What really warms up the Earth ?

The Earth works like any hot body that cools down to stay in balance: it has radiation (we talk about a black body). It is on this same principle that the nocturnal binoculars are based: they capture the infrared radiation emitted by hot bodies (human or animal) and make them “visible”.

The heat that remains on Earth corresponds to the difference between what goes in minus what comes out. Studies that focus on global warming try to quantify how each variable affects these two fluxes.

Radiative forcing quantifies the difference between the amount of energy coming in minus the amount going out. A phenomenon that has a positive radiative forcing simply means that the amount of energy left on Earth is greater than without this same phenomenon. Conversely, a negative radiative forcing corresponds to a phenomenon that decreases the heat in the atmosphere.

Be careful, aerosols have nothing to do with spray/aerosol cans – which are often complex chemical compounds. Here, an aerosol is just a small particle, of dust for example, too light to fall. “Good” for the climate but “bad” for health, they perfectly illustrate the need to conceive ecology as a set of themes that sometimes oppose each other.

The best known aerosols are those released into the atmosphere by volcanic eruptions. This dust forms a layer that reflects light rays and cools the Earth: we speak of volcanic winter. This drop in temperatures is often short-lived – and past examples show us that if this period lasts too long, many plants do not survive due to the low amount of light.

Apart from the melting of glaciers, many phenomena come into play when it comes to looking at the albedo on Earth, for example:

• Deforestation converts dark surfaces to light surfaces
• Soot is a residue from the combustion of fossil fuels and biomass. It settles on snow, increasing dark surfaces

Some of these elements are grouped under the name: short-lived gases, because of their short residence time in the atmosphere. This is the case for nitrogen oxides (better known as NOx, atmospheric pollutants), carbon monoxide (CO) and volatile organic compounds.

Note: Volcanic eruptions are counted as aerosols. They are added here to simplify the main message of these explanations.

We do not consider the total activity of the sun – because all the energy comes to us from it – but the variation compared to a reference value. These solar cycles last from 9 to 15 years. They have long been used to fuel climatosceptic arguments. Nevertheless, the latter predicted a decrease in temperature in the mid-2000s (due to the downward phase of the cycle) – which did not happen.

In this Figure (taken from the IPCC AR6 report – Figure SPM2), the Radiative Forcing has been converted into temperature for simplicity.