Probably the most hospitable environment for human beings in the Earth's long history should be one in the ages from about 200,000 years ago, when Homo sapiens were believed to appear on this planet for the first time, to the present date. Human beings should thank God for a temperate environment during this period.
The primordial atmosphere when the Earth was created about 4.6 billion years ago consisted of hydrogen and helium, of which temperature and pressure were as high as those of the Sun. It was blown away by the intense solar wind from the primitive Sun. This process lasted tens of millions of years. The solar wind was then weakened gradually, but part of the Earth's atmosphere is still being blown away today. In fact, the Earth-derived oxygen was found on the Moon lately. The surface temperature of the Earth then declined, leading to the formation of the crust. The crustal formation induced intense volcanic activities on the Earth, which released a large amount of carbon dioxide and ammonia into the atmosphere. The primordial atmosphere in those times contained a large amount of water vapor as well.
Between 4.1 billion and 3.8 billion years ago, a number of small asteroids and other astronomical objects bombarded the planets in the Solar System including the Earth. Craters found on the surface of the Moon is the evidence of the asteroid impact in those times. The surface temperature of the Earth rapidly dropped 4 billion years ago, which prompted highly concentrated atmospheric water vapor to condense, creating the primordial sea. The origin of life dates back to this time. The first photosynthetic organisms emerged 3.2 billion years ago. They flourished 2.7 billion years ago, supplying oxygen into the sea water. The dissolved oxygen in the sea water began to be released into the atmosphere around 2 billion-plus years ago. This released atmospheric oxygen reacted with ultraviolet radiation, creating the ozone layer. As the concentration of atmospheric oxygen increased, the ozone layer rose in altitude, eventually reaching the stratosphere, where the layer has ever since protected the Earth surface from DNA-damaging ultraviolet radiation by reducing the amount reaching the said surface. The Earth completed preparation of the land before being colonized by terrestrial organisms, which were former aquatic organisms adapted for a lifestyle on land.
The Earth experienced the ice ages several times between 800 and 650 million years ago, which froze the entire Earth. The concentration of oxygen in the atmosphere reached today's amount (about 21%) 600 million years ago. The Earth also experienced the ice age between 460 million and 430 million years ago, causing a mass extension of life toward the end of this age.
The concentration of carbon dioxide in the atmosphere between 500 and 400 million years ago was about 20 times as high as that of today (the concentration of carbon dioxide in the atmosphere today is 0.03%). During this period, plants and arthropods began to colonize the land. The Earth's environment was temperate 360 million years ago; in those times, large forests were formed in many places around the globe, which later became the sources of coal. Their active photosynthesis consumed carbon dioxide, reducing the greenhouse effect. This process cooled the Earth. In the meantime, the concentration of oxygen, a byproduct of photosynthesis, increased. The Earth then experienced the ice age again between 350 and 250 million years ago. Insects increased their presence 300 million years ago. Cockroaches emerged in those times. The concentration of carbon dioxide became today's value, whereas the concentration of oxygen became around 35%. The fungi that were able to digest trees then emerged. Their metabolic activity decreased the concentration of oxygen while gradually increased that of carbon dioxide.
A supercontinent Pangaea consisting of today's divided continents was formed 250 million years ago. The formation of Pangaea triggered global volcanic activities, decimating life on the Earth - about 95% disappeared. The volcanic activities released a large amount of carbon dioxide, increasing the temperature of atmosphere and sea water, which liberated a large amount of methane from methane hydrate that was formed during the ice age and held on the ocean floor. Liberated atmospheric methane generated a large amount of carbon dioxide and water vapor. This reduced the concentration of oxygen extremely. The resulting environment caused a mass extinction of life again. Only dinosaurs adapted for the reduced concentration of oxygen survived this era.
Although the concentration of oxygen decreased to 12% and that of carbon dioxide increased between several and ten times as high as that of today 200 million years ago, the climate had been temperate in those times. After that, the concentration of oxygen increased and that of carbon dioxide decreased. Dinosaurs which survived this climate change flourished 100 million years ago, but living organisms including dinosaurs became extinct 65.5 million years ago. The most supported hypothesis that accounts for this mass extinction is a drastic change of the Earth's environment because of the impact of meteorites.
A sudden global warming occurred 55 million years ago. This was because of the activity of submarine volcanoes in the North Atlantic Ocean, which released a large amount of greenhouse gasses like carbon dioxide and methane into the atmosphere. Again, methane hydrate on the ocean floor melted, liberating methane gas into the atmosphere, which increased carbon dioxide and water vapor in the atmosphere. This overall process increased the temperature of the surface of the sea by 3-4°C, and the atmospheric temperature by as much as 20°C in the subtropical zone. However, the Earth began to get cold 40 million years ago. During this time, the glacier was formed in Antarctica.
Substantial degree of global climate fluctuations with a typical period of 100,000 years were observed from around 700,000 years ago. For instance, the peak of a warm interglacial period was observed around 230,000 years ago, whereas that of a subsequent glacial period was observed around 140,000 years ago. After that, the Earth got warmer at a faster rate, and peaked between 130,000 and 120,000 years ago. Then from about 110,000 years ago, the Earth began to get colder again. The Earth slowly repeated cycles of warming and cooling, heading toward a glacial period over time. Homo sapiens emerged in Africa about 200,000 years ago, and spread over the continents around 60,000 years ago.
Afterward, the Earth experienced rapid rise of sea surface associated with global warming. During the Würm glaciation (the Last Glacial Maximum) about 20,000 years ago, the annual average temperature dropped by 7-8°C, glaciers expanded and the sea surface lowered by 100-130 m, as compared with today. Through further small repetitive cycles of global warming and cooling, the Earth got warmer over time. The Earth finally had an environment similar to the one of today between 10,000 and 8,000 years ago.
We have presented so far a brief summary of the transition of the Earth's environment. During these considerable fluctuations of the Earth's environment from global warming to cooling or vice versa, all forms of life on the Earth have struggled for their survival and repeated cycles of emergence and extinction. Human beings have thus far enjoyed their life in the current short-term temperate and hospitable environment. Habitable environmental conditions for human beings are certainly limited as compared with other living organisms which have survived a far greater range of environmental fluctuations. A challenge that God has given us is to find a way to maintain this hospitable environment on the Earth for the future, or improve it in order to maintain its hospitability.
The Industrial Revolution occurred in Britain in the 1760s, and the effect spread over the world. Since then, the Earth has entered into a new era of mass consumption of energy. The world population, which was believed to be about 300 million in 1 A.D., grew to slightly below 1 billion in 1800. It reached about 7.3 billion 210 years later, in 2010. The rapidly growing population poses a great threat to the Earth's environment. This population growth, coupled with the improvement of living standards, will inevitably require a greater consumption of energy in the future, which will result in a further increase in the production of greenhouse gases, primarily carbon dioxide. This has been a global concern that must be addressed immediately. This view has been shared by many people, including policy makers in many countries around the world.
However, some scientists warn of the possible extinction of Homo sapiens in 200 years, or in the worst case, 80 years, if this emission of greenhouse gases continues as it does today. The accumulation of carbon dioxide increases the amount of water vapor, another factor contributing to the greenhouse effect. The synergy between carbon dioxide and water vapor further increases the temperature of atmosphere and sea water. Average 2°C increase in the sea water temperature liberates a large amount of methane in methane hydrate accumulated on the ocean floor during the cooling phase of the Earth, which increases the Earth's temperature further. Liberated methane gas combines with oxygen to exponentially increase atmospheric carbon dioxide and water vapor. If the amount of carbon dioxide in the atmosphere exceeds 3%, it causes dizziness, headaches, and nausea in people. If it exceeds 7%, people lose consciousness. At this concentration, carbon dioxide not only exacerbates global warming, but also directly affects the survival of human beings.
So, what should we do? Save energy resources? Certainly. In order to reduce unnecessary wasting of energy, developed countries and major fossil-fuel consuming countries must act to reduce the emission of greenhouse gases spontaneously beyond the target set by the Paris Agreement (COP21). This effort surely includes the promotion of further improvement in the efficiency of internal combustion engines, but more than that, should include a step-by-step implementation of more immediate effective measures, such as reducing the use of gasoline-fueled cars.
We must also promote the use of alternative energy sources that replace fossil fuels. The Sun is an absolutely essential source of energy for life on the Earth. So is geothermal energy, which also keeps our planet warm. The energy of the Sun comes from nuclear fusion, whereas half of the geothermal energy comes from nuclear decay heat. Both are nuclear origins that yield an extraordinary large amount of energy for our planet. The nuclear energy sustains even the life of activists against the peaceful uses of nuclear energy and the activity of political parties promoting the abandonment of nuclear power.
Rather than obtain energy from burning fossil fuels to yield carbon dioxide, we should promote the utilization of not only nuclear power but also hydropower, solar energy, wind power, geothermal, biomass, etc. These production methods have advantages and disadvantages; therefore, they must complement each other to realize an efficient production of energy. What we expect from policy makers and business leaders in the world is to take strong leadership to promote global collaboration of technology to develop alternative energy sources that will replace oil.
We may have to face the risk of the extinction of human species because of an unavoidable environmental change on the Earth in the future, but we must prevent the extinction of our offspring because of our insatiable consumption of fossil fuels.