Inès H. et Olivia M. ; climate modelling part 2

Inès H. Olivia M.

You can use the little planet on each page to go back to the first page, and access the rest of the "genially" with the summary.

Climate modelling

Part 2

01GHG and Radiative forcing

02positive forcing

03negative forcing (part 1)

04negative forcing (part 2)

The sun emits sun rays (containing heat) towards the Earth. When those reach some surfaces, especially clear or white surfaces such as clouds, ice, sands, deserts, some of it is send back into space, approximately 30%. However, about 70% of those sun rays are absorbed by the ground. Then, the heat brought by the sun rays is re-emitted by the Earth as infrared rays (which also contain heat) in direction of the space. All of this heat would be lost if it wasn't for greenhouse gases. Indeed, when greenhouse gases are emitted they stay above the Earth and surround it. Then, those greenhouse gases stop some of the heat from infrared rays to go into the space, and "change their direction" to make them return on Earth. Greenhouse gases are natural, and, for a long time, they permitted to have a constant temperature on the surface of the planet, a temperature that allowed life to develop. However, in the last decades, anthropogenic activities have caused an increased of the greenhouse gases emissions, which, when surrouding the Earth, return too much heat on the surface of the planet, which then caused an increase of the global temperature. This can be illustrated and explained with radiative forcing, the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change.

Green houses gases and radiative forcing

Radiative forcing is a concept used to quantify a change to the balance of energy flowing through a planetary atmosphere. There are many factors that contribute this change in energy balance, such as concentrations of greenhouse gases and aerosols. Positive radiative forcing means that the Earth recieves more sunlight energy than what radiates to space. In fact, this gain of energy will cause warming. This means that changes that have a warmer effect are called positive radiative forcing. “Positive forcing” is exerted by climatic factors that contribute to the warming of Earth’s surface.

Positive forcing: CO2 fossil fuels or CO2 deforestation

Water has a better thermal capacity than air, about four times higher. Thus, water temperature "evolves" more slowly than air temperature, but that also means that the consequences can be even worse and on a longer term. However, because of its high thermal capacity, seas and oceans are a good "sink" to "absorb" the extra-green house gases emitted by anthropogenic activities. Yet, it doesn't mean that there is no consequences. Indeed, the global sea level is increasing, for two main reasons. The first is that, because of the increase of the air temperature, some ice sheets are melting, and there is an increase of liquid water. The second reason is the thermal expansion of water. When water gets warmer, the molecules that compose it get more "excited", move more, and herefore, take more space. This is what causing the increase of sea level. Since 1880, the global mean sea level have risen about 21-24 centimeters. In 2021, the sea level was 97 milimeters above the 1993 level which is a record in the period 1993-nowadays. The rise of sea level, and the ocean's heat have consequences, first on the fauna and the flora living in the ocean, which are not used to such temperatures. It also have consequences on human habitations, living close to the coast, which can get unhabitable.

Negative forcing : thermal expansion and melting balanced by ocean thermal capacity ?

Picture showing the temperature of different areas and their different amount of vegetations (from IPCC, 5th assesment report, from the Padlet)

As we can see on this picture, the temperature is lower where this is a lot of plants, while the temperature gets warmer in really "urban" areas such as downtown. The presence of plants is really useful, which is why some associations and city councils are trying to develop the vegetation in their towns.

Plants and forests are able to "store" some of the carbon dioxide emited, which means that there is less carbon dioxide going into the atmosphere, and warming the planet. The absorbation if carbon dioxide is permited thanks to the processus of photsynthesis, when the plant absorb carbon dioxide and reject dioxygen. However, this only works when the plant is growing, so when the forest is developping. When the forest is done growing, it will no longer absorb carbon dioxide. Nevertheless, vegetation is a really useful "tool" to slow down climate change, even though it is not the entire solution. Thanks to vegetation and photosynthesis, the global temperature of Earth decrease, or at least, does not increase as fast as it would do without plants. This can be seen at different scales, even local scales.

Negative forcing : vegetation and photosynthesis sink or flux