Guest Post by Wim Röst
Glacial cycles show a gradual diminishing temperature during the slide into the glacial period, but a steep increase of temperature at the start of an interglacial period. As argued here, both ‘ocean upwelling’* and the temperature of the deep ocean might play an important role.
The temperature profiles from interglacial to glacial and the one back into an interglacial is are very unequal. After a short and steep rise of temperatures into the interglacial, there is a much slower and stepwise fall of global temperatures lasting some 100,000 years. It is interesting to consider the role of the oceans in this process. Ocean upwelling and deep-sea absolute temperatures may play important roles.
The unequal temperature profile of a full glacial cycle
As figure 1 shows, after the rapid rise into an interglacial there is a long cooling period. So, how do we explain the following?
- The rapid rise of temperatures at the start of an Interglacial
- The more gradual / stepwise cooling to the lowest temperatures of the glacial cycle
Figure 1: 400,000-year δ18O
temperatures from the Vostok ice core
Thick blue lines are added. Source for the original
For the rapid rise into an interglacial, some possible explanations include the role of obliquity, insolation, albedo and ‘dust.’ All are already well described. The role of the deep sea and the role of ocean ‘upwelling’ are (as far as I know) not mentioned as a main factor. Which is surprising, as both forces are impressive in their potential effects.
The role of ocean upwelling
Upwelling is a massive force. One million cubic kilometres of water yearly rise from the deep ocean to the surface layer. A one-year halving of this upwelling makes sea surface temperatures, world-wide, rise nearly a tenth of a degree (0.09 °C). This is a huge increase. Diminishing wind diminishes upwelling. And very important: wind is quite variable.
Theoretically it is possible that at the deepest point of a glacial a weather pattern develops that diminishes wind, resulting in less upwelling and/or less mixing. As a result, the warm surface layer would rise quickly in temperature, strongly enhancing the ice melt, decreasing the albedo and so on. This could result in an interglacial.
During the interglacial the pattern of ‘less wind / less upwelling’ could have continued and the interglacial could have stayed warm for a while, at least in some cases. As figure 1 shows, some interglacials only last for a short time, others continue for a longer period. Less wind, less upwelling and mixing could have played an important role in the time an interglacial exists. More wind will activate the huge cooling potential of the deep oceans. This might start the definite cooling process of the surface of the oceans, leading to the next glacial.
The role of the deep seas
Our deep seas are ice cold. Of the total volume of the oceans, 95% have a temperature of less than 5°C. The deepest and coldest waters are near zero degree Celsius. With nearly 1.3 billion cubic kilometres of cold water, the oceans have an apparently endless cooling potential. The variable factor in upwelling and mixing is ‘wind’. Uncontrollable by man.
As argued above, there could be an important role for ‘upwelling’ in the explanation of the rapid rise into an interglacial. But what could the role of the ocean be in the cooling down into a deep glacial period?
First of all, during our current interglacial (MIS 1) the temperature of our deep ocean has risen. A rise that seems to be small, but nevertheless considerable, given the enormous mass and heat capacity of the water involved. The deep ocean temperatures, during the Holocene, have risen around two degrees Celsius (see Figure 2).
Figure 2: Deep Sea temperatures in the last 200,000 years
Source Hansen, et al., 2013.
A rise of the deep-sea temperatures of two degrees Celsius means that during the interglacial all upwelling water is two degrees warmer than during the end of the glacial period. The surface layer of the oceans only contains 72.4 million cubic kilometres of relatively warm water. Yearly one million cubic kilometres of the surface layer is ‘refreshed’ with less cold water, this means that the surface layer soon became two degrees Celsius warmer than the surface layer was during the colder glacial upwelling. This is only because of the difference in the temperature of the upwelling water.
The warmer surface layer deep ocean is one of the reasons that the Holocene can last some time. Even as other ‘interglacial promoting factors’ diminish. The cooling down of the deep sea will take time due to the enormous heat content and capacity of the oceans. There will be a big delay. Upwelling waters therefore will only slowly show lower temperatures, affecting the temperature of the surface layer more gradually.
The enormous heat content of the deep ocean is a massive potential cooling factor as well as a massive potential warming, depending on the actual temperature of the deep seas and the average surface temperatures.
At the start of a new glacial the cooling deep oceans cool the surface but slowly because the heat capacity of the oceans is almost a thousand times higher than the atmosphere’s heat capacity and expelling that much heat takes a long time. But, in the case of warming at the start of an interglacial, only the surface layer – no more than 5% of the ocean volume – must be warmed. Therefore, warming can be quick.
The fast rise of surface temperatures at the start of an interglacial suggests that diminished upwelling did play a role.
Upwelling can play a very important role in creating interglacials. A changing weather pattern resulting in less wind results in less upwelling. Since only the thin surface layer of the oceans must be warmed, a diminished inflow of cooling deep water will have huge warming effects on the surface layer.
The observed warming of the deep sea during the Holocene results in a warmer surface layer. The starting temperature of the upwelling water is two degrees higher and raises the final temperature of the surface layer by two degrees.
Because cooling the remaining 95% of the ocean water takes a long time, this (stepwise) cooling during the glacial period will take a very long period.
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About the author: Wim Röst studied human geography in Utrecht, the Netherlands. The above is his personal view. He is not connected to firms or foundations nor is he funded by government(s).
Andy May was so kind to read the original text and improve the English where necessary. Thanks Andy!