The atmosphere, a fragile balance
The atmosphere is the layer of gas, a few tens of kilometers thick, that surrounds our planet. It appeared 4.5 billion years ago at the time the Earth was formed, and has undergone significant changes since. Its composition has only been stable for around 300 million years.
Climate is the term used for all atmospheric conditions – like temperature, humidity, pressure – observed over a long period of time and in a given zone. Climate is the result of complex interactions between solar radiation, and the atmosphere and the Earth’s surface temperature, all forms of water (water vapor, clouds, rain, oceans, glaciers, etc.) and the biosphere (especially flora, fauna and human activities).
The average temperature at the surface of the Earth: a complex variable
The average temperature at the surface of the Earth has always varied as testified by ice core samples from polar ice. The causes are multiple some of which are cyclical and with very different periodicities, following space mechanics, when others are strongly linked to human activity.
Thus, different studies reveal the variations in the Earth’s inclination between 22.1 and 24.5 degrees over periods of 41,000 years and following some continent displacements. Also, the Earth’s elliptical trajectory around the sun varies according to cycles of periods between 90,000 and 100,000 years. Finally the orientation and inclination of the Earth’s axis (precession motion similar to that of a top) evolve over time with a periodicity to order of 22,000 years. These three factors do not have the same periodicity, but they combine in a complex manner and their effects add to or compensate according to the period.
In addition, the atmosphere’s composition strongly interacts with these various positions: this results in what is commonly known as the "greenhouse effect".
The greenhouse effect is a natural mechanism that existed well before the appearance of life. The Earth receives the energy of the sun in the form of radiation. The atmosphere acts like the glass pane of a greenhouse: only part of total solar radiation passes through the atmosphere (on average 235 W/m2), the rest of it is reflected towards space. The sun’s rays that pass through reflect off the Earth’s surface (deserts, glaciers, etc.), or are absorbed by the earth and oceans. They warm up and, in turn, emit radiation: infrared radiation, part of which the atmosphere sends back. The atmosphere thus first absorbs part of the radiation from the Earth, releasing some of this towards space, and re-emitting the other part towards the ground, which contributes to the warming of the lower layers of the atmosphere. It is a complex mechanism, in which the radiation "rebounds" onto particles of certain gases that make up the atmosphere.
Illustration Challenge Bibendum booklet « More air » page 7
Greenhouse gases (GHG)
Not all the gases in the atmosphere are active in the greenhouse effect. Nitrogen and oxygen, which make up almost 100 % of the atmosphere, have nothing to do with it. It is a small minority of gases, around 1 %, that are active.
The biggest natural greenhouse gas is water vapor (around 0.4 % of the atmosphere), which alone is responsible for around 60 % of the reflection of radiation from the atmosphere towards the Earth. Water vapor emissions originating in evaporation from the ground and transpiration from plants can be increased under the effect of climate warming but without a significant contribution to the greenhouse effect because these phenomena are localized in the lower levels of the atmosphere. Other gases that are naturally present in the atmosphere, representing less than 0.1 % of the atmosphere, also contribute to the greenhouse effect. In this 0.1 %, particularly active gases are to be found: carbon dioxide (CO2) weighs in at 25 %, ozone (O3) at 8 %, methane (CH4) and nitrous oxide (N2O) 6 %.
Anthropogenic greenhouse gases, those due to human activity, amplify the natural greenhouse effect, be it CO2, methane or fluorinated gases of industrial origin (PFC, HFC and SF6 formulae).
Illustration Challenge Bibendum booklet « More air » page 8 (anthropogeneous greenhouse gas)
The different greenhouse gases have neither the same lifespan in the atmosphere, nor the same warming power calculated per tonne. The "CO2 equivalent tonne" is used as a standard measure of the global warming potential of greenhouse gases as established over a reference period of 100 years. Methane, then, has a global warming potential 25 times higher than carbon dioxide.
The global anthropogenic emissions of all gases combined stood at 44 billion tonnes CO2-eq in 2005, on average 7 tonnes CO2-eq per inhabitant with a great disparity from country to country.
Temperature and CO2 concentration: strong correlation and positive retroaction
The average temperature at the surface of the Earth is the outcome of a balance between the incoming energy and the energy re-emitted. Without the greenhouse effect this temperature would be – 19°C. Today it is close to +14 °C.
CO2 is the gas with the lowest greenhouse warming potential per tonne. However, the quantities of gas emitted and the lifespan of these emissions make it the top contributor to the increase in the greenhouse effect in industrial times.
The ice core samples provide indications of surface temperature and the concentration of CO2 in the small air bubbles imprisoned in the ice as the snow accumulates, and it is possible to observe a strong correlation between the Earth’s average surface temperature and the concentration of CO2. Over the last 425,000 years, the concentration of CO2 has evolved from 180 to 280 ppm (parts per million).
Image created using data from NOAA
During the last 425,000 years, cold periods have coincided with periods during which the concentration of CO2 in the atmosphere was lower. When the atmosphere contains less CO2, the greenhouse effect diminishes and the planet cools down.
In addition, the phenomenon is cumulative: the higher the levels of CO2, the more the temperature of the oceans (which stock 85 % of the CO2) increases and the more CO2 they release into the atmosphere…this is a positive retroaction.
Climate warming since the industrial revolution
The increase in anthropogenic emissions, started during the Industrial Revolution, has been accelerating for over a century. The concentration of CO2 during 2013 has reached 400 ppm.
The phenomenon of warming the atmosphere has been observed since the beginning of the "western industrial era" and has simultaneously amplified over the last forty years. The part of anthropogenic greenhouse gases increases as the population decouples. In a century and a half, the global population has gone from 500 to 600 million to over 7 billion.
The average global temperature has increased by approximately 0.06°C per decade since 1880, i.e. a warming of 0.6°C in a century. Over the last 25 years, the trend is +0.23°C per decade.
Illustration Challenge Bibendum booklet « More air » page 8 (Trend in temperatures since the industrial revolution)
The last document published by the Intergovernmental Panel on Climate Change (IPCC) confirms this.
What future for the planet?
It is the speed of current warming which is causing concern and which raises the question of the capacity of all the ecosystems to adapt. The planet has evidently the means to achieve its former equilibrium but with a constant of time much higher than those of the evolutions provoked by industrial activity for over a century and a half.
Finding efficient actions to take action in the face of climate change is today, more than ever, an imperative.