The Book - Chapters 1 to 8
In the first eight chapters of our book Earth, Our Living Planet, we uncover hidden connections between organisms and several unique aspects of Earth: location in Solar System, atmosphere, hospitable environment, long-term climate variations, liquid water, gravitation, natural greenhouse effect, plate tectonics, and magnetic field.
1. Earth is Connected With the Solar System and the Whole Universe
The Living Earth is our home in the Solar System and the Universe. In Chapter 1 of Earth, Our Living Planet, we examine connections of organisms and ecosystems with planet Earth, the Solar System and the whole Universe. For example, most of the Earth water, which is essential to organisms, may have been brought from far away in the Solar System, by impacts between protoplanets during the formation of Earth, and/or later by asteroids and comets. Also, chemical elements such as carbon were forged in faraway stars billions of years before the formation of the Sun and Earth.
The co-evolution of Earth and its organisms and ecosystems created the whole Earth system called Living Earth, which exists since 2.5 billion years, and is marked by the development of global regulations such as that of the oxygen content of the atmosphere. The Living Earth is the last of three phases, the first of which had been the Mineral Earth, which lasted until 4.0 billion years before present and was characterized by the reduction of the huge greenhouse effect of the first atmosphere, and the Cradle Earth phase from 4.0 to 2.5 billion years before present, characterized by the emergence of organisms and the onset of photosynthesis.
Broad temporal divisions in the history of the Outer Universe (including the Solar System) and Earth pertinent to organisms and ecosystems. The geological eons and eras are grouped here in the Mineral, Cradle and Living Earth phases. Ga: billions of years.
2. The Atmosphere is Connected With Astronomical Characteristics of Earth
The Earth's atmosphere is unique among astronomical bodies of the Solar System. In Chapter 2 of Earth, Our Living Planet, we describe the connections of the atmosphere with the Earth’s mass and distance from the Sun. We explain how these two characteristics allowed Earth to acquire a significant atmosphere and retain it since more than 4 billion years.
The formation of a liquid ocean early in Earth's history protected water from being lost to space, which would have happened if water had remained as water vapour in the atmosphere. The presence of carbon dioxide in Earth's atmosphere created a natural greenhouse effect that kept Earth suitably warm for organisms. However, too much atmospheric carbon dioxide hinders organisms and ecosystems, as is the case on planet Venus where the atmosphere is dominated by carbon dioxide and surface temperature is scorching +462 °C.
Mass and distance from the Sun of planetary bodies with an atmosphere or a very rarefied atmosphere called "exosphere". The diagram shows that the presence of an atmosphere on a planetary body is related to a specific combination of its mass and distance from the Sun.
3. Earth Habitability is Connected With the Planet's Motion around the Sun
The habitability of a planet is its potential to provide environments hospitable to organisms. In Chapter 3 of Earth, Our Living Planet, we examine the conditions that make the Earth temperature generally suitable for organisms, thus determining the planet's thermal habitability. These conditions are connected with the Earth’s motion around the Sun, which governs the seasons.
We explain how the Earth's atmosphere and the planet's distance from the Sun contribute to the long-term control of temperature, and how the formation of the Earth-Moon system 4.5 billion years ago and the resulting presence of the Moon likely favoured climatic conditions suitable for organisms. We also describe the effects on organisms of latitudinal, diurnal and seasonal variations in the solar radiation flux reaching the Earth's surface, and the combined effects of the atmospheric and oceanic circulations on the redistribution of heat on the planet.
Radiation budget of the Earth System. Most of the energy that fuels the Earth System comes from the Sun, and the geothermal component from inside Earth is very small.
4. Changing Earth Environments are Connected With Geology and Astronomy
Overall habitability of Earth is the proportion of its surface that is habitable. We explain in Chapter 4 of Earth, Our Living Planet that the Earth's overall habitability is connected with geological and astronomical events and processes. The extent of the habitable environments of Earth varied during the planet's history, but never threatened the presence of organisms. However, there were periods when lower habitability caused mass mortality of species, witnessed by the five mass extinctions of the last 500 million years.
We explain how the Earth's overall habitability is naturally affected by large volcanic eruptions, Ice Ages, and snowball Earth episodes during which the planet was entirely or largely covered by ice during millions of years. We also explain how overall habitability is affected by changes to atmospheric gases originating in human activity (called "anthropogenic"). For example, humans release carbon dioxide about ten times faster than ever in the last 66 million years, causing the global temperature to risen by more than 1 °C since 1900. It is predicted that the present changes will have long-term effects on the overall habitability of the planet.
Shift in the locations of pine trees (upper panels) and non-grass prairie vegetation (lower panels) in North America from 21,000 years ago to present. Coherent pale-blue mass: ice sheet. Increasing colour intensities: increasing concentration of pollen in sediments, which is proportional to the abundance of the species in a given area. Derivative from NASA.
Phase diagram of pure water at pressures lower than 250 atm and temperatures lower than 500 °C. Dots: present pressure and temperature conditions at the surface of Mars (red), Earth (blue) and Venus (orange), and conditions on Earth about 4.4 billion years ago (black). Modified after a figure of Prof. Olivier Dequincey.
Liquid water was essential for the emergence of organisms on Earth almost 4 billion years ago, was a key factor of their takeover of the planet afterwards, and remains essential to their existence nowadays. We describe in Chapter 5 of Earth, Our Living Planet connections of the Earth's water with the outer reaches of the Solar System, from where comets and asteroids carried water in the early history of the planet.
We also describe the characteristics of Earth that contribute to keep most of its water in liquid form. For example, the concentration of atmospheric carbon dioxide reached after the early formation of planet-wide ocean contributed to create a greenhouse effect which has maintained since the temperature of the planet at a level compatible with the presence of liquid water.
5. Earth Liquid Water is Connected With the Solar System
6. The Building Blocks of Organisms are Connected With Gravitation
The theory of general relativity predicts that space-time around a rotating astronomical body would be warped, and twisted by the body’s rotation. The deformed grid around Earth is a visual analogy of the curvature of space-time caused by the presence of the planet. The curved geometry of spacetime is felt as gravity. From Gravity Probe B.
Organisms are made of chemical elements assembled into increasingly complex organic molecules. In Chapter 6 of Earth, Our Living Planet, we explain connections between the building blocks of organisms and gravitation. The latter had major roles in the availability of key biogenic elements in the Universe and the Earth’s environment, and has largely controlled the distribution of chemical elements on the planet since its early history. Gravitation also favours the circulation of chemical elements between different layers of Earth, thus ensuring their long-term availability for ecosystems.
One major component of organisms is their chemical constituents and physical structures (hardware), which includes molecules ranging from very simple (a few atoms) to highly complex DNA (more than one billion atoms). Another major component is the genetic information (software), which is carried by DNA and other nucleic acids. Gravity contributed to the creation of biological hardware and software by contributing to the formation of chemical elements in the Universe and controlling their distribution on Earth.
Main reservoirs of carbon and some of the fluxes of this chemical element in the outer envelopes of the current Earth System.
7. The Natural Greenhouse Effect is Connected With Plate Tectonics
The natural greenhouse effect has kept the Earth temperature suitable for organisms since their appearance on the planet. The suitable temperatures allowed biomasses to build up, and organisms to progressively take over the Earth System. We show in Chapter 7 of Earth, Our Living Planet how the natural greenhouse effect is connected with the Earth geological activity.
Plate tectonics is a very special characteristic of Earth, which has major effects on the long-term functioning of the Earth System. It affects the recycling and sequestration (long-term storage in natural reservoirs) of carbon and other chemical elements used by organisms. The chemical alteration of continental and seafloor silicate rocks is a key process that controls the concentration of atmospheric carbon dioxide in the long-term, and thus the global climate. The latter is also affected by volcanic activity at different timescales. Earth's habitability depends on the long-term natural greenhouse effect.
8. The Long-Term Atmosphere is Connected With the Magnetic Field
The history of the Earth atmosphere began shortly after the formation of the Earth-Moon system, and the composition of the atmosphere underwent profound changes during the following 4.5 billion years. In Chapter 8 of Earth, Our Living Planet, we describe connections of the long-term atmosphere with the Earth magnetic field, and the resulting contribution of the magnetic field to the takeover of Earth by organisms.
Planet Earth and other astronomical bodies in the Solar System, notably the Sun, have magnetic fields. Interactions between the magnetic fields of Earth and the Sun contribute to protect the Earth's atmosphere, and the latter shields the liquid water from loss to space. In addition, organisms on continents are largely protected from direct hits by cosmic rays because the magnetic field both shields the atmosphere from most cosmic rays, and protects the atmosphere whose thickness absorbs most of the cosmic radiation that passes the shield.
Polar lights above Bear Lake, Alaska, U.S.A., in January 2005. This phenomenon results from interactions between the solar wind and the Earth’s magnetic field over polar areas. Wikimedia.