effect of an elliptical orbit with the Northern Summer at the furthest point is a strong net cooling of high and mid-northern latitudes. The Southern Hemisphere in this situation would have its summer with the Earth closest the Sun and in this situation the Antarctic ice-sheet would start to melt with the Sun being stronger. That said, the Northern Hemisphere with its greater land area and mountain ranges would have snow accumulating on them over a much wider area than the melting going on at the edge of Antarctica- so the net effect of the Northern Hemisphere summer occurring when the Earth is furthest from the Sun in an elliptical orbit is likely to be a sharp global cooling leading to an Ice Age.

Usually though, it is the changing tilt of the Earth that tends to trigger Ice Ages- by weakening the spring and summer sunshine in mid and high latitudes in both hemispheres. Decreasing the tilt of the Earth also means a little more sunshine in winter, so it would (initially) make winters less cold and, it must be said, more able to carry moisture leading to heavier snowfall in areas where the high-latitude land areas still get cold enough: More snow in winter and less melting in summer means rapidly-increasing ice-sheets that reflect away the Suns heat bringing about an Ice Age in the space of as little as 100 years. This happens simultaneously in both hemispheres as Earth's tilt decreases.     

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Nice and concise It has to be said though there remains a number of uncertainties with the Milankovitch theory and research is ongoing.

What Caused the Ice Ages and Other Important Climate Changes Before the Industrial Era?

https://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-6-1.html

 

Edited by knocker
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