Climate cycles are continuously happening on Earth since its beginning estimated to be some 4.5 billion years ago. Detailing the earliest climate cycles can only be done by painting with the broadest of brushes. In more recent times, proxies, such as ice cores and oxygen isotope measurements are available for use in reconstructing these cycles. The globe has experienced glaciers in relatively recent times and in the not too distant future will surely experience glaciers again. The profound changes that take place resulting in glacial periods are of great interest.
Glacial periods are made up of two segments,—-glacial cooling and interglacial warming. Do we know what causes the glacial periods? Milankovic cycles are probably the most agreement upon cause. The Milankovic cycles are predicated upon changes in the Earth’s elliptical orbit, axial tilt and precession. However, others feel that alteration to the Sun’s Great Conveyor Belt plasma currents (and thus the Sun’s activity) are caused by the solar system’s barycenter being moved by the large gas planets’ angular momentum. There are more theories than just these two. The causes are controversial and discussion of them are for a future posting. For now, glacial periods will be the topic.
Chart courtesy of Robert A Rohde and Wiki Commons
According to this chart, Earth has gotten colder and the Antarctic has been glaciated for the past 13 million years. The Zachos proxies were calibrated against the Vostok ice cores. Zero shown on the chart is the current global temperature.
Significant growth of ice sheets did not begin in Greenland and North America until approximately 3 million years ago, following the formation of the Isthmus of Panama by continental drift. This ushered in an era of rapidly cycling glacials and interglacials.
About 1.2 million years ago, the glacial period (glacial cooling and interglacial warming) became approximately 100,000 years in length. Roughly speaking the glacial cooling happens for 80,000 years and interglacial for 20,000 years. For the current glacial period, the interglacial has existed for 11,500 years. That is a red flag raiser isn’t it. Ok, it’s a long way off, and the time is never exactly 20,000 years, but the history suggests that if the human race makes it for the next 8,500 years or so, there is going to be a very cold globe in their future.
This chart shows the nominal 80,000 cold and 20,000 warm years.
NOAA describes the features of this chart as follows:
Variations in the Earth’s orbit through time have changed the amount of solar radiation received by the Earth in each season. Interglacial periods, shown as the periods of higher temperature (shaded in yellow) in the Dome Fuji ice core from Antarctica, tend to happen during times of more intense summer solar radiation in the Northern Hemisphere. Solar radiation varies smoothly through time with a strong cyclicity of about 23,000 yrs as seen in this time-series of July incoming solar radiation at 65degreesN. Interglacial periods tend to occur during periods of peak solar radiation in the Northern Hemisphere summer.
Notice the asymmetric shape of the Dome Fuji temperature record, with abrupt warmings shown in yellow preceding more gradual cooling. Warming at the end of glacial periods tends to happen more abruptly than the increase in solar insolation.
The chart suggests that the cooling dropped rather rapidly in the early part of the glacial period followed by a steady but less abrupt decline in temperature.
As you will hear repeated in the next posting is that glacial periods seem to begin when the summers are too cool to melt all the ice that builds up each winter in the Northern Hemisphere. So perhaps the fact that the Arctic Sea ice is declining is sign that the globe in not yet ready to begin a glacial period.
The history of mankind has been written during the interglacial period we now are enjoying. But the globe averages many more cold years than it does warm years. Mankind must have ridden out a number of theses glacial periods. But there probably were not 9 billion people sharing the Earth when the last glacial period began.
***Some comments on oxygen isotope proxies: The temperature proxy for the chart used in this posting is the ratio of the two stable isotopes of oxygen O16 and O18. Calcium carbonate samples taken from the oceans’ bottom were tested to determine each isotopes content. The ratio of 18 to 16 is then calculated A water molecule containing 18 is heavier than a water molecule formed by 16. Also, 16 formed seawater evaporates more readily than 18. Consequently, the ocean surface contains greater amounts of 18 around the subtropics and tropics where there is more evaporation, and lesser amounts of 18 in the mid-latitudes where it rains more. The water vapor gradient heading from the tropics to the poles gradually becomes more and more depleted of 18. Snow falling in Canada has much less 18O than rain in Florida; similarly, snow falling in the center of ice sheets has a lighter 18/16 signature than that at its margins, since heavier 18 precipitates first. Changes in climate that alter global patterns of evaporation and precipitation therefore change the background 18/16 ratio.