“You are looking for the strings that hold it, some point of support, and they do not exist. You look at the Earth and around, the darkest darkness that man can imagine.”(Eugène Cernan, Apollo 17 mission).
This December 7, 1972, 29,000 km from Earth, the Apollo 17 astronauts had the Sun behind them when they took this photo, the first – and one of the few – to show a fully lit Earth. The trajectory of their mission allowed them to photograph at the same time the entire South Pole.
They named this photo “The Blue Marble”, in reference to the glass ball of our children’s games …
Only a few dozen kilometers of atmosphere surround our blue “ball”. Quite quickly, as one rises above the Earth’s surface, the effects of gravity diminish, the gas concentration (kilograms of gas per cubic meter of atmosphere) decreases.
To simplify, we usually consider that 9/10 of the atmospheric mass is between 0 and 16 km above sea level (area we usually call the troposphere). It is in this layer that most of the meteorological phenomena take place (cloud formation, etc.). The temperature of the troposphere is not constant: it decreases on average by 0.5 ° C when one rises by 100 m. At the top of the troposphere, the temperature is around -50 ° C.
The second layer, which is therefore between 10-15 km and about 50 km above sea level is called the stratosphere. The temperature gradually increases with altitude, up to around 0 ° C at 50 km altitude.
Beyond that, the concentration of gas in the atmosphere becomes increasingly negligible (although the gases do not really escape terrestrial attraction until above 400 km in altitude)
The main so-called “greenhouse gases” are found in these first two layers of atmosphere:
- water vapor (H2O) which is a “natural” greenhouse gas is present mainly in the troposphere;
- laughing gas (N2O), and chlorofluorocarbons (CFCs) accumulate in the troposphere, and are destroyed by ultraviolet solar radiation when they pass through the stratosphere;
- carbon dioxide (CO2) and methane (CH4) are present in both the troposphere and the stratosphere; CO2 is destroyed above an altitude of 70 km by ultraviolet radiation emitted by the sun;
- ozone, which is a little different, is 90% in the stratosphere. The remaining part, produced in the troposphere plays a significant role in greenhouse gases.
The atmosphere is relatively transparent for the part of the solar radiation that corresponds to the visible range, and it is relatively opaque for the ultraviolet as well as for long wavelength (infrared) radiation.
Visible radiation therefore fairly easily passes through the atmosphere and is partly absorbed by the earth’s surface. This energy supply causes the surface to heat up.
Depending on its temperature, the surface then emits energy towards space, in the form of infrared radiation. However, this is partly absorbed by greenhouse gases, which in turn emit infrared radiation to the surface. Thus, visible solar radiation that has reached the surface through the atmosphere is “transformed” into infrared radiation and cannot be returned to space, since it is absorbed by greenhouse gases.
The energy thus trapped is transformed into heat, which contributes to the warming of the lower layers of the atmosphere (the troposphere), where greenhouse gases are most concentrated.
For two hundred years, human activity has generated an exponential growth in greenhouse gas emissions from land:
source: IPCC Fourth Assessment report, 2007
For 40 years, scientists have alerted us. Our children now question us, and at the same time question our model …
AND WE, WHAT DO WE DO?
Iconography: “The Blue Marble”, Apollo 17 mission, December 7, 1972 © NASA