Thermal balance of the planet
The Earth's average temperature is 14° C, and its temporal and geographic variations are relatively low when compared with other known planets. Such conditions are highly favourable for life.
The Earth's atmosphere: the key to its balance
Our atmosphere is a gaseous mixture that is subject to the forces of gravity, just like Newton's apple, which prevents it from spreading out towards outer space. It began to form over 4 billion years ago. At that time, gases escaped from the Earth's mantle during its cooling: nitrogen, carbon dioxide (CO2), methane, water vapour... The water vapour gradually condensed and formed the oceans. 3 billion years ago, oxygen was still scarce, unlike CO2. The emergence of both life and photosynthesis helped accelerate the production of oxygen (O2). The atmosphere's composition eventually stabilised 300 million years ago. On a planetary scale, our atmosphere is very thin, barely 50 km. thick, with the first 10 km. holding 70% of the atmosphere's gases (1). It consists of dry air mixed with water vapour (0 to 4%). Dry air is made up of dinitrogen (N2 - 78%), O2 (21%) and other gases (1%).
The sun plays a key role in the thermal equilibrium of our planet: it provides an external energy to the Earth's system. Without it, the Earth's temperature would have gradually declined, turning the blue planet into a frozen boulder. Thanks to the sun, the Earth's temperature remains relatively constant. There is a balance between the solar radiation that is received (incoming energy) and the energy that is reflected from the ground towards outer space (outgoing energy). Some solar radiation is directly reflected, while the other part is absorbed and then re-emitted by the Earth, mainly in the form of infrared (IR) rays.
The greenhouse effect is the property of our atmosphere that captures a portion of the infrared radiation emitted by the Earth and redirects it towards the ground. Part of the heat is thereby preserved. The temperature near the ground (an average of 14° C on the Earth) is higher than it would be in the absence of greenhouse gases (-19° C (1)). Greenhouse gas (GHG) emissions confer this property to the atmosphere: their molecular structure makes them "opaque" to IR radiation. These greenhouse gases represent only 0.04% of dry air. The greenhouse effect is primarily created by water vapour (60% of the reflection towards the ground). Next is CO2 (25%), ozone (8%), methane and nitrous oxide (6%) (1). Non-gaseous particles can also affect the radiation passing through the atmosphere (ex: clouds, which are comprised of ice and water droplets).
Solar energy, terrestrial radiation and the greenhouse effect caused by the atmosphere therefore dictate the Earth's surface temperatures.
The different types of climatic variations
We spoke earlier of the Earth's average temperature (14° C). However, temperatures, as well as all atmospheric parameters (humidity, pressure, precipitation...), vary from one point of the globe to another. By "climate", we mean all of the atmospheric conditions that we observe over a long period of time in a given region. In addition to the sun and the atmosphere, climate is influenced by many factors. The presence of water, for example, is crucial: the ocean has a thermal inertia that is greater than that which is found on land, which explains why coastal areas have lower temperature ranges; water in the form of ice reflects much of the sun's rays, thereby limiting rising temperatures in the atmosphere. As for ocean currents (Gulf Stream, El Niño...), they are the source of significant energy transfers. The Earth's rotation, which drives atmospheric circulation from west to east, also has an influence on the climate.
Climates include annual variations, seasons, which are primarily caused by the inclination of the Earth's rotation axis with regards to its orbit around the sun. This tilted axis causes fluctuations in sunshine duration and in the angle of incidence of the sun's rays. However, the seasons have little influence on the Earth's average temperatures (when summer reigns in the northern hemisphere, it is winter in the south, and vice versa). Furthermore, weather conditions can vary over short scales of time and space (clouds, winds, anticyclones...). The study and prediction of these variations is called meteorology.
Paleoclimatology studies the evolution of climate during past eras, thousands, even millions of years ago. This science has shown that our planet regularly experiences ice ages, lasting an average of 100,000 years, interspersed with eras of temperate climate lasting approximately 30,000 years. This phenomenon is probably explained by variations in the Earth's orbit and in solar activity. It has also been found that there is a strong correlation between the Earth's average temperature and the concentration of greenhouse gases.
A shaky balance…
The Earth's balanced climate is valuable, and, in any case, it is probably quite rare, as it has never been observed by humans on another planet. Venus and Mars have atmospheres that are 95% CO2. Respectively, their average temperatures are of 460° C and -63° C. Mercury, which has no atmosphere at all, has a temperature that ranges from -180° C to +425° C.
The Earth's climate balance is derived from a combination of many events. The distance from the sun provides an ideal energy intake. The proportion of oceans, continents and ice areas provides a balance between the radiation that is reflected, absorbed and reemitted. The greenhouse effect and the temperature control that is provided by the atmosphere ensure of temperatures that are well-suited for life, allowing us to benefit from water in its solid, liquid and gaseous states.
The Earth's climate history, the study of other planets, the observation of the number of parameters that are involved... All of these elements help us sense the fragility of our planet's thermal equilibrium. That being said, how can we not be worried after reading the IPCC's conclusions (2)? Namely, the IPCC discovered the following: the earth's temperature is increasing at a rate never before seen; the atmospheric CO2 concentration has increased by 35% since 1750, reaching a level not seen in at least 650,000 years... The conclusion that is reached from their climate research is unequivocal: it is high time to do something.
Written by Florent Planas for "One year for the Planet" (translated by Anyword).
Find out more…
- Raphaël Trotignon, Comprendre le réchauffement climatique (Pearson, 2009)
- Intergovernmental Panel on Climate Change (IPCC), 4th assessment report (2007)
- Fondation Nicolas Hulot, Ecologuide de A à Z (Le Cherche Midi, 2004)
- If you're looking for answers to all of your greenhouse effect questions, add Jean-Marc Jancovici's excellent website to your favourites, www.manicore.com. It's a leading reference on the subject!
