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Subsections

3.2 Implementing the experimental method with SimClimat: examples

3.2.1 Role of human activities in the observed recent global warming

The experimental method begins as usual with an observation, a question and a hypothesis.

In the case of the experimental method with numerical modeling, some additional steps are necessary before carrying out the experiments. Then the experimental method continues as usual with experience, result and conclusion.
Figure 11: Screenshot of the results for a pre-industrial simulation with constant $CO_{2}$ concentration (blue) and with anthropogenic emissions that lead to the current $CO_{2}$ concentration (red). The green simulation is identical to the red one, except that the water vapor feedback has been disconnected by keeping the water vapor concentration constant. Note that for the $CO_{2}$ concentration, the green curve is hiden by the red curve.
\includegraphics[width=0.7\textwidth]{figs/Capture_retrovap_eng}

3.2.2 Climate feedbacks at play in the recent global warming

We demonstrate in the previous section that the global warming is caused mainly by the increase in the $CO_{2}$ concentration. Does $CO_{2}$ act directly on the greenhouse effect? Or are there any amplifying feedbacks? We show here how to implement the experimental method with SimClimat to quantify the role of the water vapor feedback.

Similarly, the role of other climate feedbacks can be highlighted by SimClimat. For example, by unplugging the surface albedo feedback, we can see that this feedback is positive but remains rather weak at short time scales. Finally, by unplugging the role of the ocean or vegetation in the carbon cycle, we can see that the increase in temperature is stronger. The concentration of $CO_{2}$ also increases faster. This shows that the ocean and vegetation partially mop up $CO_{2}$ human emissions, by about half.

3.2.3 Mechanisms at play in glacial-interglacial variations

Glacial-interglacial variations are characterized by large variations in temperature, in ice sheet extent, and in sea level, which can be observed in various paleoclimate records
([Masson-Delmotte and Chapellaz, 2002,Masson-Delmotte et al., 2015]). 21,000 years ago, the Earth underwent the last glacial maximum. The overall temperature was 5°C colder, a polar cap covered all of Northern Europe, and the sea level was 130 m lower. For 10,000 years, we have been in an interglacial period. There is an inter-glacial period every 100,000 years (Figure 12).

Figure 12: Variations in temperature and $CO_{2}$ concentration recorded in Vostok ice core in Antarctica.
Image vostok-temp-vs-co2

Here we propose to implement the experimental method in three steps to understand what causes glacial-interglacial variations.

 

Step 1: role of orbital parameters

The same approach can be applied to the other orbital parameters.

Figure 13: Screenshot of the results of a pre-industrial control simulation of 100,000 years (red), with minimal obliquity (blue), with minimal obliquity and constant albedo (green) and with minimal obliquity and the $CO_{2}$ solubility in the ocean that does not depend on temperature (purple). Note that in panels where the green and purple curves are absent, they are actually hidden by the red curve.
\includegraphics[width=1\textwidth]{figs/Capture_glaciaire_eng}

 

Step 2: role of summer insolation in polar regions

The same mechanism applies to other orbital parameters. The obliquity is the easiest parameter to understand: if the polar axis is more inclined, in boreal summer the sun rays hit more perpendicularly the Northern polar regions. It favors the melting of the ice sheet. Precession acts on the season for which the Earth is closest to the sun. Presently, the Earth is closest to the sun in boreal winter. If, on the contrary, the Earth is closer to the sun in boreal summer, then the Northern polar ice sheet receives more energy in summer, which favors its melting. Eccentricity is the most complex parameter because its effect depends on precession. For the present precession where the Earth is furthest from the sun in boreal summer, if the orbit becomes more eccentric, the Earth will be even further away from the sun in summer. The Northern polar ice sheet will then receive less energy in summer which favors its extension.

Note that what is important here is the energy received by the Northern polar ice sheet and not the Southern polar ice sheet (i.e. Antarctica). This is because the Northern polar ice sheet is free to extend over Europe, Siberia, North America. On the contrary, the Southern polar ice sheet is limited to the Antarctic continent and can not extend over the Southern Ocean.

 

Step 3: Why does the $CO_{2}$ concentration decreases during the glacial period?

Air bubbles trapped in ice cores show that changes in $CO_{2}$ concentration co-vary with temperature during glacial-interglacial variations (Figure 12). Why?


next up previous contents
Next: 4 Comparing SimClimat to Up: 3 Implementing the experimental Previous: 3.1 Why do we   Contents
Camille RISI 2023-07-24