Yes, by now our model has more than 100,000 lines of source code. For example: Suppose you want to include an ice sheet subcomponent that allows ice to grow on land. We must remember that we have already included terrestrial vegetation dynamics (vegetation that grows and can interact with climate) and a carbon cycle. We have to ensure that the various modules interact because if ice grows where trees were: What happens to the carbon? Does it go to the atmosphere? Does it get stored in the soils? You see, as you add more and more complicated processes, you also have to worry about their interaction with existing processes. And when you do that you have to go right back to the drawing board again.
When people want to know about the past state of the oceans, which sources of information can be used? For investigating past temperatures on land, people make use of tree rings and other proxydata that are believed to contain temperature information. What can you use for the oceans?
There are a number of techniques that are used. There are tiny critters (e.g. foraminifera) that live in the upper ocean and other different levels of the water column. And as these creatures grow, they take into their shells the chemical properties of the environment around them. When they die they end up in the sediments of the deep ocean. Year after year more material is added to the sediments. So you have the remains of these little critters in the sediments and you can measure the chemical properties of their shells. From that you can infer the conditions at the time when they lived.
For example, oxygen isotopes are a classic indicator: Calcium carbonate is the building block of the shells of many critters. There is a fractionation that occurs when the shells grow. What this means is that depending on the temperature, the amount of 18O vs. 16O that is taken up during the formation of the calcium carbonate shells changes. This can give you information about past temperatures of the ocean. And there are other examples of these isotopes that are used to infer past environmental properties.
So the main information you can get from this analysis are the temperature at earlier times?
You can also look at proxies for the amount of ice on land. If you have a lot of ice on land you have very 18O—enriched water left in the sea itself so the sediment should be enriched in 18O. Another tracer that can be detected is salinity. So you can infer various things in different regions.
Let us talk about the carbon cycle. There is large amount of CO2 stored in the oceans. How large is this amount, compared to the CO2 in the atmosphere or on land?
There is the CO2 on land (I will use gigatons (Gt) of carbon as unit), which is about 2,300 Gt. Then you have the carbon stored in all the fossil fuels we know about today; that would be 3,700 Gt. The intermediate and deep ocean contain about 37,000 Gt. And the surface sediments have another 150 Gt in them. The ocean surface contains another 900 Gt. When you compare the atmosphere that hosts about 595 Gt—that is about two orders of magnitude less than the ocean.
In rough numbers, the ocean has about 40,000 Gt of carbon and the atmosphere has about 600 Gt.
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